About this converter
A free converter for everyday and technical units, plus conversion charts, worked formulas and exact definitions you can check the results against.
16 categories
Length, weight, temperature, volume, speed, area, data, energy, power, fuel economy, pressure, angle, time, cooking, number bases and frequency, with more than 150 units in total.
Instant results
Results update as you type, with no button to press. The right-hand panel shows the value in every unit of the category at once, so you can compare at a glance.
Copy with one click
Click or tap any row in the results panel to copy that value to your clipboard. A tick appears next to the row to confirm the copy.
Links you can share
The page address updates as you convert, for example 350 °F to °C. Bookmark it or send it and it opens to the same conversion.
How the calculations work
Each category has one base unit: metres for length, kilograms for weight, litres for volume, pascals for pressure, and so on. Your value is converted into the base unit and then into the target unit. This means every unit only needs one well-checked conversion factor, and any combination of units stays consistent with every other.
Three categories need more than a multiplier. Temperature scales have different zero points, so they need an offset as well as a factor. Fuel economy mixes "distance per fuel" and "fuel per distance", which are reciprocals of each other. And dBm is logarithmic. These are handled with their own formulas, shown under the input as you convert.
Metric, imperial and US customary
The metric system (SI) is used by almost every country and is built on powers of ten. The imperial system was standardised in Britain in 1824, and the US customary system grew from older English units. The two share many names but not always sizes: a US gallon is about 3.785 litres, while an imperial gallon is about 4.546 litres, and a US pint is 16 fluid ounces against the imperial pint's 20.
Since the 1959 international yard and pound agreement, the inch, foot, yard, mile and pound have been defined exactly in metric terms in both systems. That is why the converter can give exact results for them rather than approximations.
Conversion charts
Ready-made tables for the conversions people look up most often. Each one links to the converter preloaded with that conversion, if you need a value that isn't listed.
Centimetres and inches
One inch is exactly 2.54 cm.
| cm | inches | inches | cm |
|---|---|---|---|
| 1 | 0.394 | 1 | 2.54 |
| 2 | 0.787 | 2 | 5.08 |
| 5 | 1.969 | 3 | 7.62 |
| 10 | 3.937 | 4 | 10.16 |
| 15 | 5.906 | 6 | 15.24 |
| 20 | 7.874 | 8 | 20.32 |
| 25 | 9.843 | 10 | 25.4 |
| 30 | 11.811 | 12 | 30.48 |
| 50 | 19.685 | 24 | 60.96 |
| 100 | 39.37 | 36 | 91.44 |
Height: feet and inches to centimetres
Heights in feet and inches, converted to centimetres and metres.
| Height | Centimetres | Metres |
|---|---|---|
| 4′ 10″ | 147.3 cm | 1.47 m |
| 5′ 0″ | 152.4 cm | 1.52 m |
| 5′ 2″ | 157.5 cm | 1.57 m |
| 5′ 4″ | 162.6 cm | 1.63 m |
| 5′ 6″ | 167.6 cm | 1.68 m |
| 5′ 8″ | 172.7 cm | 1.73 m |
| 5′ 10″ | 177.8 cm | 1.78 m |
| 6′ 0″ | 182.9 cm | 1.83 m |
| 6′ 2″ | 188 cm | 1.88 m |
| 6′ 4″ | 193 cm | 1.93 m |
| 6′ 6″ | 198.1 cm | 1.98 m |
Kilograms, pounds and stone
One pound is exactly 0.45359237 kg, and a stone is 14 lb. Stone is still common for body weight in the UK and Ireland.
| kg | pounds (lb) | stone |
|---|---|---|
| 1 | 2.2 | — |
| 5 | 11 | — |
| 10 | 22 | 1 st 8 lb |
| 20 | 44.1 | 3 st 2.1 lb |
| 30 | 66.1 | 4 st 10.1 lb |
| 40 | 88.2 | 6 st 4.2 lb |
| 50 | 110.2 | 7 st 12.2 lb |
| 60 | 132.3 | 9 st 6.3 lb |
| 70 | 154.3 | 11 st 0.3 lb |
| 80 | 176.4 | 12 st 8.4 lb |
| 90 | 198.4 | 14 st 2.4 lb |
| 100 | 220.5 | 15 st 10.5 lb |
Celsius and Fahrenheit
Everyday reference points. Values are exact or rounded to one decimal place.
| Celsius | Fahrenheit | Reference |
|---|---|---|
| -40 °C | -40 °F | Celsius and Fahrenheit meet |
| -18 °C | -0.4 °F | Freezer |
| -10 °C | 14 °F | |
| 0 °C | 32 °F | Water freezes |
| 4 °C | 39.2 °F | Fridge |
| 10 °C | 50 °F | |
| 15 °C | 59 °F | |
| 20 °C | 68 °F | Room temperature |
| 25 °C | 77 °F | |
| 30 °C | 86 °F | |
| 35 °C | 95 °F | |
| 37 °C | 98.6 °F | Body temperature |
| 40 °C | 104 °F | |
| 100 °C | 212 °F | Water boils (sea level) |
Oven temperatures and gas marks
Recipes round oven settings to convenient numbers, so these are the conventional kitchen equivalents rather than exact conversions.
| Gas mark | Conventional | Fan oven | Fahrenheit |
|---|---|---|---|
| Gas 1 | 140 °C | 120 °C | 275 °F |
| Gas 2 | 150 °C | 130 °C | 300 °F |
| Gas 3 | 170 °C | 150 °C | 325 °F |
| Gas 4 | 180 °C | 160 °C | 350 °F |
| Gas 5 | 190 °C | 170 °C | 375 °F |
| Gas 6 | 200 °C | 180 °C | 400 °F |
| Gas 7 | 220 °C | 200 °C | 425 °F |
| Gas 8 | 230 °C | 210 °C | 450 °F |
| Gas 9 | 240 °C | 220 °C | 475 °F |
Fan (convection) ovens circulate hot air, so recipes usually lower the temperature by about 20 °C. Check your oven manual, as some models adjust automatically.
Convert °F to °CKilometres and miles
One mile is exactly 1.609344 km. Race distances are defined in kilometres.
| km | miles | Note |
|---|---|---|
| 1 | 0.62 | |
| 5 | 3.11 | 5K race |
| 10 | 6.21 | 10K race |
| 21.0975 | 13.11 | Half marathon |
| 42.195 | 26.22 | Marathon |
| 50 | 31.07 | |
| 100 | 62.14 |
Speed limits: mph and km/h
Common speed-limit values. Road signs use whole numbers, so 30 mph is not the same as a 50 km/h zone.
| mph | km/h | m/s |
|---|---|---|
| 20 | 32.2 | 8.9 |
| 25 | 40.2 | 11.2 |
| 30 | 48.3 | 13.4 |
| 40 | 64.4 | 17.9 |
| 50 | 80.5 | 22.4 |
| 60 | 96.6 | 26.8 |
| 70 | 112.7 | 31.3 |
| 80 | 128.7 | 35.8 |
US cups, millilitres and spoons
US customary cup of 8 fluid ounces. US nutrition labels use a rounded 240 mL cup, and Australia, New Zealand and Canada often use a 250 mL metric cup.
| Cups | Millilitres | Tablespoons | Fluid ounces |
|---|---|---|---|
| ⅛ cup | 29.6 mL | 2 tbsp | 1 fl oz |
| ¼ cup | 59.1 mL | 4 tbsp | 2 fl oz |
| ⅓ cup | 78.9 mL | 5.33 tbsp | 2.67 fl oz |
| ½ cup | 118.3 mL | 8 tbsp | 4 fl oz |
| ⅔ cup | 157.7 mL | 10.67 tbsp | 5.33 fl oz |
| ¾ cup | 177.4 mL | 12 tbsp | 6 fl oz |
| 1 cup | 236.6 mL | 16 tbsp | 8 fl oz |
What one US cup weighs
Converting volume to weight depends on the ingredient's density, which is why this converter keeps cooking volumes and weights separate. These figures are typical kitchen values.
| Ingredient | Weight of 1 cup | Note |
|---|---|---|
| Water | 237 g | Same as its volume in mL |
| All-purpose flour | 120–130 g | Depends on how it is scooped |
| Granulated sugar | ~200 g | |
| Packed brown sugar | ~213 g | |
| Butter | 227 g | 2 US sticks = 8 oz |
| Honey | ~340 g | About 1.4 × the weight of water |
For baking, weighing flour is far more reliable than measuring by the cup: a loosely spooned cup and a firmly scooped cup can differ by 20% or more.
Convert ounces to gramsFuel economy: MPG and L/100 km
Higher MPG means better economy, but lower L/100 km does. UK miles-per-gallon figures use the larger imperial gallon.
| MPG (US) | L/100 km | MPG (UK) |
|---|---|---|
| 15 | 15.7 | 18 |
| 20 | 11.8 | 24 |
| 25 | 9.4 | 30 |
| 30 | 7.8 | 36 |
| 35 | 6.7 | 42 |
| 40 | 5.9 | 48 |
| 50 | 4.7 | 60 |
| 60 | 3.9 | 72.1 |
Tyre pressure: PSI, bar and kPa
Always use the pressure printed on your vehicle's door-jamb sticker or in the handbook, not the maximum shown on the tyre.
| PSI | bar | kPa |
|---|---|---|
| 26 | 1.79 | 179 |
| 28 | 1.93 | 193 |
| 30 | 2.07 | 207 |
| 32 | 2.21 | 221 |
| 33 | 2.28 | 228 |
| 35 | 2.41 | 241 |
| 36 | 2.48 | 248 |
| 38 | 2.62 | 262 |
| 40 | 2.76 | 276 |
| 44 | 3.03 | 303 |
Why a new drive looks smaller
Drive makers count in powers of 1,000. Windows counts in powers of 1,024 but still labels the result "GB", so the same drive appears smaller.
| Advertised size | What Windows reports |
|---|---|
| 128 GB | 119.2 GiB |
| 256 GB | 238.4 GiB |
| 500 GB | 465.7 GiB |
| 1 TB | 0.91 TiB (931 GiB) |
| 2 TB | 1.82 TiB (1,863 GiB) |
| 4 TB | 3.64 TiB (3,725 GiB) |
macOS switched to decimal units in 2009, so a Mac shows the advertised size.
Open the data converterInternet speed: Mbps to download speed
Broadband is sold in megabits per second. Download managers show megabytes per second, which is 8 times smaller.
| Plan (Mbps) | Top download speed | Downloaded per minute |
|---|---|---|
| 10 | 1.25 MB/s | 75 MB |
| 25 | 3.12 MB/s | 188 MB |
| 50 | 6.25 MB/s | 375 MB |
| 100 | 12.5 MB/s | 750 MB |
| 300 | 37.5 MB/s | 2.25 GB |
| 500 | 62.5 MB/s | 3.75 GB |
| 1000 | 125 MB/s | 7.5 GB |
These are theoretical maximums. Real downloads are typically 5–10% slower because of network overhead.
Convert megabitsHow to convert units by hand
Most conversions fall into one of six patterns. Once you know which one you're dealing with, you can check any result yourself.
Multiply by a factor
Length, weight, volume, area, speed, energy and most other units are simple ratios. Find how many base units are in one of your unit, and multiply.
For area and volume, the factor is squared or cubed: 1 ft = 0.3048 m, so 1 ft² = 0.3048² = 0.09290304 m².
Add an offset: temperature
Celsius and Fahrenheit put zero in different places, so you have to subtract or add as well as multiply, in the right order.
Watch out for differences. A temperature change has no offset: a rise of 10 °C is a rise of 18 °F, not 50 °F. The converter converts temperatures, not differences, so multiply a difference by 9/5 yourself.
Take the reciprocal: fuel economy
Miles per gallon measures distance per unit of fuel, while litres per 100 km measures fuel per unit of distance. Converting between them means dividing, not multiplying.
The constant 235.215 is 100 × 3.785411784 (litres per US gallon) ÷ 1.609344 (km per mile). Because the relationship is a curve, improving from 10 to 20 MPG saves far more fuel than improving from 40 to 50.
Use logarithms: dBm
Radio and network engineers express signal power in decibels relative to one milliwatt. Each 10 dB step is a factor of ten.
Useful anchors: 0 dBm is 1 mW, 30 dBm is 1 W, and adding 3 dB roughly doubles the power.
Know which "gigabyte" you mean
Storage units come in decimal (powers of 1,000) and binary (powers of 1,024) versions. The official binary names are kibibyte (KiB), mebibyte (MiB) and gibibyte (GiB), but most software still writes KB, MB and GB.
This converter uses binary values for byte units (1 KB = 1,024 B), matching what Windows reports, and decimal values for bit units (1 Mbit = 1,000,000 bits), matching how internet speeds are sold.
Divide repeatedly: number bases
To write a decimal number in another base, divide by the base repeatedly and read the remainders from last to first.
To go back, multiply each digit by the base raised to its position: 7 × 16² + 14 × 16 + 10 = 2026.
Rules of thumb for mental maths
When you don't need a precise answer, these shortcuts get you close enough. The last column shows how far off each one is.
| Converting | Shortcut | Exact factor | Accuracy |
|---|---|---|---|
| °C → °F | Double it, add 30 | × 1.8 + 32 | Exact at 10 °C; 2 °F off at 0 and 20 °C; 4 °F off at 30 °C |
| km → miles | Multiply by 5, divide by 8 | × 0.621371 | 0.6% high |
| miles → km | Multiply by 1.6 | × 1.609344 | 0.6% low |
| kg → pounds | Double it, add 10% | × 2.204623 | 0.2% low |
| pounds → kg | Halve it, subtract 10% | × 0.453592 | 0.8% low |
| metres → feet | Multiply by 3.3 | × 3.280840 | 0.6% high |
| inches → cm | Multiply by 2.5 | × 2.54 | 1.6% low |
| litres → US gallons | Divide by 4 | ÷ 3.785412 | 5.4% low |
| acres → hectares | Multiply by 0.4 | × 0.404686 | 1.2% low |
| MPG (US) ↔ L/100 km | Divide 235 by the value | 235.215 ÷ value | 0.1% off |
| Mbps → MB/s | Divide by 8 | ÷ 8 | Exact (decimal megabytes) |
| km/h → m/s | Divide by 3.6 | ÷ 3.6 | Exact |
A handy trick for distances: consecutive Fibonacci numbers are close to the mile-to-kilometre ratio. 3 miles ≈ 5 km, 5 miles ≈ 8 km, 8 miles ≈ 13 km, 13 miles ≈ 21 km.
A guide to all 16 categories
What each category measures, the units it includes, and where you're likely to need it.
📏 Length
Metres, kilometres, centimetres, millimetres, miles, yards, feet, inches, nautical miles and light-years. The metre has been defined by the speed of light since 1983, and the imperial units are defined from the metre. Useful for travel distances, DIY and furniture measurements, and product sizes listed in a different system.
Try feet to metres⚖️ Weight & mass
Tonnes, kilograms, grams, milligrams, micrograms, pounds, ounces and stone. In everyday speech "weight" means mass. A physicist would call weight the force gravity exerts on that mass, measured in newtons. Common uses include nutrition labels, postage limits, gym plates, and converting UK body weights given in stone.
Try stone to kilograms🌡️ Temperature
Celsius, Fahrenheit, Kelvin and Rankine. Kelvin starts at absolute zero, the lowest temperature possible, and is the SI unit used in science. Rankine is the equivalent absolute scale using Fahrenheit-sized degrees and still appears in some US engineering work.
Try body temperature in °F🧪 Volume
Litres, millilitres, kilolitres, cubic metres and centimetres, US gallons, quarts, pints, cups, fluid ounces and spoons, plus UK gallons and pints. The imperial gallon is about 20% larger than the US gallon, a common source of confusion when comparing fuel prices or drink recipes between countries.
Try UK to US gallons⚡ Speed
Metres per second, km/h, mph, knots, feet per second, Mach and the speed of light. One knot is one nautical mile per hour, and a nautical mile is about one minute of latitude, which is why knots are standard in aviation and at sea. Mach here uses 343 m/s, the speed of sound in dry air at 20 °C. The real speed of sound falls with temperature, so it is lower at cruising altitude.
Try knots to km/h⬛ Area
Square kilometres, metres, centimetres and millimetres, hectares, acres, and square miles, yards, feet and inches. Property listings and farmland often mix acres and hectares. One hectare is about 2.471 acres, and an acre was historically the area a team of oxen could plough in a day.
Try square feet to m²💾 Data
Bits and bytes, kilobytes through petabytes, and kilobits, megabits and gigabits for network speeds. Byte units here are binary (1 KB = 1,024 B). Bit units are decimal because that is how connection speeds are advertised. Remember that a byte is 8 bits, so a 100 Mbps connection downloads at most about 12.5 MB per second.
Try Mbps to MB🔋 Energy
Joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours, electronvolts, BTU and foot-pounds. Food labels in the US print "Calories" with a capital C, which means kilocalories, so a 250-Calorie snack contains 250,000 small calories. Electricity bills use kilowatt-hours, and heating equipment is often rated in BTU.
Try kcal to kJ⚙️ Power
Watts from milliwatts to gigawatts, mechanical and metric horsepower, BTU per hour and per second, calories per second, foot-pounds per second, dBm, and the ton of refrigeration. A ton of refrigeration, about 3.517 kW, is the cooling rate needed to melt one short ton of ice in 24 hours, and it is still used to rate air-conditioning systems.
Try horsepower to kW🌀 Pressure
Pascals, kilopascals, megapascals, bar, millibar, atmospheres, torr (mmHg), PSI, inches of mercury and kgf/cm². PSI dominates for US tyre pressures, bar is standard across most of Europe, and mmHg is still used for blood pressure. Weather forecasts use millibars or hectopascals, which are the same size.
Try PSI to bar📐 Angle
Degrees, radians, gradians, turns, arcminutes, arcseconds and NATO mils (6,400 to a full circle). Radians are the natural unit in maths and physics: one radian is the angle at which the arc length equals the radius. Gradians divide a circle into 400 and appear in some surveying equipment.
Try degrees to radians⏱️ Time
Nanoseconds through centuries, plus Planck time, about 5.39 × 10⁻⁴⁴ seconds, the scale at which current physical theories are expected to break down. Years here are Julian years of 365.25 days, the convention used in astronomy, and months are an average twelfth of that year.
Try days to hours🍳 Cooking
Millilitres, teaspoons, tablespoons, fluid ounces, US and metric cups, pints, quarts and gallons, plus the informal pinch, dash and smidgen. These use the definitions printed on many US measuring-spoon sets: a dash is ⅛ teaspoon, a pinch is 1/16 and a smidgen is 1/32.
Try tablespoons to mL🔢 Number base
Decimal, binary, octal, hexadecimal, base 32, a base 64 digit set, and Roman numerals. Programmers use hexadecimal for memory addresses and colour codes, since two hex digits describe exactly one byte. Large whole numbers are handled exactly, and Roman numerals cover 1 to 3,999.
Try 2026 in Roman numerals〰️ Frequency
Microhertz through terahertz, RPM and revolutions per second, radians per second, beats per minute and frames per second. It covers audio engineering, engine and motor speeds, music tempo and video frame rates in one place. A tempo of 120 BPM, for example, is exactly 2 Hz.
Try RPM to Hz⛽ Mileage
L/100 km, km/L, US and imperial MPG, miles per litre, km per US gallon and millilitres per mile. The relationship is reciprocal rather than linear, so it can't be converted with a single multiplier. It's useful for comparing a car's official figures across regions, since Europe uses L/100 km, the US uses MPG (US) and the UK uses MPG (imperial).
Try UK MPG to US MPGExact definitions behind the numbers
Many units that look approximate are defined exactly by law or international agreement. The converter uses these exact values, which is why its results match official references.
| Unit | Exact value | Defined by |
|---|---|---|
| Inch | 25.4 mm | International yard and pound agreement, 1959 |
| Foot / yard / mile | 0.3048 m / 0.9144 m / 1,609.344 m | International yard and pound agreement, 1959 |
| Pound | 0.45359237 kg | International yard and pound agreement, 1959 |
| Nautical mile | 1,852 m | International Hydrographic Conference, 1929 |
| US gallon | 231 in³ = 3.785411784 L | US customary definition |
| Imperial gallon | 4.54609 L | UK Weights and Measures Act 1985 |
| Acre | 43,560 ft² = 4,046.8564224 m² | Follows from the international foot |
| Light-year | 9,460,730,472,580,800 m | International Astronomical Union (Julian year × speed of light) |
| Speed of light | 299,792,458 m/s | Fixed when the metre was redefined, 1983 |
| Standard atmosphere | 101,325 Pa | General Conference on Weights and Measures, 1954 |
| Bar / torr | 100,000 Pa / 101,325 ÷ 760 Pa | Defined from the pascal and atmosphere |
| Calorie (thermochemical) | 4.184 J | Thermochemical convention; the International Table calorie is 4.1868 J |
| BTU (International Table) | 1,055.05585262 J | Derived from the International Table calorie |
| Kilowatt-hour | 3,600,000 J | 1,000 W for 3,600 s |
| Electronvolt | 1.602176634 × 10⁻¹⁹ J | Exact since the 2019 SI revision fixed the elementary charge |
| Metric horsepower (PS) | 735.49875 W | 75 kilogram-force metres per second |
| 0 °C | 273.15 K | SI definition of the Celsius scale |
Mechanical horsepower (550 foot-pounds per second, about 745.7 W) and the ton of refrigeration (12,000 BTU per hour) are exact in their own units, so their watt values carry the rounding of those conversions.
SI and binary prefixes
Prefixes turn a base unit into a larger or smaller one. The newest four, ronna, quetta, ronto and quecto, were added in November 2022.
| Prefix | Symbol | Factor |
|---|---|---|
| quetta | Q | 10³⁰ |
| ronna | R | 10²⁷ |
| yotta | Y | 10²⁴ |
| zetta | Z | 10²¹ |
| exa | E | 10¹⁸ |
| peta | P | 10¹⁵ |
| tera | T | 10¹² |
| giga | G | 10⁹ |
| mega | M | 10⁶ |
| kilo | k | 10³ |
| hecto | h | 10² |
| deca | da | 10¹ |
| Prefix | Symbol | Factor |
|---|---|---|
| deci | d | 10⁻¹ |
| centi | c | 10⁻² |
| milli | m | 10⁻³ |
| micro | μ | 10⁻⁶ |
| nano | n | 10⁻⁹ |
| pico | p | 10⁻¹² |
| femto | f | 10⁻¹⁵ |
| atto | a | 10⁻¹⁸ |
| zepto | z | 10⁻²¹ |
| yocto | y | 10⁻²⁴ |
| ronto | r | 10⁻²⁷ |
| quecto | q | 10⁻³⁰ |
Binary prefixes, defined by the IEC in 1998, use powers of 2: kibi (Ki) = 2¹⁰ = 1,024, mebi (Mi) = 2²⁰, gibi (Gi) = 2³⁰, tebi (Ti) = 2⁴⁰ and pebi (Pi) = 2⁵⁰. Case matters in SI symbols: "mW" is a milliwatt, "MW" is a megawatt, a billion times larger.
How today's units came to be
- 1791The French Academy of Sciences proposes the metre as one ten-millionth of the distance from the North Pole to the equator. Surveyors spend most of the 1790s measuring the meridian through Paris to fix its length.
- 1875Seventeen nations sign the Metre Convention in Paris, creating the International Bureau of Weights and Measures.
- 1889A platinum-iridium cylinder kept near Paris becomes the International Prototype of the Kilogram, the world's definition of mass for the next 130 years.
- 1959The US, UK, Canada, Australia, New Zealand and South Africa agree on exact metric values for the yard and the pound, ending small differences between their national units.
- 1960The metric system is formally named the International System of Units (SI).
- 1983The metre is redefined as the distance light travels in a vacuum in 1/299,792,458 of a second.
- 2019On 20 May, the kilogram, ampere, kelvin and mole are redefined using fixed constants of nature, and the physical prototype kilogram is retired as the standard.
Conversion mistakes that made history
The Mars Climate Orbiter (1999). Software supplied by one team reported thruster impulse in pound-force seconds, while NASA's navigation software expected newton-seconds. The small error accumulated over the months-long flight, and the spacecraft was lost as it arrived at Mars in September 1999.
The "Gimli Glider" (1983). Air Canada Flight 143, a Boeing 767, ran out of fuel in mid-air after the fuel load was calculated using a pounds-per-litre figure instead of kilograms-per-litre, leaving it with less than half the fuel needed. The crew glided the aircraft to a landing at a former air-force base in Gimli, Manitoba, and no one was killed.
Medicine doses at home. Confusing teaspoons with tablespoons, or using kitchen spoons of varying size, is a well-known cause of dosing errors with liquid medicines. The American Academy of Pediatrics recommends measuring children's liquid medicines only in millilitres, using the syringe or cup supplied.
Frequently asked questions
Is this converter accurate enough for professional or scientific use?
Where a unit has an exact legal or SI definition, the converter uses that exact value (see exact definitions). Results are shown to up to 8 significant figures, which is enough for everyday, academic and most engineering work. For regulatory, legal or safety-critical work, always check against the standard that applies in your field.
Why do some results differ slightly from other converters?
Usually it's rounding, or a different version of the same unit. There are several "calories" (4.184 J or 4.1868 J), several "cups" (236.6 mL, 240 mL or 250 mL), and Mach 1 depends on the air temperature. This page states which version it uses in each category guide above.
Why does 1 KB equal 1,024 bytes here?
Byte units in this converter follow the binary convention used by Windows and many other programs, where each step is 1,024. Strictly speaking these are kibibytes, mebibytes and so on. Hard drive makers and macOS use decimal units (1 KB = 1,000 B), which is why a "1 TB" drive shows as about 931 GB in Windows.
What's the difference between a US gallon and an imperial gallon?
A US gallon is 3.785 L and an imperial (UK) gallon is 4.546 L, so the imperial gallon is about 20% larger. The Volume category includes both, and the Mileage category has separate US and imperial MPG units.
Why is fuel economy handled differently?
MPG and km/L measure distance per unit of fuel, while L/100 km measures fuel per unit of distance. One is the reciprocal of the other, so the converter divides rather than multiplies. A simple multiplier would give results that go the wrong way.
Can I convert a cup of flour to grams?
Not with a unit converter alone, because a cup measures volume and a gram measures mass. The answer depends on the ingredient's density: a cup of water weighs about 237 g, but a cup of flour weighs roughly 120–130 g. The cup weights chart lists typical values for common ingredients.
How do I convert a temperature change rather than a temperature?
Differences don't use the 32-degree offset. Multiply a Celsius difference by 9/5 to get the Fahrenheit difference, so a 5 °C rise is a 9 °F rise. A change of 1 °C is the same as a change of 1 K.
Why do Roman numerals stop at 3,999?
Standard Roman numerals have no symbol above M (1,000), and a fourth M breaks the usual rule of repeating a symbol at most three times. Larger values were historically written with a bar over the numeral to multiply it by 1,000, which the converter does not support.
Can I share or bookmark a conversion?
Yes. The page address updates as you convert, for example ?c=length&from=km&to=mi&v=10. Copy it from your browser's address bar to share the exact conversion.
Does this tool store or send my data anywhere?
The converter runs entirely in your browser, and nothing you type is sent to a server. The only thing it saves is your light or dark theme choice, stored in your browser. See the Privacy Policy for full details about the site.
Does it work offline?
Once the page has loaded, conversions keep working even if your connection drops, because all calculations happen on your device. It isn't packaged as an installable offline app.
About this page
CONVERT is an independent, free unit converter. Conversion factors are taken from the published SI definitions and from NIST's guide to SI units, and are checked against the charts on this page. Last reviewed: September 2026. Found an error or want a unit added? Get in touch.