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Unit converter

Convert length, weight, area, volume, temperature, speed, pressure, energy, power, data, fuel economy and time with exact factors.

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LengthWeightAreaVolumeTemperatureSpeedPressureEnergyPowerData storageFuel economyTime
Fromcentimetre (cm)
1
Toinch (in)
0.3937007874
DecimalsAuto (up to 10 significant digits)

Formula: in = cm × 0.393700787402

1 cm = 0.3937007874 in

Common values: centimetre to inch

cmin
1 cm0.3937007874 in
2 cm0.7874015748 in
5 cm1.968503937 in
10 cm3.937007874 in
20 cm7.874015748 in
25 cm9.842519685 in
50 cm19.68503937 in
100 cm39.37007874 in
250 cm98.42519685 in
500 cm196.8503937 in
1,000 cm393.7007874 in

How it works

Pick a category tab, choose the two units and type in either box: the other box updates as you type, in both directions. The swap button flips the units, the Decimals control sets how many places are shown, and the table underneath lists common values for the pair you picked, so you can read off several conversions at once.

Every factor is either exact by definition or derived from exact definitions. Since the international yard and pound agreement of 1959, an inch is exactly 25.4 mm and a pound exactly 0.45359237 kg; a US gallon is 231 cubic inches (3.785411784 L) and an imperial gallon 4.54609 L. Pressure, energy and power units derive from those plus standard gravity (9.80665 m/s²), the thermochemical calorie (4.184 J) and the International Table BTU (1055.05585262 J), as listed in NIST Special Publication 811 (Guide for the Use of the SI), Appendix B. Temperature is converted with offsets, not a single factor, and fuel economy with a reciprocal, because litres per 100 km and miles per gallon run in opposite directions.

The converter shows the formula it is using for the selected pair, so you can check it or reuse it in a spreadsheet. Nothing is sent to a server: the numbers are computed in your browser.

What each category covers

  • Length: mm, cm, m, km, in, ft, yd, mi, NM.
  • Weight: mg, g, kg, t, oz, lb, st, short ton, long ton, oz t, ct.
  • Area: cm², m², ha, km², sq in, sq ft, sq yd, ac, sq mi.
  • Volume: mL, L, m³, metric cup, US tsp, US tbsp, US fl oz, US cup, US legal cup, US pt, US qt, US gal, in³, ft³, imp fl oz, imp pt, imp gal.
  • Temperature: °C, °F, K, °R.
  • Speed: m/s, km/h, mph, kn, ft/s.
  • Pressure: Pa, hPa, kPa, MPa, bar, mbar, atm, kgf/cm², psi, mmHg, Torr, inHg.
  • Energy: J, kJ, MJ, cal, kcal, Wh, kWh, BTU, ft·lbf, eV.
  • Power: W, kW, MW, hp, PS, BTU/h, kcal/h, TR.
  • Data storage: bit, kbit, Mbit, Gbit, B, kB, MB, GB, TB, PB, KiB, MiB, GiB, TiB, PiB.
  • Fuel economy: km/L, L/100 km, mpg (US), mpg (UK).
  • Time: ns, µs, ms, s, min, h, d, wk, mo, yr, decade, century.

How to use it

  1. Choose a category tab (Length, Weight, Temperature, ...). Arrow keys move between tabs.
  2. Pick the unit to convert from and the unit to convert to.
  3. Type a value in either box; the other box updates immediately.
  4. Use the swap button to reverse the direction and the Decimals menu to round the result.
  5. Read further values from the common-values table, or copy the result line.

Frequently asked questions

How accurate are the conversions?

The factors are exact wherever an exact definition exists (inch, foot, mile, pound, ounce, US and imperial gallons, standard atmosphere, calorie, BTU) and computed in double precision. Results are shown with up to 10 significant digits in Auto mode; choose fewer decimals to round.

Why is temperature not a simple multiplication?

Celsius and Fahrenheit have different zero points, so the conversion needs an offset: °F = °C × 9/5 + 32. The converter goes through kelvin internally and shows the textbook formula for the pair you choose.

Can I type numbers with commas or Arabic digits?

Yes. "1,234.5", "1 234,5" style input with a single decimal comma, exponents such as 1e6, and Arabic-Indic digits (١٢٣) are all read correctly. A comma followed by exactly three digits is treated as a thousands separator.

Which gallon and which cup does it use?

Both are listed separately: US gallon (3.785 L) and imperial gallon (4.546 L); US customary cup (236.59 mL), US legal cup used on nutrition labels (240 mL) and metric cup (250 mL). Pick the one your source uses.

Does it work offline?

Once the page has loaded, yes. All conversion factors are part of the page and the calculations run locally.

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