How to Convert Metric and Imperial on Site

What you need

A tape measure marked in both units, a pencil and paper, and the converters on this site. Most modern tapes show feet and inches on one edge and metres and centimetres on the other — use the edge that matches the drawing, and convert only when you have to.

Step 1 — Know the exact conversions

Three numbers cover most site work: 1 ft = 0.3048 m exactly, 1 in = 25.4 mm exactly, and 1 mile = 1.609344 km. Write them down or bookmark the converters — do not trust a rounded figure from memory, because rounding errors add up over a long run. The reverse figures follow from the same numbers: 1 m = 3.28084 ft, and 1 mm = 0.03937 in.

Step 2 — Feet to metres

Multiply feet by 0.3048. The feet to metres converter does it and shows the other units too: 10 ft is 3.048 m, which is 304.8 cm, 120 in and 3.3333 yd. For a quick estimate on site, 1 ft is roughly 30 cm — but use the exact figure for cutting, because the rough one is wrong by 5 mm per foot.

Step 3 — Inches to feet

Divide inches by 12. The inches to feet converter gives you the answer as a decimal and as feet and inches: 78 in is 6.5 ft, which is 6 ft 6 in — and 1.9812 m, or 198.12 cm. Saying "six foot six" is how carpenters talk; writing 6.5 ft is how calculators work. Know both.

Step 4 — Miles, kilometres and the length converter

For longer distances, 1 mile is 1.6093 km, and 100 ft is 30.48 m. The length converter handles any pair of units — miles, kilometres, feet, metres, inches, millimetres and yards — using the exact constants, with no rounding until the displayed result. Use it whenever the units on the drawing and the units on your tape do not match.

Step 5 — Convert before you cut

Measure in one unit, convert, then mark the cut — never convert in your head while the saw is running. Write the converted number on the material next to the mark, and check it against the original: if 10 ft became 3.048 m, the maths is right; if it became 30.48 m, a decimal point slipped. One check takes ten seconds; one wrong cut takes an hour.

Worked example

10 ft is 10 × 0.3048 = 3.048 m (304.8 cm, 120 in, 3.3333 yd). 78 in is 78 ÷ 12 = 6.5 ft, which is 6 ft 6 in (1.9812 m, 198.12 cm). 1 mile is 1.6093 km, and 100 ft is 100 × 0.3048 = 30.48 m. Those are exactly the numbers the feet to metres converter, the inches to feet converter and the length converter give you.

Common mistakes

• Using 1 ft = 0.3 m. The rounded figure is wrong by 5 mm per foot — over a 10 ft run that is 50 mm, which is a gap you can see. Use 0.3048.
• Mixing units in one measurement. A length written as "10 ft 6 in" is fine; a length written as "10.6 ft" means something else entirely (10 ft 7.2 in). Keep the units straight.
• Converting after cutting. Convert first, mark second, cut third. A conversion done after the cut is a confession, not a measurement.

Safety

Converting is safe work, but the tape measure is not: a snapping tape can cut, and a long tape across a site is a trip hazard. Roll it up when you are done, and mind steps and edges while you measure. Never mark or drill into a wall until you have checked for wires and pipes behind it.

Try these calculators

• Length Converter — convert between miles, kilometres, feet, metres, inches and more.
• Feet to Meters Converter — feet to metres, centimetres, inches and yards.
• Inches to Feet Converter — inches to feet, feet and inches, and metric.