Length is the quantity most often quoted across two systems at once: recipes use inches, overseas forms want feet, shoe specs are in centimetres, and nautical charts and flight levels each keep their own habits. This tool lines up four metric units (kilometre down to millimetre), four imperial ones (mile, yard, foot, inch) and the nautical mile in a single table — pick the unit your number is already in and the other eight appear at once, each with its own copy button. The arithmetic is a multiplication and a division, and your figures stay in this tab rather than being submitted anywhere. The page itself does pull in third-party resources such as advertising, and those requests know nothing about what you typed.
How to use it
- The Value field starts at 1. Replace it with your own number — it is a numeric field, so a decimal point and a leading minus sign are fine, but a string carrying a unit suffix or a thousands separator will not be taken.
- The From unit dropdown starts on Meter. Set it to **the unit your number is already in** — if you measured 170 centimetres, choose Centimeter, not the unit you want to end up with.
- The grid underneath lists all eight remaining units, so there is nothing else to select. Each card shows the unit name above the converted value, with a copy button on its right.
- To go the other way, just change From unit to whatever you were converting into. The number in the Value field is kept, not cleared.
- Clear the field, or leave anything that cannot be read as a number, and every card falls back to a dash — the copy button then hands you an empty string.
When it comes in handy
Fill in a height field on a foreign form
Visa applications, gym sign-ups and clothing size charts routinely ask for feet or inches. For 170 cm the Inch card reads 66.92913386, which divided by 12 is 5 ft 6.93 in; the Foot card shows 5.577427822 directly.
Reconcile race distances
A half marathon is 21.0975 km, or 13.10937873 miles — yet US listings print it as 13.1 miles, which converts back to 21.0824064 km, about 15 metres short. The famous 26.2 for a full marathon is the same rounding.
Buy appliances, furniture or worktops from abroad
A 55-inch television measures 139.7 cm across the diagonal; a 60-inch counter is 152.4 cm long. Convert before ordering and you will know whether the wall you measured can actually take it.
Read aviation and marine figures
A cruising altitude of 35,000 ft is exactly 10,668 m, and one nautical mile is 1852 m — 1.150779448 statute miles, or 6076.115486 feet. Charts quoting horizontal distance in nautical miles and vertical distance in feet on the same page is entirely normal.
Limits and behavior
- Every imperial constant here is a **definition**, not a measurement: 1 inch = 25.4 mm, 1 foot = 0.3048 m, 1 yard = 0.9144 m and 1 mile = 1609.344 m were all fixed by the 1959 international yard and pound agreement, and the nautical mile was set at a round 1852 m in 1929. The table therefore contributes no rounding error of its own; any wobble in the last digits can only come from floating-point arithmetic.
- The US survey foot (1200/3937 m) is not offered. It differs from the international foot by roughly 3.2 mm per mile — irrelevant for everyday dimensions, but older US cadastral maps and some geodetic coordinates are stated in it, which matters when you compare data from different eras.
- Output is always a single decimal number, never a compound "5 ft 6 in". If you need the compound form, take the whole part of the Foot card and multiply the fraction by 12, or simply read the Inch card instead.
- There is no Taiwanese chi (about 30.3 cm), no li, no light year or astronomical unit, and no speed units at all — a knot is one nautical mile per hour, which is a speed, and this table only handles distance.
- Very small results switch to scientific notation: one millimetre in miles displays as 6.213711922e-7. That is simply how JavaScript prints numbers below one millionth as text, not an overflow or an error.
- Negative input is accepted: −5 m returns −16.40419948 ft. A negative length is physically meaningless on its own, but it is exactly what you want when the number is a coordinate delta or an elevation difference.
Frequently asked questions
Why do I get 5.577427822 feet when I wanted "5 ft 6 in"?
Height in English-speaking countries is spoken as a compound of feet and inches, but a unit converter returns a plain decimal. The two are one step apart: the whole part, 5, is the feet, and the fraction 0.577427822 multiplied by 12 gives 6.93 inches. Reading the Inch card is faster — 170 cm is 66.92913386 in, and 66.93 divided by 12 is 5 remainder 6.93, the same answer. As a rule, keep the centimetre figure as your source of truth: rounding "5 ft 6 in" back into centimetres can easily be more than a centimetre out.
Is an inch really exactly 25.4 mm, rather than an approximation?
Exactly. In 1959 the United States, United Kingdom, Canada, Australia, New Zealand and South Africa agreed to define the yard as precisely 0.9144 m; feet, inches and miles all follow from that, which makes 1 in = 25.4 mm a definition rather than a measurement. Before then national inches differed slightly, and old engineering drawings occasionally state which one they used. Modern specifications no longer have to.
Why is a nautical mile such an odd number of metres?
Because it was never chosen as a length. It is the distance covered by one minute of arc (1/60 of a degree) along a meridian. The Earth is not a perfect sphere, so that arc grows from the equator towards the poles and countries historically used slightly different values. A 1929 conference settled on a round 1852 m, which is what this tool uses. The practical payoff is that one minute on a chart's latitude scale is one nautical mile, so you can step distances off the margin with dividers.
Can I use this for area or volume — square feet, for instance?
No, and this is where unit conversion most often goes wrong: a length ratio cannot be carried over to an area. One foot is 0.3048 m, but one square foot is 0.3048² = 0.09290304 m² — the factor has to be squared, and cubed for volume. Use the area converter for area, though note it does not currently stock square feet either. Volume units are not on the site yet.
Is the "inch" on a screen or a bicycle rim the same inch as here?
The unit is identical; what it measures is not, and that is where confusion starts. A television or laptop size is the screen **diagonal**, so converting it gives you neither the width nor the height. A 26" or 29" bicycle rim is a rim standard, and the real outside diameter with a tyre fitted is larger. A4 paper, meanwhile, is defined metrically (210 × 297 mm), which is why it converts to the unmemorable 8.27 × 11.69 in. The numbers are right; just be sure which edge you are converting.