How to Convert Length Units (With the Exact Factors)

One mile, eight ways

Enter a length in any unit and every other unit fills in. No second dropdown — showing them all costs nothing, and you usually want two anyway: the metric figure for a form and the imperial one to picture it.

The conversions are exact, not approximate

This is worth knowing because plenty of converters get it slightly wrong.

Since the international yard and pound agreement of 1959, the yard is defined as exactly 0.9144 metres. Everything imperial follows from that by definition:

Unit	Exact value
1 yard	0.9144 m
1 inch	25.4 mm
1 foot	0.3048 m
1 mile	1609.344 m

These are definitions, not measurements. The conversion introduces no error of its own — any imprecision in the answer arrived with your own figure.

Plenty of tools round the mile to 1609 or the yard to 0.91. It's close enough for a quick sum, and the error grows with distance. This one uses 1609.344 throughout.

The check that proves it's right

Put in 1 mile and look at the results:

• 5,280 feet
• 1,760 yards
• 63,360 inches
• 1,609.344 metres

Those first three are the classic figures every schoolchild once memorised, and they come out exactly. If a converter gives you 5,279.99 feet in a mile, its factors are rounded somewhere.

The metric side is the easy half

Pick your unit from the measured in dropdown and the rest follow. Within metric, every step is a factor of ten, which is the entire reason the system exists:

1 kilometre	1,000 metres
1 metre	100 centimetres = 1,000 millimetres
1 centimetre	10 millimetres

So converting metres to centimetres is moving a decimal point two places, and metres to kilometres is three the other way. There's nothing to remember beyond which direction.

Imperial has no such rule — 12 inches to a foot, 3 feet to a yard, 1,760 yards to a mile. Each relationship is its own fact, which is why the mile-to-feet figure is one people memorise rather than derive.

Why there's no weight or volume in the dropdown

A single list mixing length, weight and volume lets you ask for kilograms expressed in metres — and get a confident number back for a question that has no answer.

That's worse than not offering it. One dimension per tool means every conversion on the page is a real one.

The mistake that catches everyone

5 ft 6 in is not 5.6 feet.

Six inches is half a foot, so it's 5.5 feet. The wrong version hides well because the number looks plausible and lands close — about a centimetre and a half out — which is exactly enough to matter on a cut and not enough to notice on a screen.

Convert the inches to a fraction of a foot first, or work entirely in inches and convert once at the end.

Where unit errors actually bite

• Ordering material — check whether the supplier quotes metric or imperial before you commit to a quantity
• A drawing scaled in mm read as cm — a factor of ten, and surprisingly easy to miss
• Nautical miles used where statute miles were meant. A nautical mile is 1852 m exactly — a genuinely different unit, defined from the earth's circumference rather than the yard. Confusing the two is about a 15% error, and it's not offered here for exactly that reason.
• Mixing units inside one calculation instead of converting everything first

Precision you can actually rely on

The results show three decimal places. That's enough to render the exact conversions honestly — a mile really does come back as 5,280 feet — without filling the screen with digits nobody needs.

It does mean very small results get rounded at that width. One millimetre expressed in miles is 0.0000006 of one, which displays as 0.000. That isn't the converter failing; it's a reminder that the sensible move is to convert into a unit where your answer is a number you can read. Millimetres belong in millimetres.

Rough factors for doing it in your head

When you don't need the exact answer:

• Metres → feet: ×3.3 (a little over 3)
• Km → miles: ×0.6 — or halve it and add a tenth
• Miles → km: ×1.6
• Inches → cm: ×2.5 (slightly under the true 2.54)

Fine for judging whether a thing will fit through a door. Not fine for ordering the door.

Print the length conversion reference — it has the exact definitions, the within-system relationships, and the mental shortcuts on one sheet.