Length Converter
Convert between metric and imperial lengths instantly, with the exact factor shown.
What this tool does
To convert a length, multiply by the exact factor between the two units - for example, 1 inch is exactly 2.54 centimetres, and 1 mile is exactly 1.609344 kilometres. This tool does that conversion for nine units (millimetres, centimetres, metres, kilometres, inches, feet, yards, miles and nautical miles) at once: enter a value, pick the two units, and the result and the exact factor both update instantly.
Converts a length between nine metric and imperial units - millimetres, centimetres, metres, kilometres, inches, feet, yards, miles and nautical miles.
Shows the exact conversion factor used, not just the answer, so you can see the working and reuse it elsewhere.
How to use it
- Enter the value you want to convert.
- Pick the unit you're converting from and the unit you're converting to.
- Read the result, or press Swap units to reverse the conversion.
Why use this converter
Length units are the ones people meet most - metric and imperial are both in everyday use - and chains of conversions (inches to feet to metres) multiply small rounding errors. Converting directly between any two units with the exact factor avoids that.
Why we built it
We built the length converter to cover nine units with the exact factor shown, not a rounded approximation. For common pairs use the feet to meters converter, the miles to km converter or the mm to inches converter.
Related tools worth a look: the Yards to Metres Converter, the Area Converter and the Acres to Hectares Converter.
Who this tool is for
- Students and teachers
- DIYers, builders and crafters
- Travellers and online shoppers
When to use it
- Reading dimensions on plans, products and listings that use the other system.
- Converting road, running or flight distances between kilometres, miles and nautical miles.
- School, engineering or craft work that mixes systems.
- Checking that something will fit before you buy it.
Where the numbers come from
Defined constants, not measurements.
Every factor here is an internationally defined exact value, not a rounded approximation. The inch was fixed at exactly 2.54 centimetres by the 1959 international yard-and-pound agreement between the US, UK, Australia, Canada and South Africa, which is why a mile is exactly 1.609344 kilometres and a foot is exactly 0.3048 metres - both follow from that single fixed inch. The nautical mile is defined as exactly 1,852 metres, based on one minute of latitude, which is why it doesn't convert to a round number of statute miles or kilometres.
Metric vs imperial, at a glance
Metric units scale by powers of ten - a metre is 100 centimetres, a kilometre is 1,000 metres - which is what makes metric arithmetic simple. Imperial units don't share a common base: a foot is 12 inches, a yard is 3 feet, and a mile is 5,280 feet, each a separate historical definition. That's the real reason imperial conversions need a calculator where metric ones often don't, and it's also why mixing the two systems in one project is where measurement mistakes usually happen.
What you give it, and what you get back
| Item | Details |
|---|---|
| Value | A number in the "from" unit. |
| From and To | Any of nine units: millimetres, centimetres, metres, kilometres, inches, feet, yards, miles and nautical miles. |
| Output | The converted length and the factor used. |
Issues you may run into
| What you see | Why it happens | What to do |
|---|---|---|
| Area or volume result is wrong | Squared and cubed units need squared and cubed factors. | Use the area or volume converter. |
| A nautical mile gives an odd answer | A nautical mile (1,852 m) is not a statute mile. | Check which mile your source means. |
| Very long decimals | Results are rounded for display. | The underlying factor is exact. |
Assumptions and limitations
- This converts lengths only. For area (square units) or volume (cubic units), use the dedicated area and volume converters - squaring or cubing a length factor by hand is a common source of error.
- Nautical miles are used for sea and air navigation; if you're converting a distance for a nautical or aviation context, confirm which mile is meant before using the answer.
- Very large or very small results are shown with practical rounding for readability - the internal calculation itself is exact.
Frequently asked questions
How many centimetres are in an inch?
Exactly 2.54 centimetres. This is a defined constant, not a rounded figure - it was fixed by international agreement in 1959, and every other imperial-to-metric length conversion (feet, yards, miles) is derived from that same exact number.
How do I convert miles to kilometres?
Multiply the number of miles by 1.609344. This tool does it instantly and shows the factor, but that's the exact figure to use for a manual calculation too - it follows directly from the fixed 2.54 cm inch.
What is the difference between a mile and a nautical mile?
A statute mile is 1,609.344 metres, defined from the inch. A nautical mile is a separate, older definition - exactly 1,852 metres, based on one minute of latitude on the Earth's surface - and is used for sea and air navigation rather than everyday distances.
Why isn't 1 foot exactly 30 cm?
Because the foot is defined from the exact 2.54 cm inch (1 foot = 12 inches = 30.48 cm), not rounded to a convenient metric number. 30 cm is a common approximation, but it's about 1.6% short of a true foot.
Can I convert between metric and imperial length units here?
Yes - that's the main purpose of this tool. Pick any of the nine units (millimetres to nautical miles, metric or imperial) as your 'from' and 'to' unit, and it converts directly between them.
Are these conversion factors exact or rounded?
Exact. Every factor used - the 2.54 cm inch, the 1,609.344 m mile, the 1,852 m nautical mile - is an internationally defined constant, not a measured or rounded approximation.
Sources and accuracy
Exact internationally defined constants: the inch fixed at 2.54 cm by the 1959 international yard-and-pound agreement; the nautical mile fixed at 1852 m.
Figures last checked against the source on . Standards and rates can change — if you are relying on this for something that matters, confirm against the source above. Think a figure is wrong or out of date? Send a correction.