How the converter works: one base unit per category
Each of the ten categories has a base unit — metre, kilogram, kelvin, square metre, litre, metre per second, second, byte, pascal and joule. Your number is multiplied by the source unit’s factor to reach that base, then divided by the target unit’s factor. Because every unit in a category shares the same base, the whole table is converted from a single input, not just the pair you picked.
Temperature is the exception. Celsius, Fahrenheit, Kelvin and Rankine do not share a zero point, so they are converted with an affine formula — multiply by a slope, then add an offset — instead of a single factor. Temperature has its own code path, and every result is checked against absolute zero before it is displayed.
Every factor is labelled exact or approximate. Exact means fixed by definition: 1 in = 0.0254 m and 1 lb = 0.45359237 kg from the 1959 international yard and pound agreement, 1 nmi = 1852 m, 1 atm = 101,325 Pa. Approximate factors — Mach, mmHg, inHg, month, year, decade — depend on conditions or on an averaging convention and carry a ≈ badge in the unit table.
The definitions behind the numbers
- Length: 1 in = 0.0254 m, 1 ft = 0.3048 m, 1 yd = 0.9144 m, 1 mi = 1,609.344 m, 1 nmi = 1,852 m — all exact (NIST SP 811, Appendix B).
- Mass: 1 lb = 0.45359237 kg, 1 oz = 1/16 lb = 28.349523125 g, 1 stone = 14 lb = 6.35029318 kg, 1 grain = 64.79891 mg.
- Volume: 1 US gallon = 3.785411784 L, 1 imperial gallon = 4.54609 L, 1 US fl oz = 29.5735295625 mL, 1 UK fl oz = 28.4130625 mL.
- Pressure: 1 atm = 101,325 Pa exactly, 1 bar = 100,000 Pa, 1 psi ≈ 6,894.757 Pa, 1 Torr = 101,325/760 Pa.
- Energy: 1 cal (thermochemical) = 4.184 J, 1 kcal = 4,184 J, 1 kWh = 3.6 MJ, 1 BTU (IT) = 1,055.05585262 J.
- Data: 1 kB = 1,000 B but 1 KiB = 1,024 B (IEC 80000-13); 1 byte = 8 bits, so 1 Mbit = 125,000 bytes.
- Time: the second is defined by the caesium-133 hyperfine transition; minute, hour, day and week are exact multiples of it, while month (30.4375 d) and year (365.25 d) are averaging conventions.
These are the values published in the BIPM SI Brochure and NIST Special Publication 811 — not a rounded “about 2.2 pounds to the kilo”. Rounding happens only on display, at the number of significant digits you pick: Auto shows 6, and 2, 4 or 10 are available.
Worked examples
Height. 180 cm ÷ 2.54 = 70.8661 in. Divide by 12 to split off whole feet: 5 feet with 10.866 inches left over, so 180 cm is 5 ft 10.9 in. The converter shows the decimal 5.90551 ft — multiply the fractional part by 12 whenever you need feet-and-inches.
Storage. A “500 GB” SSD holds 500 × 10⁹ bytes, because manufacturers use decimal units. Your operating system divides by 1,024 three times: 500,000,000,000 ÷ 1,073,741,824 = 465.66 GiB, which Windows labels “465 GB”. Nothing is missing from the drive; the two labels measure different units.
Speed. 60 mph × 0.44704 = 26.8224 m/s, and 26.8224 × 3.6 = 96.56 km/h. Read the other way, a 100 km/h motorway limit is 62.14 mph — close to a US 60 mph limit, but not the same number, which matters when you are the one holding the ticket.
Conversions people look up most
- 1 in = 2.54 cm · 1 cm = 0.3937 in · 30 cm = 11.81 in
- 5 ft 10 in = 177.8 cm · 6 ft = 182.88 cm · 1 mi = 1.609344 km
- 1 kg = 2.20462 lb · 70 kg = 154.32 lb · 1 lb = 453.592 g · 1 oz = 28.3495 g
- 20 °C = 68 °F · 37 °C = 98.6 °F · 180 °C = 356 °F · −40 °C = −40 °F
- 1 acre = 4,046.86 m² · 1 hectare = 2.4711 acres = 10,000 m²
- 1 US gallon = 3.7854 L · 1 UK pint = 568.26 mL · 1 US cup = 240 mL
- 1 bar = 14.5038 psi · 32 psi = 2.206 bar · 1 atm = 14.696 psi
- 1 GB = 1,000 MB but 1 GiB = 1,024 MiB · 1 Gbit/s = 125 MB/s
Pairs that look identical but are not
MB vs MiB. Storage capacity and network speeds are decimal (1 MB = 1,000,000 bytes); RAM and most operating systems are binary (1 MiB = 1,048,576 bytes). IEC 80000-13 introduced the kibi/mebi/gibi prefixes in 1998 precisely because “kilobyte” was being used for both, and the gap grows with scale: 2.4% at kB, 7.4% at GB, 10% at TB.
Bits vs bytes. Internet plans are sold in bits per second, files are measured in bytes. A 1 Gbit/s line moves at most 125 MB/s, and a 100 Mbit/s line about 12.5 MB/s — before protocol overhead. The data tab lists bit units in their own group so the two can never be mixed up by accident.
US vs imperial volume. A US gallon is 3.785 L and a UK gallon 4.546 L — about 20% larger. A US pint is 473 mL against a UK pint of 568 mL, and a US fluid ounce (29.57 mL) is slightly larger than a UK one (28.41 mL). British and American recipes are not interchangeable at these volumes.
Tons. A metric tonne is 1,000 kg, a US short ton 907.18474 kg and a UK long ton 1,016.0469088 kg — a 12% spread between the extremes. Freight quotes, emissions figures and material orders all use “ton” without saying which, so the unit table lists the three separately.
Common mistakes and their fixes
Converting a difference as if it were a reading. This tool converts points on a scale. A room at 10 °C is 50 °F, but a 10 °C rise is an 18 °F rise (10 × 9/5), with no +32 offset. The same trap appears with pressure: gauge pressure is measured from atmospheric, absolute pressure from vacuum, and the two differ by 101,325 Pa.
Rounding too early. Convert once at full precision and round at the end. Chaining rounded steps (cm → in → ft, rounding each time) can shift a result by several millimetres over a long distance. The precision selector only changes what is shown; the conversion itself always runs at full double precision.
Mass and weight, power and energy. Kilograms and pounds here are units of mass (the avoirdupois pound, not pound-force). A kilowatt is a rate and a kilowatt-hour is an amount, so the energy tab converts kWh, MJ and kcal but cannot turn kW into kWh — you need a duration for that.
Conversions this table cannot do. Fuel economy is reciprocal, not proportional — mpg and L/100 km multiply to a constant instead of scaling together — so it is not a unit pair. Of the traditional Korean units only the pyeong (坪) is in the table — under Area, defined exactly as 400/121 m² ≈ 3.305785 m² (the Japanese tsubo is the same size). The rest you convert by hand first: 1 don = 3.75 g, 1 geun = 600 g for meat.
Where these conversions get used
Cooking and shopping. American recipes are written in cups, tablespoons and Fahrenheit; European ones in grams and Celsius. The volume tab keeps US and UK spoons and cups apart, and the temperature tab covers oven settings — 180 °C is 356 °F, which US recipes round to 350 °F.
Travel, sport and health. Speed limits, running distances, luggage allowances and body measurements all cross the metric/imperial line. A 23 kg checked bag is 50.7 lb, a marathon is 26.219 miles, and a 5K is 3.107 miles.
Engineering and IT. Datasheets mix psi with bar, BTU with kW, and MB with MiB. Because every unit in a category is shown at once, you can read a spec sheet in the units it was written in and the ones your team uses side by side, and copy any row of the table.