Convert m to mm

The metre has been defined since 1983 as the distance light travels in 1/299,792,458 of a second. It is not a physical object any more; the platinum bar in Paris was retired.

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.

Going from metres to millimetres moves the decimal point 3 places to the right and changes nothing else. There is no factor to remember and no rounding to decide: the digits stay in the same order and only their position changes.

1,234 m is 1234000 mm — same digits, moved along. That is worth knowing because it is the one kind of conversion you can check at a glance: if the digits of the answer are not the digits you started with, something other than the conversion has happened to them.

Hand a joiner, a cutting service or a machine shop a dimension in metres and it will be converted before anything happens to it, because the tools, the schedules and the software all work in millimetres. Doing the conversion yourself is not politeness; it is keeping control of the rounding, since the person converting for you does not know which figures matter and which were approximations.

The conversion is a shift of three places and the result should be an integer. 2.4 m is 2400 mm, 0.718 m is 718 mm, 12.5 m is 12500 mm. If a converted figure still has a decimal on it, that is information: either the dimension is genuinely a machined one held to a tenth of a millimetre, or somebody measured to a precision the job cannot hold.

The classic version of the decimal-point failure is not a typed number at all. STL, the format most three-dimensional printing and cutting workflows run on, stores raw coordinates with no unit declaration anywhere in the file. A model built in metres and opened by software assuming millimetres arrives a thousand times too small, and the same file opened by software assuming inches arrives at a third size again.

Nothing in the file will warn you, because from its point of view a cube of side 2 is a cube of side 2. The only defence is to check one known dimension immediately after import, before any work is done on the model: pick something whose size you are certain of, measure it in the receiving program, and confirm the number matches. That check takes seconds and it is the entire safeguard.

Construction and furniture dimensions are written as whole millimetres, and that is a decision about tolerance rather than about precision. A stud wall is not built to a tenth of a millimetre, so writing 2400.0 promises something the trade cannot deliver and invites an argument about a figure nobody intended to be exact.

Machined work is the opposite: a bearing seat written as 40.02 mm has two decimals because it will be measured to two decimals, and dropping them turns a controlled fit into a loose hole. The number of decimal places on a drawing is read as a statement of intent, so a converted metre figure should be rounded to the precision the process actually holds, and no further.

A cut list is where a metre dimension turns into a set of millimetre figures that must add up. Convert the stock length and each piece, then account for the saw. A blade removes roughly 3 mm per cut, so five pieces taken from one board lose four kerfs and about 12 mm, and a list that fills the board exactly on paper comes up short by the last cut.

Cutting order matters as much as the arithmetic. Take the longest pieces first, because an offcut can always become a short piece and a short piece can never become a long one, and leave the smallest components until the stock is already reduced. A converted cut list should include the offcuts as line items — that is the column that shows whether the plan is real.

A dimension without a tolerance is incomplete, and converting from metres is a good moment to add one because the metre figure almost certainly came from something measured rather than specified. A drawing normally carries a general tolerance in a note, covering every dimension not individually marked, and then marks the handful of features that need something tighter.

Which features need it is decided by function, not by size. A cabinet's overall height can wander a couple of millimetres without anybody noticing; the spacing between the holes that carry its shelf pins cannot, because two panels that disagree by a millimetre give a shelf that rocks. Convert every dimension, then mark the two or three that are actually holding the thing together.

Every conversion in this direction adds three digits, which makes a scale check almost free. Domestic and furniture dimensions land in the hundreds or low thousands of millimetres: a worktop depth is 600, a door is around 2000, a sheet of board is 2400. A figure in the tens is somebody who forgot to convert, and a figure in the hundreds of thousands is somebody who converted twice.

Do the check against an object rather than against a rule, because the eye catches what the arithmetic does not. Read your converted number and ask what it would be if it were a real thing — 45 mm is a matchbox, 450 mm is a shelf, 4500 mm is a wall. If the answer does not match the thing you are making, stop before the file goes anywhere.

A converted metre figure often has to be broken into a series of smaller millimetre figures — shelf positions up a side, holes along a rail, studs across a wall — and how those are written decides whether the last one lands where it should. Dimensioning each step from the previous one lets every small error add to the next, so a run of ten spacings each a millimetre generous finishes a centimetre out.

Taking every dimension from a single datum face instead keeps the errors independent: each position is right or wrong on its own and none of them inherits the one before. It also makes the piece easier to mark out and to check, since a tape hooked once over the end gives all ten figures without being moved. The total converted length becomes the last dimension on the list rather than the sum of everything above it.

The conversion is only safe if it stays converted, and a number leaves your hands the moment the file does. Formats differ in how much they protect you: some carry an explicit unit field, some record a scale factor, and some carry nothing at all. Knowing which of those you are exporting is part of the job, and when the format cannot state its unit, the covering message must.

A drawing sent as a document rather than a model has a related failure. Print at actual size and state the scale, because a sheet printed with fit-to-page is no longer at any scale, and anybody measuring off it gets a plausible wrong answer. The conventional protection is the instruction not to scale from the drawing, with the dimensions treated as the only authority on the sheet.