Length

Inches, Feet, Meters, and Kilometers Explained

Length is the most common everyday conversion. Recipes rarely need it, but furniture, maps, running routes, and building plans do. A length converter is useful when the source document and the destination document refuse to share a system. This guide states the exact relationships used on Measurewell Atlas and shows where rounding quietly changes an answer.

Metric length units

The meter is the SI base unit of length. On this site, millimeters, centimeters, meters, and kilometers are related by tens:

1 cm = 10 mm
1 m = 100 cm = 1000 mm
1 km = 1000 m

Accessible text: one centimeter equals ten millimeters. One meter equals one hundred centimeters. One kilometer equals one thousand meters. Those are exact. Converting 100 centimeters to meters is not an approximation; it is 1 meter. The length converter uses the meter as its canonical internal unit so every other pair is a ratio of two factors to the meter.

Milli, centi, and kilo are prefixes, not separate physical ideas. A millimeter is a thousandth of a meter. Mixing “milli” with a non-length base, such as milligrams, is a different dimension and belongs in the mass converter.

US customary length units

The inch, foot, yard, and mile used here are the international definitions tied to the meter:

1 in = 25.4 mm exactly
1 ft = 12 in = 0.3048 m
1 yd = 3 ft = 0.9144 m
1 mi = 5280 ft = 1609.344 m

Accessible text: one inch equals twenty-five point four millimeters exactly. One foot equals twelve inches, which is zero point three zero four eight meters. One yard equals three feet. One mile equals five thousand two hundred eighty feet, which is one thousand six hundred nine point three four four meters.

Older US survey feet exist in some land records. This calculator does not implement a separate survey foot. If you are reading a historic plat, check which foot that document used before treating the result as legal area. Area conversions that square the international foot are handled on the area converter.

Inches and centimeters

Because 1 inch is 25.4 mm, 1 inch is 2.54 cm exactly. That is the number printed on many dual-scale rulers. To convert inches to centimeters, multiply by 2.54. To convert centimeters to inches, divide by 2.54.

Worked example: 12 inches to centimeters

12 × 2.54 = 30.48 cm. In base units: 12 × 0.0254 m = 0.3048 m, and 0.3048 / 0.01 = 30.48 cm. Reverse: 30.48 / 2.54 = 12. Load it in the converter with 12 in → cm.

Feet and meters

One foot is 0.3048 m exactly because it is 12 × 0.0254 m. People often remember “about 30 centimeters” or “about 3.28 feet in a meter.” Those phrases are rounding aids, not definitions. 1 / 0.3048 ≈ 3.280839895 ft per meter. If you round that to 3.28 too early and then multiply a long span, the error accumulates.

Worked example: 5 feet to meters

5 × 0.3048 = 1.524 m. Reverse: 1.524 / 0.3048 = 5 ft. Try 5 ft → m. For a room that is 12 ft by 14 ft you would convert each length first, then multiply if you need area; do not convert the product of feet as if it were still linear. That is an area problem.

Miles and kilometers

One mile is 1609.344 m, so 1 mile = 1.609344 km exactly under these definitions. One kilometer is 1000 / 1609.344 ≈ 0.621371192 miles. Road signs often round to 1.6 km per mile or 0.62 miles per kilometer. That is acceptable for a glance and poor for a tabulated conversion chart.

Convert 1 mile to kilometers in the length converter, or 1 kilometer to miles with the units swapped. The swap button inverts the pair without retyping the number, which is the practical way to check an inverse.

Exact conversion relationships

Exactness here means the definition is a terminating decimal or a simple integer ratio, not that every displayed result is an infinite-precision decimal. JavaScript Number cannot represent every real. The engine still avoids rounding until display. Identity conversions return the original value. A round trip from inches to centimeters and back should recover 12, not 11.999999 due to a sloppy factor.

The chain used internally is always to meters and back. That prevents a web of handwritten pairwise constants from drifting. The same engine fills the length table on the conversion charts page.

Common rounding mistakes

Using 2.5 cm per inch instead of 2.54 is a two-percent error. Using 3 ft per meter instead of 3.2808… is worse. Using 1.6 km per mile instead of 1.609344 is a small relative error that becomes visible on a long route. Rounding 0.3048 m per foot to 0.3 m is a six-percent error and should never appear in a conversion table.

Another mistake is converting a squared quantity with a linear factor. Square feet to square meters needs 0.09290304, which is 0.3048 squared. The length converter will not offer square feet; that pair lives in the area tool.

Practical measurement examples

A 36-inch table width is 36 × 2.54 = 91.44 cm. A 5-kilometer loop is 5 / 1.609344 ≈ 3.106856 miles. A 2-meter doorway is 2 / 0.3048 ≈ 6.56168 ft, often described as a bit over 6 ft 6 in once you convert the leftover 0.56168 ft × 12 ≈ 6.74 in. Those last inches are a display choice; the converter reports decimal feet unless you convert the remainder yourself.

For more on how factors chain across categories, read how to convert units of measurement. For names that look identical in the US and the UK but are not, read metric vs imperial units.