Length Converter
Converts between metric and imperial length units using exact definitions rather than rounded approximations. Most of the time the difference is irrelevant; in surveying, machining, and anything cumulative it is not.
How to use it
- Enter a value and pick the unit it is in.
- Pick the unit you want.
- Read the converted value.
The inch is defined in metric
Since the international yard and pound agreement of 1959, the inch is defined as exactly 25.4 millimetres. Not approximately — exactly, by definition. Every imperial length unit derives from it: a foot is exactly 0.3048 metres, a yard exactly 0.9144, and a mile exactly 1609.344.
This means every metric-to-imperial length conversion is exact, and any residual error comes from rounding in the calculation rather than from the units themselves. A conversion factor of 2.54 centimetres per inch is not an approximation of a messy physical relationship; it is the definition.
Before 1959 the definitions differed slightly between countries, which is why very old survey data can disagree with modern figures by a small but non-zero amount. The United States retained a separate survey foot, defined as 1200/3937 metres, for land surveying, differing from the international foot by about two parts per million. Across a mile that is roughly an eighth of an inch; across a long baseline in a state plane coordinate system it is metres. That unit was officially deprecated at the end of 2022, but historical records and older datums still carry it.
Where rounding actually causes problems
Using 2.5 centimetres per inch instead of 2.54 is a 1.6 percent error. On a single measurement of a few inches that is invisible. Where it stops being invisible is anywhere the error accumulates or where tolerance is tight.
A worked case: a run of 40 cabinet units nominally 24 inches wide, converted at 2.5 centimetres, gives 60 centimetres each rather than 60.96. Over 40 units that is 38.4 centimetres of accumulated shortfall — well over a foot. This is the same class of error that produces a wall that does not quite meet a fixed opening.
Machining tolerances make it worse. A part specified to plus or minus 0.001 inch is a tolerance of 0.0254 millimetres. Any rounding in the conversion consumes a meaningful fraction of that budget before the machine has done anything, which is why drawings should carry dimensions in one unit system with the conversion done once, at full precision, rather than converted repeatedly through a chain of drawings.
Fractional inches
Imperial construction and woodworking work in binary fractions — halves, quarters, eighths, sixteenths, sometimes thirty-seconds. Converting these mentally is a matter of knowing the decimal equivalents rather than doing division under pressure.
The useful anchors: 1/16 is 0.0625 inch or 1.5875 mm; 1/8 is 0.125 or 3.175 mm; 1/4 is 0.25 or 6.35 mm; 1/2 is 0.5 or 12.7 mm; 3/4 is 0.75 or 19.05 mm.
A rough field approximation worth having is that 1 millimetre is a little under 1/25 inch, so 1/16 inch is about 1.6 mm and 1/32 inch is about 0.8 mm. That is close enough to judge whether a metric drawing and an imperial tape are going to agree.
The reverse direction is where mixed-unit projects go wrong. Metric sheet goods and imperial framing do not align: 1220 by 2440 mm plywood is not 4 by 8 feet, it is about 48.03 by 96.06 inches. Small, but enough to matter when sheets are meant to butt on a stud layout.
Nominal sizes are not measurements
A great many stated dimensions are names rather than sizes, and no conversion will fix that because the number was never the measurement.
A nominal two-by-four in North America measures 1.5 by 3.5 inches after drying and planing. A one-by-six is 0.75 by 5.5. Nominal pipe size is a designation whose relationship to any physical dimension varies with schedule — a 2-inch pipe has an outside diameter of 2.375 inches. Screen sizes are diagonals, not widths, and drill bits below a certain size are named by gauge or letter rather than dimension.
The rule is to convert measured dimensions and look up nominal ones. Converting 2 inches to 50.8 millimetres for a two-by-four gives a precise answer to the wrong question.
At a glance
| Inch | Exactly 25.4 mm by international definition |
|---|---|
| Foot | Exactly 0.3048 m |
| Mile | Exactly 1609.344 m |
| Transmitted | Nothing |
Frequently asked questions
Is 1 inch exactly 2.54 cm?
Yes, exactly, by the 1959 international agreement. Every imperial length unit derives from that definition, so the conversions are exact.
What is a survey foot?
A United States land-surveying unit defined as 1200/3937 metres, differing from the international foot by about two parts per million. Deprecated at the end of 2022 but still present in historical data.
Why does rounding to 2.5 cm per inch matter?
It is a 1.6 percent error that accumulates. Across 40 cabinet units of 24 inches it produces over a foot of total shortfall.
How do I convert a two-by-four?
You do not — it is a nominal name, not a measurement. The actual section is 1.5 by 3.5 inches, or 38 by 89 mm.
Read more
Unit conversion done properly — Most conversions are exact multiplications. The ones that are not cause the expensive mistakes.