Enter one length and see it in every other unit at once — metric, imperial, nautical, surveying and astronomical. Factors are the exact legal definitions, not rounded approximations, and heights come out in feet-and-inches rather than a decimal you have to convert again.
Length Converter
LiveThe conversions people actually need
| Conversion | Exact factor | Quick mental estimate |
|---|---|---|
| Inches → cm | × 2.54 | × 2.5, then add 2% |
| cm → inches | ÷ 2.54 | ÷ 2.5, then subtract 2% |
| Feet → metres | × 0.3048 | ÷ 3, then subtract 10% |
| Metres → feet | × 3.28084 | × 3, then add 10% |
| Miles → km | × 1.609344 | × 1.6 — or use Fibonacci: 5 mi ≈ 8 km, 8 mi ≈ 13 km |
| km → miles | ÷ 1.609344 | × 0.6, then add 1% |
| Yards → metres | × 0.9144 | Subtract 10% — close enough for most purposes |
The Fibonacci trick for miles and kilometres is genuinely useful and not a coincidence: the golden ratio is 1.618 and the mile-to-kilometre factor is 1.609, so consecutive Fibonacci numbers approximate the conversion to within half a percent. 3→5, 5→8, 8→13, 13→21, 21→34 all work.
Common heights and distances
| Imperial | Metric | Where it comes up |
|---|---|---|
| 5'0" | 152.4 cm | Human height — the most-searched length conversion by a wide margin |
| 5'4" | 162.6 cm | |
| 5'8" | 172.7 cm | |
| 6'0" | 182.9 cm | |
| 6'2" | 187.9 cm | |
| 6'6" | 198.1 cm | |
| 1 mile | 1.609 km | Running and road distances |
| 5 km | 3.107 miles | The standard park run |
| 26.2 miles | 42.195 km | Marathon — the metric figure is the official one |
| 1 nautical mile | 1.852 km | Aviation and shipping. Defined as one minute of latitude |
Inch fractions to millimetres — the shop reference
The conversion table used on site and at the bench. Every sixteenth, exact to three decimal places.
| Fraction | Decimal inch | Millimetres | Thou |
|---|---|---|---|
| 1/16" | 0.0625" | 1.587 mm | 62.5 thou |
| 1/8" | 0.1250" | 3.175 mm | 125.0 thou |
| 3/16" | 0.1875" | 4.762 mm | 187.5 thou |
| 1/4" | 0.2500" | 6.350 mm | 250.0 thou |
| 5/16" | 0.3125" | 7.938 mm | 312.5 thou |
| 3/8" | 0.3750" | 9.525 mm | 375.0 thou |
| 7/16" | 0.4375" | 11.112 mm | 437.5 thou |
| 1/2" | 0.5000" | 12.700 mm | 500.0 thou |
| 9/16" | 0.5625" | 14.287 mm | 562.5 thou |
| 5/8" | 0.6250" | 15.875 mm | 625.0 thou |
| 11/16" | 0.6875" | 17.462 mm | 687.5 thou |
| 3/4" | 0.7500" | 19.050 mm | 750.0 thou |
| 13/16" | 0.8125" | 20.637 mm | 812.5 thou |
| 7/8" | 0.8750" | 22.225 mm | 875.0 thou |
| 15/16" | 0.9375" | 23.812 mm | 937.5 thou |
| 1/1" | 1.0000" | 25.400 mm | 1000.0 thou |
One sixteenth is 1.5875 mm, one thirty-second 0.794 mm, one sixty-fourth 0.397 mm. A thou is 0.0254 mm — so 1/64" is about 15.6 thou.
Choosing a fraction precision
Snapping a metric measurement to an inch fraction always introduces error. How much matters entirely on what you are making:
| Precision | Worst-case error | Appropriate for |
|---|---|---|
| 1/8" | ±1.59 mm | Rough carpentry, framing, landscaping — anything where a saw kerf exceeds the tolerance anyway |
| 1/16" | ±0.79 mm | General construction, cabinetry, trim. The default on most tape measures |
| 1/32" | ±0.40 mm | Fine joinery, furniture, close-fitting woodwork |
| 1/64" | ±0.20 mm | Machining and metalwork — though at this level most shops work in thou or millimetres directly |
The calculator flags the actual rounding error whenever it exceeds 0.005 mm, so you can see immediately whether the fraction you picked is fine enough. Below about 1/64", fractions stop being the right tool — decimal inches or millimetres carry the precision more honestly.
When "equivalent" sizes are not equal
A trap that catches people converting between systems in trade contexts: many nominal sizes are designations, not measurements, and the metric and imperial versions were never intended to match exactly.
| Nominal size | Actual imperial | Actual metric |
|---|---|---|
| 2×4 timber | 1.5" × 3.5" | 38 × 89 mm — not 50 × 100 |
| 1/2" drywall | 0.5" | 12.7 mm, sold as 12.5 mm in metric markets |
| 1/2" pipe | Neither ID nor OD is 0.5" | DN15 — a designation, not a dimension |
| A4 vs Letter | Letter 8.5" × 11" | A4 210 × 297 mm — 6 mm narrower, 18 mm taller |
Convert the measured dimension, never the nominal name. A drawing that says "2×4" and one that says "38×89" describe the same piece of wood; a drawing that says "50×100" describes something else entirely.
Where unit confusion actually causes damage
Length conversion sounds trivial until it isn't. Three documented cases worth knowing:
| Incident | What happened |
|---|---|
| Mars Climate Orbiter, 1999 | One team supplied thrust data in pound-force seconds, the other expected newton-seconds. The spacecraft entered the atmosphere at the wrong altitude and was destroyed — a $327 million loss to a unit mismatch |
| Gimli Glider, 1983 | A Boeing 767 was fuelled using pounds where the calculation required kilograms, giving less than half the fuel needed. It ran dry at 41,000 feet and glided to an emergency landing |
| Space Mountain, Tokyo, 2003 | An axle was manufactured to an imperial spec after the drawings had been converted to metric, leaving it 2 mm undersized. It broke mid-ride |
The common thread is not that the arithmetic was hard — it is that nobody stated the unit. Writing the unit beside every number, in every document and every message, prevents essentially all of it.
The units, and where each is still used
| Unit | Exact value | Still used for |
|---|---|---|
| Thou / mil | 0.0254 mm | Machining tolerances, PCB trace widths, plastic film thickness |
| Hand | 101.6 mm | Horse height, universally, in every country |
| Fathom | 1.8288 m | Water depth on nautical charts |
| Chain | 20.1168 m | Land surveying, and the length of a cricket pitch |
| Furlong | 201.168 m | Horse racing distances |
| Nautical mile | 1,852 m | Aviation and shipping — one minute of latitude, which is why it exists |
| Point | 0.3528 mm | Typography — 72 points to the inch |
| CSS pixel | 0.2646 mm | Web design — defined as 1/96 inch, independent of screen resolution |
| Astronomical unit | 149,597,870,700 m | Distances within the solar system — the Earth-Sun distance |
| Light year | 9.461 × 10¹⁵ m | Distances between stars. A measure of length, not time |
| Parsec | 3.086 × 10¹⁶ m | Professional astronomy — about 3.26 light years |
The inch has not always been the same length
Conversions between imperial and metric are exact today because they were defined to be, and that definition is newer than most people assume:
| Definition | From | One inch |
|---|---|---|
| International inch | 1959 | Exactly 25.4 mm by international agreement. What every conversion on this page uses |
| US survey inch | Pre-1959 | 25.400051 mm. Retained for land survey until it was officially retired at the end of 2022 |
| Imperial inch | Pre-1959, UK | 25.399977 mm. Differed from the US inch by about two parts in a million |
Two parts in a million sounds negligible and is not, at survey distances. Over a mile the US survey foot and the international foot differ by about 3 mm; over 100 miles, by roughly 300 mm — enough to move a property boundary. State plane coordinate systems in the US still carry the distinction, which is why the retirement took decades and a formal announcement.
Where a unit error has actually cost something
| Case | The error | Outcome |
|---|---|---|
| Mars Climate Orbiter 1999 | Pound-force seconds sent where newton seconds were expected | The spacecraft entered the atmosphere far too low and was destroyed. Around $327 million |
| Gimli Glider 1983 | Fuel loaded in pounds when the aircraft needed kilograms | A Boeing 767 ran out of fuel at 41,000 feet and glided to a landing. Nobody died |
| Space Mountain, Tokyo 2003 | An axle specified in inches, ordered in millimetres | An undersized axle broke on a moving train. No injuries, and the ride closed for months |
| Laufenburg bridge 2003 | Two countries using different sea-level references | The two halves met 54 cm apart in height and one side had to be lowered |
Common mistakes
Frequently asked questions
How many centimetres in an inch?
Exactly 2.54 — not approximately. Since the international yard and pound agreement of 1959, the inch has been defined as 25.4 mm, which makes every imperial length unit exactly derivable from the metre. There is no rounding anywhere in the chain, so 12 inches is exactly 30.48 cm and a mile is exactly 1,609.344 m.
How do I convert my height to centimetres?
Convert the whole thing to inches first, then multiply by 2.54. For 5 ft 9 in: (5 × 12) + 9 = 69 inches, × 2.54 = 175.3 cm. The common error is converting feet and inches separately and adding — which works, but only if you remember the feet part is 60 inches, not 5. Enter your height in inches above and the calculator handles it.
Why is a nautical mile different from a regular mile?
Because it was defined from the Earth rather than from a body part. One nautical mile is one minute of latitude — 1/60th of a degree — which makes chart navigation straightforward: a degree of latitude is always 60 nautical miles. The statute mile traces back to the Roman mille passus, a thousand paces, and has no geographic relationship at all.
What is a CSS pixel, and is it a real length?
Yes — the CSS specification defines it as exactly 1/96th of an inch, or 0.2646 mm. It is deliberately independent of your screen's physical pixels, which is why a 16px font looks about the same size on a phone and a desktop despite wildly different pixel densities. The device translates CSS pixels to hardware pixels itself.
Is a light year a measure of time?
No, distance — the distance light covers in one year, roughly 9.46 trillion kilometres. The confusion is understandable given the name. Professional astronomers more often use the parsec, which is about 3.26 light years and derives from parallax measurement rather than from travel time.
Why do some conversions show so many decimals?
Because the underlying factors are exact and the calculator does not hide precision you might need. Machining and surveying work to thousandths; a carpenter does not. Take as many digits as your task requires and ignore the rest — but round only at the end, never partway through a chain of conversions.
Which countries still use imperial?
Only the United States, Liberia and Myanmar have not adopted metric as the official system. Britain is formally metric but uses miles on roads and pints in pubs, and Canada mixes both — road signs in kilometres, human height in feet. That partial adoption is exactly why conversion errors persist in countries that consider themselves metric.
Does this work offline?
Yes. Every factor is fixed by definition and the conversion runs in your browser, so once the page has loaded no connection is needed. Nothing you enter is transmitted or stored.
Related converters
See the full list of Unit Converters, or try:
- Area Converter — square metres, feet, acres and hectares
- Volume Converter — litres, gallons and cubic units
- Weight Converter — kilograms, pounds, stones and ounces
- Speed Converter — km/h, mph, knots and m/s
- Feet & Inch Fraction Calculator — arithmetic in construction format