Unit conversion tool
Length converter — meters, cm, km, inches, feet, yards, miles
Enter a value in any unit to instantly convert meters, centimeters, kilometers, inches, feet, yards, and miles.
- Exact NIST/international definitions, listed on the page
- Converts through metres, so no rounding accumulates between units
- Every unit updates as you type — no convert button
About this length converter
What this tool will not do
- Length only. No weight, temperature, area, volume or speed.
- International foot and yard only. The US survey foot — fractionally longer, and deprecated at the end of 2022 — is not offered. For legacy survey data, use a surveying tool.
- No nautical miles, furlongs, or astronomical units. Seven everyday units, nothing more.
- Double-precision arithmetic, 12 decimal places displayed. There is no arbitrary-precision mode; beyond the range of a double the result becomes Infinity.
Instant, exact conversions between metres (m), centimetres (cm), kilometres (km), inches (in), feet (ft), yards (yd) and miles (mi) using international standard definitions (NIST/BIPM).
How to use (3 steps)
- 1) Enter a numeric value in any unit field (m, cm, km, in, ft, yd, mi). The tool accepts negative and zero values.
- 2) The converter updates all other unit fields in real time.
- 3) Read the converted values. Results are shown to up to 12 decimal places with trailing zeros trimmed, so exact conversions such as 1 in = 2.54 cm display cleanly rather than as 2.540000000000.
How it works — principle and formulas
- Canonical approach: every input is first converted to metres (m) using exact international factors, then converted from metres to each target unit. This avoids chaining/rounding errors.
Exact factors (authoritative definitions from NIST and international agreement):
- * 1 cm = 0.01 m
- * 1 km = 1000 m
- * 1 in = 0.0254 m (exact). Source: NIST
- * 1 ft = 0.3048 m (exact). Source: NIST HB 44 Appendix C
- * 1 yd = 0.9144 m (exact). Source: international agreement / NIST
- * 1 mi = 1609.344 m (exact)
Conversion formulas (use these exact factors in code):
* m_value = V * factor_to_m[U] - where V is the input number and U is the input unit.
* output[T] = m_value / factor_to_m[T] - where T is any target unit. Example: input 1 in -> m_value = 1 * 0.0254 = 0.0254 m; output cm = 0.0254 / 0.01 = 2.54 cm (exact).
Rounding & display notes (from report):
- * Internal computation uses IEEE double precision.
- * Values are displayed to 12 decimal places with insignificant trailing zeros trimmed. There is no precision selector — every conversion uses this single format.
- * Values at or above 1e12, or below 1e-9, switch to scientific notation with 12 significant digits.
Notes and attribution
- This converter uses the international (1959) definitions of the foot and yard. The US survey foot is not supported — if you are working with legacy survey data, use a dedicated surveying tool.
- Authoritative sources for these definitions: NIST — SI units: length and NIST Handbook 44, Appendix C (unit conversion tables).
FAQ
1. Q: How do I convert inches to centimeters?
A: Multiply inches by 2.54 (because 1 in = 0.0254 m and 1 cm = 0.01 m). Example: 1 in = 2.54 cm. Source: NIST.
2. Q: Is 1 inch exactly 2.54 cm?
A: Yes. The inch is defined as exactly 0.0254 m, so 1 in = 2.54 cm exactly. Source: NIST.
3. Q: What is the difference between the international foot and the US survey foot?
A: The international foot is defined as exactly 0.3048 m, and that is the definition this tool uses. The US survey foot is fractionally longer (1200/3937 m) and appears only in legacy surveying data; it was officially deprecated at the end of 2022. This converter does not offer it. Source: NIST HB 44 Appendix C.
4. Q: Can I convert negative lengths?
A: Yes. Negative inputs are accepted and the sign is preserved in all output units.
5. Q: Why do conversion results sometimes differ between sites?
A: Differences come from (a) using different base definitions (e.g., survey vs international foot), (b) rounding at display time, and (c) chained conversions that round at each step. This tool always converts through metres as the canonical unit, so no intermediate rounding accumulates, and it displays 12 decimal places rather than truncating early.
6. Q: How many feet are in a mile?
A: 5,280 ft in 1 mi (since 1 mi = 1609.344 m and 1 ft = 0.3048 m, and 1609.344 / 0.3048 = 5280 exactly).
7. Q: How precise are these conversions?
A: The underlying definitions are exact (per NIST/international agreement). Arithmetic is IEEE double precision and results are shown to 12 decimal places, which is well within double precision for everyday lengths.
8. Q: What if I need to convert very large or very small numbers?
A: Values at or above 1e12, or below 1e-9, are shown in scientific notation with 12 significant digits. Beyond the range of a double-precision float the result becomes Infinity; there is no arbitrary-precision mode, so use a computer-algebra tool if you need one.