Convert ft to mm

A foot is twelve inches, and it is the unit heights and room sizes are quoted in across the United States. Aviation quotes altitude in feet almost everywhere, including in countries that are metric for everything else; China is the exception, where controllers clear aircraft in metres.

Millimetres are what engineering drawings and datasheets use, because a whole number of them is precise enough for almost anything made by machine and avoids a decimal point that can be lost in a photocopy.

The factor is 304.8, and almost nobody carries that around. Rounded to 300 it is off by 1.6 % — which stays invisible on small numbers and turns into a whole unit somewhere around 100 ft.

That is the number worth knowing before you round: not the error itself, but where it stops being ignorable. Below that point the shorter factor is the sensible one; above it, use the field above, which never rounds until it prints.

The foot has been defined in metric terms since the international yard and pound agreement of 1959, and in this page's units that comes out at 304.8 mm to one. The definition is exact by stipulation — it was not measured, and it cannot be refined by measuring it better. The decimal above is the whole of it, with nothing rounded away at the end.

Before that agreement the US and the British versions differed slightly, and old survey figures still carry the older definition. For anything outside land surveying the difference is far below the precision anybody is working to.

Every foot dimension converts to a millimetre figure ending in a multiple of 0.8, because a foot is exactly 304.8 mm and nothing about that is approximate. One foot is 304.8, two is 609.6, three is 914.4, four is 1,219.2, eight is 2,438.4, ten is 3,048 exactly. Only multiples of five feet come out whole, which is why converted dimensions on a drawing are so recognisable.

That trailing tenth is exact and almost always meaningless. A wall built to eight feet is not 2,438.4 mm; it is 2,438 mm give or take five, because that is what a tape, a saw and a floor deliver. Keeping the tenth on a cutting list gives a joiner a number they cannot cut to and cannot check, and it hides which digits were measured behind digits that were computed.

Plywood, plasterboard and MDF exist in two nearly identical sizes. The imperial sheet is 8 × 4 feet, which is 2,438.4 × 1,219.2 mm. The metric sheet that replaced it is 2,440 × 1,220 mm — 1.6 mm longer and 0.8 mm wider, chosen because those are round numbers rather than because anything needed to change. Both are sold as "eight by four" in the same builders merchant.

One sheet either way, nobody notices. Across a run of five sheets the metric ones are 8 mm longer, which is enough to push the last joint off a stud and turn a full sheet into a cut one. When a drawing specifies imperial sheets and the supplier delivers metric, the difference belongs in the setting-out rather than in the tolerance, because it is systematic and always in the same direction.

Structural timber is where the naming and the measuring came apart entirely. A "two by four" has not measured two inches by four for a long time: the dressed size is 1.5 × 3.5 inches, which is 38.1 × 88.9 mm, and it is sold across Europe and Canada as 38 × 89 mm. Converting the name gives 50.8 × 101.6 mm, which is a piece of timber that does not exist.

The rule that follows is to convert the measured size and never the nominal one. This applies to timber, to pipe, to sheet gauges and to anything else where an inch number has become a product name. If the figure came off a tape measure it converts; if it came off a label it has to be looked up first, and the two are not distinguishable from the number alone.

An eight-foot ceiling is 2,438.4 mm; the standard metric ceiling is 2,400 mm, which is 7 feet 10.5 inches. The two are treated as the same height in conversation and are 38 mm apart, which is a visible band of daylight above a door frame or a run of wall units. Anything ordered to one height and installed under the other has to be scribed or packed.

Doors carry the same fossil. A common British internal door is 1,981 × 762 mm, which is 6 feet 6 inches by 2 feet 6 to the nearest millimetre — an imperial door with a metric label, still in production decades after the sizes stopped being described in feet. Converting the opening rather than the door name is what prevents an order that is 19 mm out.

Rounding one dimension is harmless and rounding a repeated one is not. Sixteen-inch stud centres are 406.4 mm; metric framing uses 400 mm. Bay by bay the 6.4 mm is nothing, and by the twelfth stud the two layouts are 76.8 mm apart — more than three inches, and enough that a sheet joint lands in the middle of a bay with no timber behind it.

The way out is to convert the overall dimension first and divide it in metric, rather than converting each increment and adding them up. Set out the wall length in millimetres, decide how many bays fit, and let the spacing come out as whatever it comes out as. That keeps the total right and puts the small error where nobody can see it, which is the opposite of what converting increment by increment does.

The exactness of 304.8 says nothing about the measurement it is applied to. A dimension read off a tape in feet and inches is good to about an eighth of an inch on a good day, which is 3.2 mm; a dimension scaled off a drawing is good to whatever the scale rule allows, often 50 mm at 1:100. Converting either one produces a number with a tenth of a millimetre on the end, and the tenth is arithmetic rather than information.

The honest way to write it down is to convert precisely and round once, at the end, to the resolution the original measurement had. Eight feet becomes 2,438 mm, not 2,438.4. Twelve feet six becomes 3,810 mm, which is genuinely exact and can be written as such. Stating which figures are exact and which are rounded is worth a line on the drawing, because the next person cannot tell them apart.

American civil engineering and surveying dimension in decimal feet — tenths and hundredths — while American architecture dimensions in feet and inches, and the two notations look alike on a page. A site plan reading 8.75 means eight and three quarter feet, which is 8 feet 9 inches and 2,667 mm. Read as feet and inches it becomes 8 feet 7.5 inches and 2,629 mm, a 38 mm error introduced by a decimal point.

The tell is the number of digits after the point. Feet-and-inches notation never goes past 11 in the inch position, so a figure of 8.75 or 12.4 on a drawing is decimal feet, while 8-9 or 8′ 9″ is feet and inches. Where a set of drawings mixes a civil site plan with an architectural floor plan, both conventions are present in the same package and each dimension has to be read in the one its own sheet uses.