Unit Converter

A unit converter that works every conversion from the definition, at a precision that matches your input rather than a fixed number of decimals. It doubles as a temperature converter, with the offset Celsius and Fahrenheit need.

Value

What kind of unit

Length: from

Length: to

Notice: Not for measuring or calculating medication. Incorrect dosing can cause serious injury. Use the dosing device supplied with the medication and follow the label or the directions of a pharmacist or physician.

In inches3.93701 in

Conversion factor Exact by definition. The inch has been exactly 2.54 cm and the pound exactly 0.45359237 kg since the 1959 international yard and pound agreement, so these are not measurements that can drift.: 1 centimeter = 0.3937007874 inches
The other way: 1 inch = 2.54 centimeters
Working: 10 x 0.3937007874 = 3.93701
10 cm in every unit: 100 mm 10 cm 0.1 m 0.0001 km 3.937 in 0.32808 ft 0.10936 yd 0.000062137 mi 0.000053996 nmi

centimeters to inches

cm	in
1	0.393701
2	0.787402
5	1.9685
10	3.93701
25	9.84252
50	19.685
100	39.3701

How the Unit Converter works

Unit conversion, metric or imperial, is one multiplication, and the only reason it fills so many web pages is that there are a lot of units.

The structure that makes it simple: pick one base unit per kind of measurement (the meter for length, the kilogram for mass, the liter for volume) and define every other unit by its ratio to that base. A conversion is then from-unit's ratio divided by to-unit's ratio. An inch is 0.0254 meters and a centimeter is 0.01, so an inch is 2.54 centimeters. There is no table of pairs to maintain and no individual cell that can be wrong.

The factors themselves are definitions rather than measurements, which is worth knowing because it means they do not drift. Since the 1959 international yard and pound agreement, an inch is exactly 2.54 cm, a pound is exactly 0.45359237 kg, and a mile is exactly 1,609.344 meters. These are not approximations that might be refined; they are the definitions of the imperial units in terms of metric ones. The US survey foot, which differed from the international foot by two parts per million, was finally retired in 2022.

Temperature is the exception and needs a different mechanism. Every conversion above works because the units share a zero: zero meters is zero feet. Temperature scales have different zeros, so the conversion is affine: a ratio and an offset. F = C x 9/5 + 32. The 9/5 is the relative size of a degree and the 32 is the gap between the zero points. That is why 0 C is 32 F rather than 0 F, and why a 10-degree Celsius rise is an 18-degree Fahrenheit rise while 10 C is not 18 F. Intervals scale; values need the offset. This converter goes through Kelvin, the only one of the common scales whose zero is absolute.

Some unit names hide real ambiguity, and these are the ones to be careful with.

The gallon: a US gallon is 3.785 liters and an imperial gallon is 4.546, a 20% difference. A US fluid ounce is 29.57 ml and an imperial one is 28.41. Recipes and fuel economy figures crossing the Atlantic routinely mean different things.

The ton: a US short ton is 2,000 lb, a UK long ton is 2,240 lb, and a metric tonne is 1,000 kg or about 2,205 lb. Construction and shipping quotes frequently do not say which.

Weight versus mass: a kilogram is mass and a pound as commonly used is also mass, so converting between them is legitimate. Strictly, weight is a force and would be measured in newtons, but in everyday use nobody means that, and refusing to convert kg to lb on those grounds is pedantry rather than precision.

Data storage: a gigabyte is 10^9 bytes and a gibibyte is 2^30, which is 1,073,741,824. Drive manufacturers use the decimal form and operating systems have traditionally reported the binary one, and that alone explains why a 1 TB drive shows as about 931 GB. Neither is lying.

Months and years have no fixed length, so any conversion involving them is an average. This converter uses the Gregorian year of 365.2425 days and a month of one twelfth of that, and flags the result as approximate, the only factor here that is.

On precision, one deliberate choice: the output scales its significant figures with the magnitude rather than using a fixed number of decimals. Fixed decimals fail in both directions: two decimals turn 1 mm into 0.04 inches, losing the answer, while six imply a precision the input never carried.

Unit Converter FAQ

How many centimeters are in an inch?

Exactly 2.54, by definition since 1959. That makes a foot exactly 30.48 cm and a mile exactly 1,609.344 meters. These are definitions of the imperial units in terms of metric ones, not measured approximations, so they will not change.

How do I convert Celsius to Fahrenheit?

Multiply by 9/5 and add 32. So 20 C is 36 + 32 = 68 F. Going back, subtract 32 then multiply by 5/9. The offset is needed because the two scales have different zero points, which is also why -40 is the one temperature where they agree.

Why is a US gallon different from a UK gallon?

They were defined separately. A US gallon is 231 cubic inches, or 3.785 liters, based on the old English wine gallon. An imperial gallon is 4.546 liters, redefined in 1824 as the volume of ten pounds of water. The 20% gap affects fuel economy figures and recipes alike.

Why does my 1 TB hard drive show as 931 GB?

Because two different units share an abbreviation. The manufacturer's terabyte is 10^12 bytes; the operating system has traditionally reported in tebibytes of 2^40 bytes, which is about 10% larger. 10^12 bytes is 931 of those. Both figures are correct for the unit each is using.

Common conversions

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