Tinker 
How to use it
Type a length, pick the unit you have and the unit you want. The main result shows the converted value, rounded to six decimal places, and the list underneath shows the same length in all nine units so you can compare at a glance.
The formula
Every unit is defined by an exact number of metres: 1 inch = 0.0254 m, 1 foot = 0.3048 m, 1 yard = 0.9144 m, 1 mile = 1,609.344 m and 1 nautical mile = 1,852 m. The converter multiplies your value by the metres-per-unit of the starting unit, then divides by the metres-per-unit of the target unit.
Because these definitions are exact by international agreement (since 1959 for the inch, foot, yard and mile), results are exact apart from the final rounding.
Worked example
A 5 km race in miles: 5 × 1,000 = 5,000 m. Then 5,000 ÷ 1,609.344 = 3.106856 miles. Going the other way, 10 feet is 10 × 0.3048 = 3.048 m, and one mile is 1,609.344 ÷ 0.3048 = exactly 5,280 feet.
Practical tips
For quick mental estimates: 1 inch ≈ 2.5 cm, 1 metre ≈ 3.3 feet, 1 km ≈ 0.62 miles. Use the exact figure from the calculator when cutting materials or filling in forms. Nautical miles are used at sea and in aviation and are not the same as ordinary (statute) miles.
Frequently asked questions
How many centimetres are in an inch?
Exactly 2.54 cm. The inch has been defined as 25.4 mm since 1959.
How many feet are in a metre?
About 3.28084 feet, since one foot is exactly 0.3048 m.
Is a nautical mile longer than a mile?
Yes. A nautical mile is 1,852 m, while a statute mile is 1,609.344 m, so a nautical mile is about 1.15 miles.
Why are some results rounded?
Conversions like km to miles produce endless decimals, so results are rounded to six decimal places, which is more precise than nearly any real measurement.