Free online tool
Length Converter
Convert any length to every common unit — metric and imperial — instantly.
Metric vs imperial at a glance
The metric system is used almost everywhere in the world and scales in tidy powers of ten: 10 mm in a cm, 100 cm in a m, 1000 m in a km. The imperial system, still standard in the United States, uses historic units (inches, feet, yards, miles) that don't share a clean ratio. That's why quick conversions are so handy — reading a spec in one and needing to picture it in the other is a daily annoyance for engineers, designers, and travellers.
How this tool works
Enter any value, pick its unit, and see it converted to every other supported unit simultaneously. All calculations happen in your browser using standard conversion factors, so results are precise and instantaneous.
+Are the ratios exact?
Yes — 1 inch = 25.4 mm exactly since 1959, and every derived value uses that definition.
+Can I convert nanometers or light-years?
Not in this version. It covers the units used in everyday design, construction, and travel.
Converting length units without accumulating rounding drift
Length conversions look simple, but stacking approximate factors — like converting inches to centimeters to millimeters in three separate steps — quietly compounds rounding error. This tool always converts from a single fixed base unit to avoid that.
The conversion factors themselves are internationally standardized, but knowing where they come from helps explain why some conversions are exact and others are only close approximations.
The exact factors involved
One inch equals exactly 25.4 millimeters by international agreement since 1959 — not a rounded approximation, an exact definition. From there, one foot is 304.8 mm, one yard is 914.4 mm, and one mile is 1,609.344 meters exactly. Metric-to-metric conversions (mm, cm, m, km) are all clean powers of ten, so those never introduce rounding error at all.
Where people get caught out
Nautical miles are a different animal entirely: 1 nautical mile = 1,852 meters, unrelated to the statute mile used on land, so mixing the two in navigation or aviation contexts produces real, sometimes dangerous errors of about 15%.
Worked example
Converting 5 feet 9 inches to centimeters: 5 x 12 + 9 = 69 inches total, 69 x 2.54 = 175.26 cm. Doing it the other way, 175 cm / 2.54 = 68.9 inches, which rounds to 5 feet 8.9 inches — close but not identical to the original 69 inches, illustrating why round-tripping a conversion rarely lands you back exactly where you started.
A brief history of the inch and the meter
The meter was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole, later refined through physical prototype bars, and today defined by the distance light travels in a fixed fraction of a second. The inch, by contrast, traces back to informal body-based measures before being locked to the metric system in 1959, which is exactly why 25.4 mm is treated as exact rather than measured.
Everyday situations where this matters
Furniture shopping between US and European retailers, checking whether carry-on luggage meets an airline's centimeter limit when you only know the inch measurement, following a recipe or DIY plan written in the 'wrong' unit system, and comparing running distances given in miles against a route measured in kilometers are all places where a wrong or rounded conversion factor can cause a real mismatch.
Precision needs vary by field
A carpenter cutting lumber to the nearest sixteenth of an inch cares about a level of precision that would be meaningless to someone converting a road-trip distance from miles to kilometers, where rounding to the nearest whole unit is perfectly fine. Engineering and manufacturing contexts, by contrast, often need conversions carried to several decimal places because a small rounding error compounded across hundreds of parts can produce a component that doesn't fit its assembly.
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