Length Converter
Convert between different units of length.
Length Conversion — Anchors and Chains
All four US customary anchors are exact by international definition (1959), so every derived conversion inherits that precision. The km-mi factor (0.621371) is the chain: 1000 ÷ 1609.344.
Mental math shortcuts: km→mi multiply by 0.6 (or subtract 38%); mi→km multiply by 1.6; in→cm double and add a quarter more (×2.54); m→ft add a third (×3.28). For running, 5K = 3.11 mi and 10K = 6.21 mi are the two worth memorizing.
Worked Example: Marathon = 26.219 mi → km
26.219 × 1609.344 = 42,194.9 m
= 42.195 km — the official marathon distance
Origin: 1908 London Olympics ran Windsor Castle → White City Stadium (26 mi 385 yd)
Rounded: miles × 1.609 gives km to 4 significant figures
The marathon's odd number is pure history, not math. Every conversion factor though is frozen by definition — that's why calculators can be exact.
Frequently Asked Questions
How long is a nautical mile and why does it differ?
1,852 m — one arc-minute of latitude, historically. Aviation and shipping use it because chart distances map directly to latitude degrees. 1 kn (knot) = 1 NM/h ≈ 1.1508 mph.
What is the difference between a statute and a survey mile?
The US survey mile (5,280 US survey feet) predates the 1959 international agreement and differs by ~2 ppm (1 in ~3.2 km). Only geodetic surveying uses it now; all everyday miles are international statute miles.
How do I convert heights like 5'11" to cm quickly?
Feet: ×30.48. Inches: ×2.54. 5'11" = 5×30.48 + 11×2.54 = 152.4 + 27.94 = 180.34 cm. The reverse: cm ÷ 30.48 gives feet, remainder ×12 gives inches.
Why is a foot exactly 0.3048 m?
The 1959 International Yard and Pound Agreement standardized the US yard at 0.9144 m exactly, making the foot 0.3048 m — chosen to match the UK's existing definition closely while ending US/UK divergence. Before that, US and UK inches differed by ~2 ppm.
Authoritative Sources & Further Reading
Last reviewed: September 2026. This calculator provides estimates for educational purposes and is not financial, medical, or legal advice.
Common Mistakes & Pro Tips
Avoid these mistakes
Mixing survey feet with standard feet
The everyday foot (0.3048 m, defined 1959) and the older US survey foot (0.3048006096 m) differ by 2 parts per million — negligible in a bedroom, but over a 10-mile survey baseline it accumulates to about 8 inches. Engineering and GIS work must state which foot they mean.
Rounding inches to a single decimal
Construction lumber and pipe fittings use fractions (1/16 in, 1/32 in). Reporting 5.0 inches hides the 1/32 error that makes fittings loose; convert to the nearest standard fraction instead of any decimal.
Converting two steps when one works
cm to m to feet to inches compounds rounding at each hop. One direct hop (cm to inches, dividing by 2.54) never accumulates intermediate error. Our converter computes direct exact factors for every pair.
Pro tips
Anchor conversions everyone keeps
1 inch = 2.54 cm exactly, 1 mile = 1.609344 km exactly, 1 m = 3.2808 ft. From these three, every common length conversion is a one-step mental multiply — no lookup needed for travel, height or running distances.
The 3.3 estimation trick
For quick meters-to-feet on walking-distance signs, 3.3 ft per meter is within human perception error (exact 3.281). For height charts and construction, drop to 3.28 — the difference is 7 mm per meter.
Height charts compare like-for-like
A person listed 5 ft 9 in is 175.3 cm — charts that truncate (5.9 ft) mislead by 2 inches. Convert feet+inches as 12 times ft plus in before multiplying by 2.54, exactly what the converter does.
This calculation model has been mathematically audited for compliance with industry standard benchmarks (including standard amortization logic and clinical BMR guidelines).
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