Convert cm to mm

Centimetres are the everyday metric length — body measurements, paper, furniture — and they are the one metric unit that has no direct customary counterpart, which is why so much converting happens through them.

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 centimetres to millimetres moves the decimal point 1 place 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 cm is 12340 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.

centi- and milli- are not part of the unit — they are a multiplier bolted onto the front of it, and the same one means the same thing on every unit it appears on.

Here that is a hundredth of a metre against a thousandth of a metre: 1 power of ten apart, and the unit underneath both of them is the same metre. Nothing about the quantity changes — only how many of it you are counting in one go.

Cut-to-size suppliers almost universally ask for whole millimetres, and the field will often refuse anything else. That is not pedantry: it is the one design decision that makes a size unambiguous. A field accepting 122.5 has to distinguish it from 1225 by unit alone, and a customer who types the number without the unit has told the factory two different things depending on who reads it.

The consequence for you is that the conversion happens before the form, not on it. Multiply by ten, confirm you have gained exactly one digit, and type the integer. A centimetre figure with one decimal always becomes a millimetre figure with the same digits and no point at all, which makes the check trivial: 122.5 has four digits, 1225 has four digits, and anything with three or five is a mistake.

Almost no window, door or alcove is a true rectangle, and the discrepancy is usually several millimetres across an ordinary domestic opening. Measure the width at the top, the middle and the bottom, and the height at the left, the centre and the right. Six figures, all in centimetres, all converted, and the differences between them tell you more about the job than any single number does.

Which of the six goes on the form depends on where the piece sits. Something fitted inside the recess takes the smallest width, because a blind sized to the widest point will jam at the narrowest. Something mounted over the opening takes the largest, plus the overlap the supplier specifies. Averaging is the one option that is always wrong, and it is the instinctive one.

Rounding is a decision about which error you can live with, and made-to-measure work is not returnable, so it deserves a moment. For a piece that will be trimmed, scribed or planed on site — a worktop against an uneven wall, a shelf into an alcove — round up, because material can be removed and cannot be added back.

For anything that must pass through or drop into an opening, round down and take the clearance the supplier asks for. Glass in particular is ordered several millimetres under the aperture so it can be bedded, and a pane cut to the exact measurement will not go in at all. Where the instructions state a deduction, apply theirs and not yours; applying both is how a blind ends up ten millimetres narrow.

Clearances in joinery and fitting are small numbers with visible consequences. An internal door is typically hung with a couple of millimetres of gap at the sides and top so it does not bind when the frame moves with humidity. A drawer runs on a few millimetres each side. A kitchen appliance sits in a housing sized several millimetres larger than the machine so it can be pulled out for service.

Those numbers do not translate comfortably into centimetres, which is a large part of why the trade does not use them. Two millimetres of door clearance is 0.2 cm, and a figure written that way reads like a decimal that could be tidied away. In millimetres it reads as a specification, which is what it is.

Design and cutting software will happily work in centimetres, and it is worth switching the document to millimetres before you draw anything rather than converting at export. Print artwork is specified in millimetres throughout — 3 mm of bleed beyond the trim line, and a safe margin of 5 mm or so inside it, within which nothing important should sit.

Laser and CNC work is the same discipline for a harder reason: the file is what gets cut, and a document set up in centimetres that is exported to a machine expecting millimetres produces a part ten times too small. The unit is not a display preference in that workflow. It is part of the geometry, and it should be declared once, at the start, in the unit the machine speaks.

Every conversion in this direction adds a digit, and that gives a free check that costs nothing. A domestic window is three or four digits in millimetres — 600, 1225, 1800. A figure with two digits is somebody who forgot to convert; a figure with five is somebody who converted twice. Scan the column of numbers on the order and confirm they are all the same length before you submit.

Then read the confirmation when it arrives. Suppliers restate the sizes they are cutting to, and that email is the last point at which the order is an argument rather than a piece of glass. Comparing the restated millimetre figures against your original centimetre notes takes under a minute and catches both your transcription errors and theirs.

Some materials are named by a size they do not have, and converting your measurement accurately will not save you from that. Planed timber sold as fifty by a hundred is nearer 47 by 97 mm, because the nominal figure describes the sawn section before the planer took a couple of millimetres off each face. The name is the stock it was cut from; the millimetres are what turns up.

Sheet goods and glass carry the same distinction under the word nominal. A board sold as 18 mm may measure a little under, and a manufacturing tolerance is quoted alongside the nominal thickness precisely because it is not exact. When a converted centimetre measurement has to work against one of these, take the millimetre figure off the actual material with a gauge rather than from the label.

If more than a couple of sizes are going onto a form, stop converting and read the tape's millimetre marks directly. Every metric tape has them; the centimetre numbering is the labelling, not the resolution. Recording 1225 in the first place removes the conversion, the transcription and the rounding decision in one move, and it makes your notes match the order.

The habit is worth keeping for anything that will be repeated. Kitchens, wardrobes and window schedules involve dozens of dimensions that will be handled several times each, and a note book in the supplier's unit is a note book that can be read straight onto the form. Centimetres are for describing the result to somebody afterwards.