Length Unit Converter

Length Conversion: The Modern Meter, Exact Imperial Factors, and Nautical Units

How the meter is defined today

The meter is no longer a physical bar. Since 2019 its definition is: the distance light travels in vacuum in 1/299792458 of a second. The speed of light is fixed at the exact value 299792458 m/s, so the definition is exact and reproducible anywhere without a physical artifact.

This change does not touch everyday conversion — every traditional unit keeps the same metric equivalent it always had.

Imperial units are exactly defined

The 1959 international agreement fixed the international foot at exactly 0.3048 m, from which all related units follow deterministically:

These values are exact (zero error), so converting them never loses precision.

One historical exception remains: the US survey foot equals 1200/3937 m ≈ 0.3048006096 m, a hair longer than the international foot (about 2 ppm, roughly 3 mm per mile). US land surveying used it for a long time, so the same plot computed under the two feet yields slightly different areas. The US has scheduled the survey foot for retirement after 2023, converging on the international foot.

Why a nautical mile is 1852 meters

The nautical mile is tied to Earth's geometry: 1 nautical mile equals the average meridian arc of 1 minute of latitude (1/60 of a degree). The 1852 m figure is the internationally agreed value (countries previously differed slightly).

This definition is genuinely useful: on a nautical chart, one minute of latitude equals one nautical mile. You measure the latitude difference between two points with a divider and read the distance directly — no conversion needed. It also explains why maritime and aviation speed is measured in knots (1 knot = 1 nautical mile per hour = 1.852 km/h).

Regional Chinese units

These are the modern market (市制) units. The "chi" in classical texts, different dynasties, and trades such as carpentry (营造尺) varies widely; do not blindly convert across sources — the same character "尺" can mean a different length elsewhere.

Astronomical and microscopic scales

A light year is a unit of distance, not time — the single most common科普 (popular-science) mix-up.

Practical conversion caveats

• Significant figures. If your source has only 3 significant figures, giving 10 digits of result is meaningless. Match the output to the real precision of the input.
• "Inch" means different things per domain. A screen "inch" is a diagonal; a pipe "inch" is an inner diameter (and varies by standard). Respect the trade convention rather than converting naively.
• Chain calculations in one step. Converting mile → foot → meter accumulates rounding error per step. This tool converts in a single pass, so there is no intermediate loss.
• Read the unit together with the number. Whether 3.5 is mm or cm changes the result tenfold, and the number alone never tells you. Confirm the unit before importing data.

The tool covers meter, kilometer, decimeter, centimeter, millimeter, micron, nanometer, mile, yard, foot, inch, nautical mile, light year, and astronomical unit, plus the market li/chi/cun — all converted locally in the browser.