Length Converter: Inches, Feet, CM, MM and Miles

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How to use this tool

Enter an amount, pick the unit it is in and the unit you want. The answer updates as you type. Below it the same length appears in millimetres, centimetres, metres, inches, feet, yards and miles at once, plus a feet and inches breakdown, which saves converting twice when a drawing and a tape measure disagree.

The coloured box underneath is the practical one. It works out where your length falls on a tape measure divided in sixteenths, and tells you whether it lands on a mark or how far off it is in millimetres. Green means you can mark it exactly. Red means you are estimating, and it says by how much.

One definition underneath everything

Every imperial length on this page comes from a single line in the 1959 International Yard and Pound Agreement: the yard is exactly 0.9144 metres. That one number fixes all the others.

Divide it by three and a foot is exactly 0.3048 m. Divide by thirty six and an inch is exactly 0.0254 m, which is where the familiar 2.54 cm comes from. Multiply by 1760 and a mile is exactly 1609.344 m. None of these are measurements or approximations, they are consequences of the definition, so the conversion contributes no error of its own. Any error in a converted figure came from the input or from rounding the output.

The same agreement is why a pound is exactly 0.45359237 kg, and the reason both conversions share the same six signatory countries and the same date.

Why inches and millimetres never line up

A tape measure divides the inch by repeated halving: halves, quarters, eighths, sixteenths. A metric rule divides the metre by ten. Those two habits do not overlap, and the consequence is stronger than most people expect.

One sixteenth of an inch is 1.5875 mm. Two sixteenths is 3.175 mm. Keep going and nothing lands on a whole millimetre. Not at half an inch, which is 12.7 mm. Not at a full inch, which is 25.4 mm. The first sixteenth mark that sits exactly on a whole millimetre is the eightieth one, and that is 5 inches, or 127 mm exactly. Between zero and five inches the two rules share no mark whatsoever.

This matters on site. If a drawing says 300 mm and you are working to a tape in sixteenths, the closest you can get is 11 and 13/16 inches, which is 299.7 mm. You are 0.3 mm short and there is no mark that does better. Knowing that in advance is more useful than discovering it after the cut.

A worked check you can repeat on paper

Take 300 mm to inches. Divide by 25.4 and you get 11.811024 inches. Split off the whole inches and 0.811024 of an inch remains. Multiply by 16 and that is 12.976 sixteenths, so the nearest mark is 13/16 and you are just under it.

Check it the other way. 11 and 13/16 inches is 11.8125 inches. Multiply by 25.4 and you get 299.9875 mm. That is 0.0125 mm under 300, which for a timber cut is nothing and for a machined part may not be.

Now the height case, since it is the one people look up most. 5 feet 2 inches is 62 inches. Multiply by 2.54 and you get 157.48 cm exactly. Because the inch to centimetre factor is exact and 62 is a whole number, that answer has no rounding in it at all.

Mistakes that catch people out

Reading a decimal foot as feet and inches. 5.5 feet is 5 feet 6 inches, not 5 feet 5 inches. Survey and grade sheets often work in decimal feet, so multiply the fractional part by twelve rather than treating it as inches.

Using the wrong foot on survey work. The US survey foot was 1200 divided by 3937 metres, about two parts per million longer than the international foot. That is 3.2 mm over a mile and 0.2 m over 100 km. It was officially retired at the end of 2022, but older plans still carry it, so check the vintage of your data before converting coordinates.

Assuming a nautical mile is a mile. A nautical mile is exactly 1852 m, about 15 percent longer than a statute mile. Marine and aviation distances use it, and treating them as statute miles understates every figure.

Rounding partway through a chain of conversions. Going from millimetres to inches to feet with rounding at each step accumulates error quickly. Convert once from the original figure and round only at the end.

Popular length conversions

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How this was checked

Every factor was derived from the 0.9144 metre yard rather than copied from a table, then checked against its published value. The inch came out at exactly 2.54 cm, the foot at 0.3048 m, the mile at 1609.344 m, and the integer relationships held: twelve inches to the foot, three feet to the yard, 5280 feet and 1760 yards to the mile.

Round tripping was checked across every pair of the nine main units, all 81 combinations. Converting a value out and back returns the original to within about three parts in a hundred thousand billion, which is floating point noise rather than a real difference.

The five inch claim in the diagram was not assumed either. A sixteenth of an inch is 127 over 80 millimetres, so a whole number of millimetres requires the sixteenth count to be a multiple of eighty. The first is the eightieth sixteenth, which is 5 inches and 127 mm, and a search through the first four hundred sixteenths confirmed that nothing before it lands exactly.

Related Calculators and Reference

See also the Paper Size Calculator for standard sheet sizes in millimetres and inches without doing the conversion yourself.

• Feet and Inches to CM Converter if you are converting a height and want feet and inches entered as two separate boxes.
• Decimal to Fraction Converter for turning a decimal inch into a tape measure fraction, and seeing which decimals can never land on a mark.
• Length and Distance Converter with Survey Units for chains, links and the retired US survey foot.
• Kilograms to Pounds Converter for the mass half of the same 1959 agreement.
• Kmh to Mph Converter for the speed built on the mile defined here.
• Area Converter for these lengths squared.
• CBM and Dimensional Weight Calculator: cubic metres, volumetric weight and chargeable weight for a shipment.
• Tire Size Calculator: overall diameter and sidewall height from the size on the tire, in inches and millimetres.
• Wheel Offset Calculator: convert wheel offset in millimetres to backspacing in inches.
• Shoe Size Converter: turn a foot length into shoe sizes.
• Ring Size Calculator: turn a finger circumference into ring sizes.
• Bra Size Calculator: turn a tape reading into band and cup sizes.
• NIST Special Publication 811 is the primary reference for the exact factors used here.