Feet and Inches to Meters

How the feet and inches to metres conversion works

Enter the feet and the inches separately, the way a tape or a plan gives them, and the calculator turns the pair into one length in metres: m = (ft + in ÷ 12) × 0.3048. It shows the same length in centimetres and millimetres, as decimal feet, and as a total in inches. The arithmetic adds no error of its own. The displayed figures are rounded only so they can be read.

Why 0.3048 is exact

Since 1959 the inch has been defined as 25.4 mm, which makes a foot 0.3048 m and a yard 0.9144 m, with no rounding anywhere in the chain. NIST's Office of Weights and Measures sets out the chain on its SI units of length page, and its US survey foot page confirms that 1 foot = 0.3048 metre exactly in every application from 2023. Before then US land surveys used a slightly longer survey foot of 1200/3937 m, about 2 parts per million more, or 0.6 mm over 1,000 ft. That difference matters only for state plane coordinates and old land records. For a building dimension the international foot is the only one in use.

Worked examples

An 8 ft ceiling is (8 + 0 ÷ 12) × 0.3048 = 2.4384 m, or 2,438.4 mm. Someone 5 ft 6 in tall is 5.5 × 0.3048 = 1.6764 m. A standard 6 ft 8 in door is 80 in × 25.4 = 2,032 mm. Sheet material 4 ft wide is 1.2192 m, and studs at 16 in centres are 406.4 mm apart. Those last two are why imported plasterboard and ply in metric countries come in 1,200 mm widths with 400 mm and 600 mm stud spacing: the metric modules are close to, but not the same as, the imperial ones, and a 1,200 mm sheet on 16 in studs will not land on a stud edge.

Fractions of an inch

A US tape measure is marked in sixteenths of an inch, so the inches box takes decimals: 1/2 in is 0.5, 1/4 in is 0.25, 1/8 in is 0.125 and 1/16 in is 0.0625. Each sixteenth is 1.5875 mm, which is why the millimetre result is shown to one decimal place: 5 ft 7 3/8 in is entered as 5 ft 7.375 in and comes out at 1,711.3 mm. Inches of 12 or more are accepted too, so 8 ft 14 in comes out the same as 9 ft 2 in, 2.7940 m.

Going back from metres

The reverse uses the same factor. Divide the metres by 0.3048 to get decimal feet, keep the whole number as feet, and multiply what is left by 12 for the inches. A 1.8 m opening is 1.8 ÷ 0.3048 = 5.9055 ft, so 5 ft and 0.9055 × 12 = 10.866 in, which is 5 ft 10 7/8 in to the nearest eighth. Putting 5 ft 10.875 in back into this calculator gives 1,800.2 mm, so the rounding to an eighth costs 0.2 mm. A 2.4 m ceiling works out at 7 ft 10 1/2 in the same way.

Decimal feet and the feet-inches split

Plans in the US write dimensions as 5'-6", with an apostrophe for feet and a double quote for inches. The decimal-feet result, 5.5000 ft here, is what a spreadsheet or a surveying instrument expects. Not every split gives a tidy decimal: 5 ft 7 in is 5.5833 ft with the 3 repeating, so the calculator keeps four places and works from the exact inches, not from the rounded decimal.

Where feet and inches still turn up

US construction runs on feet and inches: plan dimensions, ceiling and door heights, framing layouts and lumber lengths. Metric countries meet them more often than they might expect. Heights of people and rooms are still often given in feet and inches in the UK, screen sizes are quoted in inches around the world, and American plans, products and equipment arrive with imperial dimensions that have to be turned into millimetres for local drawings, suppliers and building control. The reverse happens when a US contractor reads a European detail sheet. Either way the factor is the same.

Precision and rounding in the results

Metres are shown to four decimal places, a resolution of 0.1 mm; centimetres to two places, which is the same 0.1 mm; and millimetres to one place, so a sixteenth of an inch still shows. That is finer than a steel tape can be read on site, where 1 mm is a good result. NIST's Guide for the Use of the SI (Special Publication 811) covers how to round converted values sensibly: keep as many digits as the original measurement justifies, and no more.

Using this with other BuildMetricLab tools

Once a dimension is in metres it goes straight into area and volume work: room areas in square metres, concrete in cubic metres, tile coverage. The cubic yards to cubic metres converter handles volumes and the acres to hectares converter handles land. All BuildMetricLab tools run in your browser with no sign-up, and the formula is on the page so the maths can be checked.