About & Accuracy
Toolyard converts by pivoting through one base unit per category. Every factor below is the defined value, not a rounded approximation.
Where the factors come from
- Length and weight. The international yard and pound agreement of 1959 defines the inch as exactly 0.0254 m and the pound as exactly 0.45359237 kg. Every imperial length and mass figure here follows from those two definitions.
- Temperature. Converted by formula rather than a multiplier, because the scales do not share a zero point.
- Volume. US customary. A US gallon is 3.785411784 L by definition; an imperial gallon is 4.54609 L and is not used here.
- Data. Decimal (SI) units, so 1 kB is 1,000 bytes — the convention used by drive manufacturers and network speeds.
- Time. A year is taken as 365 days and a week as 7 days. Calendar months vary in length, so they are handled by the date tools rather than the unit converter.
Rounding
Calculations run in double-precision floating point and results are displayed to seven significant digits, with trailing zeros removed. Very large and very small results switch to scientific notation rather than printing a wall of zeros. The rounding is for display only — it never feeds back into a subsequent step.
Base unit and factors by category
Length
Metric and imperial distance units. The inch has been defined as exactly 2.54 cm since 1959, so every conversion on this page is exact rather than approximate.
| Unit | Symbol | In metres |
|---|---|---|
| Millimetres | mm | 0.001 |
| Centimetres | cm | 0.01 |
| Metres | m | 1 |
| Kilometres | km | 1,000 |
| Inches | in | 0.0254 |
| Feet | ft | 0.3048 |
| Yards | yd | 0.9144 |
| Miles | mi | 1,609.344 |
Weight
Mass units. The pound is defined as exactly 0.45359237 kg, and the ounce as one sixteenth of that, so these conversions are exact.
| Unit | Symbol | In kilograms |
|---|---|---|
| Milligrams | mg | 0.000001 |
| Grams | g | 0.001 |
| Kilograms | kg | 1 |
| Tonnes | t | 1,000 |
| Ounces | oz | 0.028349523125 |
| Pounds | lb | 0.45359237 |
| Stone | st | 6.35029318 |
Temperature
Temperature scales do not share a zero point, so they convert by formula rather than by a single multiplier.
| Unit | Symbol | In celsius |
|---|---|---|
| Celsius | °C | by formula |
| Fahrenheit | °F | by formula |
| Kelvin | K | by formula |
Volume
Volume units. Fluid ounces, cups, pints, quarts and gallons here are US customary, which are smaller than their imperial counterparts.
| Unit | Symbol | In litres |
|---|---|---|
| Millilitres | mL | 0.001 |
| Litres | L | 1 |
| Cubic metres | m³ | 1,000 |
| US fluid ounces | fl oz | 0.029573529562 |
| US cups | cup | 0.2365882365 |
| US pints | pt | 0.473176473 |
| US quarts | qt | 0.946352946 |
| US gallons | gal | 3.785411784 |
US customary measures. A US gallon is 3.785 L; an imperial gallon is 4.546 L.
Area
Area units for rooms, plots and land. An acre is defined as 4,046.8564224 m² exactly.
| Unit | Symbol | In square metres |
|---|---|---|
| Square centimetres | cm² | 0.0001 |
| Square metres | m² | 1 |
| Square kilometres | km² | 1,000,000 |
| Square inches | in² | 0.00064516 |
| Square feet | ft² | 0.09290304 |
| Acres | ac | 4,046.8564224 |
| Hectares | ha | 10,000 |
Speed
Speed units for driving, running and wind. A knot is one nautical mile per hour, which is why it does not line up neatly with mph.
| Unit | Symbol | In metres per second |
|---|---|---|
| Metres per second | m/s | 1 |
| Kilometres per hour | km/h | 0.277777777778 |
| Miles per hour | mph | 0.44704 |
| Knots | kn | 0.514444444444 |
| Feet per second | ft/s | 0.3048 |
Time
Durations from milliseconds to years. A year here is 365 days; leap years and calendar months are handled by the date calculators instead.
| Unit | Symbol | In seconds |
|---|---|---|
| Milliseconds | ms | 0.001 |
| Seconds | s | 1 |
| Minutes | min | 60 |
| Hours | h | 3,600 |
| Days | d | 86,400 |
| Weeks | wk | 604,800 |
| Years | yr | 31,536,000 |
One year is taken as 365 days and one week as 7 days. Calendar months vary in length and are not offered here.
Data
Digital storage in decimal (SI) units, the same convention drive manufacturers and network speeds use: 1 kB is 1,000 bytes.
| Unit | Symbol | In bytes |
|---|---|---|
| Bits | b | 0.125 |
| Bytes | B | 1 |
| Kilobytes | kB | 1,000 |
| Megabytes | MB | 1,000,000 |
| Gigabytes | GB | 1,000,000,000 |
| Terabytes | TB | 1,000,000,000,000 |
| Petabytes | PB | 1.0000 × 10^15 |
Decimal units: 1 kB = 1,000 bytes. Windows reports storage in binary units (1 KiB = 1,024 bytes), which is why a 1 TB drive shows as roughly 931 GB.
Energy
Energy units for food, electricity and physics. The "calorie" on a nutrition label is a kilocalorie — 1,000 of the calories used in physics.
| Unit | Symbol | In joules |
|---|---|---|
| Joules | J | 1 |
| Kilojoules | kJ | 1,000 |
| Calories | cal | 4.184 |
| Kilocalories | kcal | 4,184 |
| Watt hours | Wh | 3,600 |
| Kilowatt hours | kWh | 3,600,000 |
A food Calorie (capital C) is a kilocalorie. The thermochemical calorie of 4.184 J is used here.
Pressure
Pressure units for tyres, weather and diving. Tyre placards are usually in psi or bar; barometers report in hPa or mmHg.
| Unit | Symbol | In pascals |
|---|---|---|
| Pascals | Pa | 1 |
| Kilopascals | kPa | 1,000 |
| Bar | bar | 100,000 |
| Pounds per square inch | psi | 6,894.75729317 |
| Standard atmospheres | atm | 101,325 |
| Millimetres of mercury | mmHg | 133.322387415 |
The millimetre of mercury used here is the conventional 133.322387415 Pa, defined from mercury density and standard gravity. It is very slightly larger than the torr, which is defined as exactly 1/760 of an atmosphere — so 760 mmHg is 1.00000014 atm, not exactly 1.
Found a wrong figure?
Conversions are generated from the table above, so an error in a factor would show up across every page using it. If a result looks wrong, check the factor here first — it is the single source the whole site is computed from.