Why use this calculator
Two measurement systems are still in active use, and the boundary between them runs straight through ordinary life: building plans in millimetres, television screens in inches, running distances in kilometres, aviation altitudes in feet and marine charts in nautical miles. Most online converters use factors rounded to four or five figures, which is invisible on a single conversion and obvious after you multiply it by a hundred rafters or a kilometre of pipe.
Everything here uses the exact defined values, so an inch is precisely 25.4 millimetres and a mile is precisely 1,609.344 metres. The feet-and-inches row exists because that is how heights, doorways and timber lengths are actually spoken, and a decimal answer of 5.9 feet is not five feet nine inches.
How to use it
- Type the measurement exactly as it appears on your source into the value box, whether that is a dimension on an architectural drawing, a screen size from a product page, a distance from a map or a height from a passport.
- Then pick the unit that number is already in from the dropdown, checking the symbol printed beside it, since millimetres and metres are easy to confuse on a plan and inch marks are often shown as double quotes.
- Every other unit is returned at once, with the ones you did not select highlighted.
- If you are working with a person's height or a timber length, read the dedicated feet-and-inches row rather than trying to interpret the decimal feet figure.
What each result means
- Millimetres
- The working unit of construction and engineering drawings in metric countries, where whole numbers avoid decimals.
- Centimetres
- The everyday metric unit for body measurements, clothing sizes, furniture and packaging dimensions.
- Metres
- The SI base unit of length, defined from the distance light travels in a fixed fraction of a second.
- Kilometres
- The metric unit for road, running and geographic distances across most of the world.
- Inches
- The imperial unit used for screens, pipe bores, fasteners and North American construction.
- Feet
- The decimal feet equivalent, used in surveying and aviation where fractions are inconvenient.
- Feet and inches
- The same length spoken the way heights and timber lengths actually are, with whole feet and remaining inches.
- Yards
- Three feet, still used for fabric, sports fields and some landscaping materials.
- Miles
- The statute mile of 5,280 feet, used on roads in the United States and the United Kingdom.
- Nautical miles
- One minute of latitude, fixed at 1,852 metres and used for marine and aviation navigation worldwide.
The formula
Every conversion routes through the metre as the base unit, so the result is value × (metres per source unit) ÷ (metres per target unit).
The exact defining factors are 1 inch = 0.0254 m
1 foot = 0.3048 m
1 yard = 0.9144 m
1 mile = 1,609.344 m
1 nautical mile = 1,852 m
Feet and inches are recovered by taking the whole number of feet from the total inches and leaving the remainder, since 1 foot = 12 inches exactly
How this number is used in practice
Construction is where the two systems collide most expensively. Australian and British drawings are dimensioned in millimetres, timber is sold in metric lengths, but plumbing and many fasteners still carry nominal imperial sizes, so a half-inch pipe and a fifteen-millimetre pipe are not the same thing despite being used interchangeably in conversation.
On a North American site the arithmetic is fractional rather than decimal, and imported materials arrive in metric, which is why a converter that gives the exact figure rather than a rounded one matters when the error compounds over a run.
Travel and sport supply the other everyday cases. A marathon is exactly 42.195 kilometres, which is 26.2188 miles, and runners who train in one unit and race in the other need the conversion to pace correctly. Aviation quotes altitude in feet and distance in nautical miles almost everywhere, including in countries that are otherwise fully metric, because the nautical mile is tied to a minute of latitude and makes chart work straightforward.
Marine navigation uses the same unit for the same reason, which is why a knot is one nautical mile per hour.
Retail and manufacturing hide the conversion inside product names. A 55 inch television is nominally 139.7 centimetres on the diagonal, a 26 inch bicycle wheel is a rim standard rather than a measurement, and lumber sold as two by four finishes at about 38 by 89 millimetres.
Knowing the exact conversion is the first step in working out whether the nominal figure is a real dimension at all.
Worked examples
100 cm, the standard metric metre stick
Exactly 1 metre, 39.3701 inches and 3 ft 3.37 in.
A 6 foot doorway height
1.8288 metres, 182.88 cm and 72.0000 inches.
A marathon distance of 42.195 km
26.218757 miles and 22.783477 nautical miles.
A 55 inch television screen diagonal
139.70 cm, or 1.3970 metres.
When people use it
- Reading an imported building plan dimensioned in a unit your tape does not carry
- Converting a height between centimetres and feet and inches for a form or a passport
- Checking that a piece of furniture or an appliance will fit a measured space
- Translating running, cycling or swimming distances between kilometres and miles
- Interpreting marine and aviation distances given in nautical miles
Tips
- One inch is exactly 25.4 millimetres by definition, not a measured approximation. Every other imperial length follows from it.
- For mental arithmetic, a mile is about 1.6 kilometres and a kilometre is about 0.62 miles; five miles is almost exactly eight kilometres.
- Decimal feet are not feet and inches. A height of 5.75 feet is 5 ft 9 in, not five feet seventy-five.
- A nautical mile is not a statute mile. At 1,852 metres it is about fifteen per cent longer, which matters on any chart.
- Keep the full precision until the final step; rounding an inch to 25 mm introduces a 1.6 per cent error that compounds over a long run.
- Nominal sizes such as two by four timber or half-inch pipe are names, not measurements. Convert the finished dimension instead.
What it assumes, and where it stops being accurate
- All imperial units are the international definitions agreed in the yard and pound agreement, not the older US survey foot.
- Lengths are treated as exact geometric values with no allowance for material tolerance, thermal expansion or tape stretch.
- Negative lengths are rejected and treated as zero, since a physical length below zero has no meaning.
- The nautical mile is the international value of 1,852 metres; some historical charts used slightly different national values.
- Values are capped at one billion of the source unit so a shared link cannot request an unbounded conversion.
- Displayed figures are rounded for readability, so reversing a rounded output will not always return the exact original.
Terms used on this page
- Metre
- The SI base unit of length, defined by fixing the speed of light at 299,792,458 metres per second.
- International inch
- The inch as defined since the yard and pound agreement: exactly 25.4 millimetres.
- Nautical mile
- Exactly 1,852 metres, originally one minute of arc of latitude along a meridian.
- Statute mile
- The land mile of 1,760 yards or 5,280 feet, equal to 1,609.344 metres.
- Nominal size
- A trade name for a product size, such as two by four timber, that does not match its finished dimensions.
Questions people ask
- How many centimetres are in an inch?
- Exactly 2.54 centimetres, or 25.4 millimetres. This is a defined value rather than a measurement: since the international yard and pound agreement the inch has been fixed in terms of the metre, so the conversion carries no uncertainty at all and any calculator that shows 2.539 or 2.5 is simply rounding.
- Why does my height convert to a strange decimal in feet?
- Because decimal feet and feet-and-inches are different notations. A height of 1.78 metres is 5.8399 decimal feet, which is 5 feet and 0.8399 of a foot. Multiplying that fractional part by twelve gives about 10.1 inches, so the height is 5 ft 10 in. This calculator does that step for you in the feet and inches row.
- What is the difference between a mile and a nautical mile?
- A statute mile is 5,280 feet or 1,609.344 metres and is used on roads. A nautical mile is 1,852 metres, defined as one minute of latitude, and is used at sea and in the air because it maps directly onto a chart. A nautical mile is about fifteen per cent longer, and a knot is one nautical mile per hour.
- Is the US survey foot different from the international foot?
- Slightly. The survey foot was defined as 1200/3937 of a metre, about two parts per million longer than the international foot of 0.3048 metres. The difference is invisible on a building but reaches roughly three millimetres over a kilometre, which matters in land surveying. The survey foot has now been retired in favour of the international foot.
- Why do plans use millimetres instead of centimetres?
- Because millimetres let every dimension be written as a whole number, which removes the decimal point and with it the most common transcription error on a drawing. A door written as 2040 cannot be misread the way 204.0 can. Architectural and engineering practice in metric countries is to dimension exclusively in millimetres for that reason.
- How accurate do conversions need to be on site?
- Work to one more significant figure than your tolerance, then round once at the end. A cabinet needs about a millimetre, a slab about five, and a road survey far less. The risk is not the final rounding but the chain: converting, rounding, multiplying and converting back several times can move a dimension by more than the tolerance you were protecting.
Use this with
Length Converter answers one part of the question. These pick up where it stops: Area Calculator, Weight Converter, Temperature Converter and Aspect Ratio Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- NIST Special Publication 811: Guide for the Use of the International System of Units. Definitive table of exact conversion factors between SI and customary units
- NIST: SI base unit: metre. The metre defined from the fixed speed of light
- BIPM: The International System of Units (SI Brochure). The defining document for the SI base units and their definitions
Length Converter produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.