Length Unit Converter

A length unit converter quickly transforms one length measurement into another. It supports 16 common length units including metric (kilometer, meter, centimeter, millimeter), imperial (mile, yard, foot, inch), and Chinese traditional units (li, zhang, chi, cun). Whether for engineering measurements, scientific calculations, or everyday size conversions, this tool provides precise results instantly. All calculations happen locally in your browser, keeping your data private and secure. Length conversion is used constantly in daily life, engineering, education, travel, construction, and product specifications. Meters, kilometers, centimeters, millimeters, inches, feet, yards, and miles have fixed relationships, but rounding and precision depend on the task. A tiny difference may not matter for a rough travel estimate, yet it can matter for furniture, shipping dimensions, machining, maps, or scientific data. The converter gives quick results, while formal specifications should state the unit, precision, and rounding rule explicitly.

Every length conversion in the tool collapses to a single multiplication by an exact rational factor relative to the SI metre. Since 2019 the metre has been defined by fixing the speed of light at c = 299,792,458 m/s, so 1 m is the distance light travels in vacuum during 1/299,792,458 of a second. Every other supported unit is then anchored to the metre by an exact conversion: 1 in = 0.0254 m (international yard and pound agreement, 1959), 1 ft = 12 in = 0.3048 m, 1 yd = 3 ft = 0.9144 m, 1 mi = 1,609.344 m, and 1 nmi = 1,852 m exactly (International Hydrographic Organization). Metric prefixes are decimal powers of ten over the metre: km = 10^3, dm = 10^-1, cm = 10^-2, mm = 10^-3, μm = 10^-6, nm = 10^-9. Chinese 市制 units follow GB 3102.1-1993: 1 里 = 500 m, 1 丈 = 10/3 m, 1 尺 = 1/3 m, 1 寸 = 1/30 m. Astronomical scales tie back to the same SI base: 1 AU = 149,597,870,700 m exactly (IAU 2012 redefinition), 1 light-year ≈ 9.4607 × 10^15 m, and 1 parsec ≈ 3.0857 × 10^16 m. The arithmetic is trivial; the trap is floating-point precision. IEEE 754 doubles carry roughly 15-17 significant decimal digits, so multiplying by 1/0.3048 introduces a sub-femtometre rounding error that does not matter for plumbing but starts to matter when crossing nanometres to light-years in the same calculation. For engineering output the result should be rounded to the tolerance the source drawing actually carries, not to the full double-precision string.

• SI base: 1 m is defined by fixing c = 299,792,458 m/s; every other supported unit is anchored to the metre by an exact conversion factor.
• Imperial exact factors (1959 agreement): 1 in = 0.0254 m, 1 ft = 0.3048 m, 1 yd = 0.9144 m, 1 mi = 1,609.344 m.
• Nautical mile: 1 nmi = 1,852 m exactly (IHO), about 15% longer than a statute mile; matters for waypoint timing and fuel planning.
• Chinese 市制 (GB 3102): 1 里 = 500 m, 1 丈 = 10/3 m, 1 尺 = 1/3 m, 1 寸 = 1/30 m, so 3 chi exactly equal 1 metre by definition.
• Astronomical anchors: 1 AU = 149,597,870,700 m exactly (IAU 2012), 1 ly ≈ 9.4607 × 10^15 m, 1 pc ≈ 3.0857 × 10^16 m ≈ 3.26 ly.
• Precision: IEEE 754 doubles hold 15-17 significant digits, so crossing nm and ly in one chain can drop the last decimal places; round to the source tolerance, not the full double.