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Length Converter

Measurement

Free online length converter. Instantly convert between millimetres, centimetres, metres, kilometres, inches, feet, yards, miles, and nautical miles.

Reviewed by the thecalcu.com team · Last updated June 19, 2026

From
To
All conversionsfor 1 Kilometres (km)
Millimetres (mm)1000000
Centimetres (cm)100000
Metres (m)1000
Kilometres (km)1
Inches (in)39370.079
Feet (ft)3280.8399
Yards (yd)1093.6133
Miles (mi)0.62137119
Nautical Miles0.5399568

What is a Length?

A length converter translates a measurement from one unit of distance into its exact equivalent in another. Length is the one measurement almost everyone has to convert at some point, road distances in kilometres versus miles, a person's height in centimetres versus feet-and-inches, fabric or lumber measured in metres versus yards, because the metric and imperial systems coexist globally rather than one having fully replaced the other.

This converter covers the full range most people need: millimetres, centimetres, metres, and kilometres on the metric side; inches, feet, yards, and miles on the imperial/US side; plus nautical miles for maritime and aviation use. Every conversion is bidirectional and updates as you type, with no button to press.

All length conversions share one property that makes them simpler than most unit families: they're linear. There's a single multiplication by a fixed factor relative to a common base (the metre), no offsets, no branching formula like temperature requires. If your project also involves mass or weight, the Weight & Mass Converter uses the same straightforward approach.

Why Use a Length Converter?

Length conversions look trivial until you're doing several of them under time pressure, and that's exactly when a wrong multiply-versus-divide slip happens. The kilometre-to-mile factor (0.621371) is easy to invert by accident, and getting it backwards doesn't give you a slightly-off answer, it gives you a number that's off by roughly a factor of 2.6, which is obvious in hindsight but not always obvious mid-calculation.

A few places this comes up regularly: reading a speed limit or distance sign in an unfamiliar country, checking a height or body measurement recorded in the "wrong" system for a form, comparing a race distance (a 10K versus a 10-mile route) when planning training, or cross-checking a construction or shipping spec written in a different unit than your own drawings use. In each case, the cost of a manual error is higher than the ten seconds it takes to let the converter do it.

Formula & Methodology

Every length conversion runs through a shared base unit, the metre, using a two-step formula:

Result = (Input × F_from) ÷ F_to

Where F_from and F_to are each unit's conversion factor to metres (its "toBase" value):

- mm = 0.001
- cm = 0.01
- m = 1 (base)
- km = 1,000
- in = 0.0254
- ft = 0.3048
- yd = 0.9144
- mi = 1,609.344
- nmi = 1,852

Worked example, the marathon distance, 42.195 km, converted to miles:

Result = (42.195 × 1,000) ÷ 1,609.344 = 42,195 ÷ 1,609.344 ≈ 26.219 miles

That matches the IAAF-defined marathon distance exactly, which is a good sanity check that the formula is doing what it should.

All the factors above are exact by international definition, the inch has been fixed at precisely 25.4 mm since 1959, and the nautical mile at exactly 1,852 m by the International Hydrographic Organization. Nothing here is rounded or approximated.

What Insights Does the Length Converter Give You?

The converted value is shown to up to eight significant figures, which matters more than it sounds, a rough mental conversion of "1 km ≈ 0.62 miles" is fine for a quick estimate, but the actual figure (0.621371) is what you want feeding into anything downstream, like fuel calculations or pace splits.

The reference table shows all nine supported units at once for your current input, which beats converting one pair at a time when you need to cross-check a measurement against several systems simultaneously, a 25 mm bolt is also 2.5 cm, 0.9843 in, and 0.0820 ft, all visible together.

The swap control flips the From and To units instantly without re-entering the value, useful when you receive a number in an unfamiliar unit and want to see it in the one you actually think in.

How to use this Length calculator

  1. Choose your From unit from the left dropdown, units are grouped as Metric, Imperial/US, and Nautical, and the default opens on Kilometres.

  2. Type your value into the left input. Decimals and large numbers both work fine, and the To field updates as you type.

  3. Choose your To unit on the right, Miles is the default, but any of the nine units is selectable.

  4. Enter a number in the To field instead, if that's easier, the conversion runs both directions, so typing 100 miles fills in the kilometre equivalent automatically.

  5. Use the swap button between the two panels to reverse the direction in one click while keeping your current value.

  6. Check the reference table underneath for the full breakdown across all nine units, with the active unit highlighted and numbers shown in monospace for easy comparison.

Quick Reference

Common conversions at a glance:

From To Multiply by
km mi × 0.621371
mi km × 1.609344
cm in ÷ 2.54
ft m × 0.3048
in cm × 2.54

Related conversions you might also need: the Feet & Inches to Centimetres Converter for height specifically, or the Speed Converter if you're working with distance-per-time units rather than distance alone.

Frequently Asked Questions

It's a tool that turns a measurement in one unit of length into its exact equivalent in another, kilometres to miles, centimetres to inches, and so on. Rather than looking up or remembering a conversion factor, you type a value once and every supported unit updates instantly. Length converters exist because the world runs on two incompatible measurement systems (metric and imperial) at the same time, and most people work across both without realizing it.
Exactly 1,000. The metric prefix 'kilo-' always means a factor of 1,000, so 5 km equals 5,000 m and 500 m equals 0.5 km. That fixed ratio is what makes metric conversions so much simpler than imperial ones, there's no odd number to memorize.
Multiply the kilometre value by 0.621371, so 100 km × 0.621371 = 62.1371 miles. A rough mental shortcut is dividing by 1.6, which gets you within about 1% of the exact answer. For anything that actually matters, use the converter rather than the shortcut.
They're close but not equal, one metre is about 1.0936 yards, so a metre runs roughly 9.4% longer than a yard. It's a small enough gap to cause real errors if you're eyeballing a conversion rather than calculating it, especially over long distances where that 9% compounds.
Twelve, always, it's a fixed, internationally defined relationship, not an approximation. One foot also equals exactly 30.48 cm. Heights are commonly written as feet-and-inches in the US and metric elsewhere, which is why this is one of the more frequently searched length conversions.
Every conversion goes through a common base unit, the metre. Result = (Input × toBase of the From unit) ÷ toBase of the To unit. Converting 5 miles to kilometres, for instance: (5 × 1,609.344) ÷ 1,000 = 8.04672 km. Length has no non-linear cases like temperature does, it's pure multiplication, every time.
Divide by 2.54, since one inch is defined as exactly 2.54 cm. 175 cm ÷ 2.54 = 68.9 inches, or 5 feet 8.9 inches. This shows up constantly on clothing size charts, where US and European sizing use different unit systems entirely.
A nautical mile is defined as exactly 1,852 metres, corresponding to one minute of arc along the Earth's surface. It's the standard unit in maritime navigation and aviation, a vessel's speed in knots is nautical miles per hour, not statute miles per hour, which trips people up if they don't sail or fly.
Yes, a mile is about 61% longer. One mile works out to 1.609344 km exactly, so distances quoted in miles will always look like a bigger number when converted to kilometres, and vice versa. It's the source of a lot of confusion on road signs near international borders.
Multiply by 0.3048. A 10-foot ceiling comes out to 10 × 0.3048 = 3.048 m. Construction and architecture drawings mix both systems constantly, especially on projects involving imported materials specified in one unit and local building codes written in the other.
Yes, pick Millimetres as the From unit and Centimetres as the To unit, and the result updates as you type. It's a simple divide-by-10 relationship (250 mm = 25 cm), but for engineering drawings and hardware specs where a decimal error matters, letting the tool do it removes the risk entirely.
Nine, across three groups: Metric (millimetres, centimetres, metres, kilometres), Imperial/US (inches, feet, yards, miles), and Nautical (nautical miles). Every pair converts bidirectionally, and the reference table below the converter shows all nine simultaneously for whatever value you've entered.
Also known as
cm to inchesmetres to feetkm to milesinches to cmfeet to metreslength unit convertermm to inchesmeters to feet converteryards to metresnautical miles converterdistance unit converterconvert length unitskm to mkilometers to mileskilometres to miles calculator