Convert mm to cm millimetres ↔ centimetres Free Online Secure
Type a value in millimetres and read it in centimetres at once. The factor is exact by definition, the arithmetic happens in your browser, and nothing is sent anywhere.
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Common values — Convert mm to cm (millimetres ↔ centimetres)
| mm | cm |
|---|---|
| 1 | 0.1 |
| 2 | 0.2 |
| 3 | 0.3 |
| 5 | 0.5 |
| 10 | 1 |
| 20 | 2 |
| 50 | 5 |
| 100 | 10 |
Where these units come from
The metre was first defined in 1793 as a ten-millionth of the distance from the equator to the North Pole — a definition nobody could check without surveying a quarter of the planet. It has been redefined four times since, each time to something steadier: a platinum bar, then a prototype kept in Paris, then a wavelength of krypton light, and since 1983 the distance light travels in vacuum in 1/299,792,458 of a second. The inch stopped being a separate thing in 1959, when six countries agreed to define it as exactly 25.4 mm. That agreement is why every conversion on this page between a metric and an imperial length is exact rather than measured: both sides now descend from the same definition.
Where you meet them
Millimetres are the working unit of engineering drawings and of anything made in a workshop; centimetres belong to tailoring, furniture and school rulers; metres to rooms, fabric and running tracks. Feet and inches survive in aviation altitudes worldwide, in American and British construction, and in the diagonal of every screen you have ever bought. Nautical miles are not a quaint survival — one is a minute of latitude, which is why ships and aircraft still navigate in them.
Questions people ask
Why do engineering drawings use millimetres?
Because a millimetre is small enough that nearly every dimension of a manufactured part is a whole number or a short decimal, which leaves no room for the arithmetic mistakes that fractions invite. Drawings in millimetres state the unit once and then drop it, and tolerances are written the same way, in hundredths or thousandths of a millimetre.
Where is the centimetre used in everyday life?
For anything a person measures against their own body: height, clothing sizes, the length of a sleeve, the width of a shelf, the dimensions of a parcel. A centimetre is roughly the width of a fingernail, which makes it easy to picture. It is large enough to avoid long strings of digits and small enough that you rarely need a decimal.
Is the answer exact?
Where both units are defined in terms of one another by whole numbers, yes — the page says so above the figure, and the only rounding is in how many digits are shown. Where the ratio has no finite decimal, the page shows twelve significant digits and says the relation is approximate rather than pretending otherwise.
Does anything I type leave my device?
No. The arithmetic runs in your browser; nothing is uploaded and there is no server to upload it to. Load the page once and it keeps working with the network switched off.
Ten small steps to the centimetre: how to convert mm to cm
The millimetre and the centimetre are neighbours on the metric ladder, so close that people often convert between them without noticing: a drill bit of 8 mm makes a hole of eight tenths of a centimetre, a tile of 300 mm is a tile of 30 cm. Yet a slip of one decimal place changes a measurement by a factor of ten, and that is enough to ruin a cut, an order or a recipe. Whenever a figure in millimetres has to be read in centimetres, you must convert mm to cm, and this page does it as you type.
The rule is a division by ten, which moves the decimal point one place to the left. It is exact and quick, but because the two units are so close, it is easy to forget which way to move the point. The sections below give the rule with examples, short portraits of both units, a comparison, a tour of practical situations and a list of the mistakes to avoid.
How to convert mm to cm: divide by 10
The rule
One centimetre is exactly 10 millimetres. To convert mm to cm, divide the number of millimetres by 10 or move the decimal point one place to the left. 10 mm is 1 cm, 25 mm is 2.5 cm, 7 mm is 0.7 cm, 300 mm is 30 cm and 1,250 mm is 125 cm. In the other direction, multiply by 10.
Examples
A screw of 40 mm is 4 cm long. A phone 7.8 mm thick is 0.78 cm. A plank 18 mm thick is 1.8 cm. A ruler of 150 mm is 15 cm. A plasterboard sheet of 2,400 mm is 240 cm. Each result keeps the same digits, with the point one place to the left.
Mental picture
To convert mm to cm by eye, look at a ruler: the numbered marks are centimetres, and the small marks between them are millimetres, ten to each centimetre. A length of 37 mm is three numbered marks and seven small ones, which reads 3.7 cm.
Trailing zeros
A whole number of centimetres loses its trailing zero: 60 mm is 6 cm, not 6.0 cm. Keep the zero only if it shows a real precision, for example a measurement of 60 mm made to the nearest millimetre.
The millimetre
A thousandth of a metre
The millimetre is one thousandth of a metre. The prefix milli comes from the Latin for a thousand. It is the main unit of mechanical drawings, hardware and electronics, and it is also used for rainfall and for the size of camera sensors.
Where it fits
Anything that must be measured to better than a centimetre is given in millimetres: the thickness of a board, the diameter of a bolt, the width of a pipe. A tolerance of half a millimetre is routine in workshops.
The centimetre
A hundredth of a metre
The centimetre is one hundredth of a metre and ten millimetres. The prefix centi is from the Latin for a hundred. It is the everyday small length of the metric world: height of people, clothing sizes, furniture, books, photographs.
Why it is popular
Centimetres are large enough to avoid long numbers and small enough to avoid decimals for many everyday objects. A table of 75 cm is easier to say than one of 750 mm, and a person of 175 cm easier than 1,750 mm.
Millimetre and centimetre compared
A centimetre is ten millimetres. The same length gives a number ten times smaller in centimetres. Use millimetres when you need precision of less than a centimetre, centimetres when the size is the main point.
When writing for a mixed audience, choose one unit for each kind of dimension and stay with it. A shelf of 80 cm length and 18 mm thickness is a normal way to write; a shelf of 800 mm by 1.8 cm would look odd.
Where you will need it
Shopping for materials
Boards are labelled by thickness in millimetres but by width and length in centimetres: 18 mm by 30 cm by 240 cm. Tiles are 300 by 600 mm, which you convert to 30 by 60 cm to compare with a catalogue in centimetres. A quick convert mm to cm avoids ordering the wrong size.
Photography and printing
Camera sensors are 36 by 24 mm, 3.6 by 2.4 cm. A print of 150 by 100 mm, 15 by 10 cm, is the usual postcard size. A frame of 200 by 250 mm is 20 by 25 cm. Printers list paper in millimetres, picture frames in centimetres.
Rain and snow
Rainfall is in millimetres, snow depth in centimetres. A fall of 30 mm of rain is 3 cm of water, enough to fill a shallow dish. Fresh snow of 30 cm holds about 30 mm of water, so the same number means different things.
School and science
Pupils measure with a ruler in centimetres but record in millimetres for precision: 7.3 cm is 73 mm. Teachers set exercises that go both ways to strengthen the idea of tenths.
3D printing and models
Print dimensions
Slicers use millimetres, but you may describe your model in centimetres. A model 220 mm tall is 22 cm. A layer height of 0.2 mm is 0.02 cm. A print bed of 220 by 220 mm is 22 by 22 cm. If you are preparing a design for a friend with a different printer, write the dimensions in both units.
Scale models
A model at 1:24 shrinks a car of 450 cm to 18.75 cm, 187.5 mm. A figure of 1:87 scale reduces 10 m, 1,000 cm, to 11.5 cm, 115 mm. Model makers prefer millimetres for parts.
History of the units
The decimal ladder
The French commission of the 1790s planned a system in which each unit is ten times the one below it. The centimetre and millimetre followed from the metre by hundredths and thousandths. The three units have been used together since the early 19th century.
The ruler
School rulers in centimetres with millimetre divisions became standard in Europe in the 19th century. They are among the first scientific instruments a child uses.
Fashion and sewing
Seams and notions
A seam allowance of 15 mm is 1.5 cm. Buttons are 10 to 25 mm wide. Zips are 200 to 700 mm. A hem of 30 mm is 3 cm. A pattern may use millimetres for small details and centimetres for body measurements, which is why a seamstress needs the conversion at hand.
Knitting needles
Needles are sized in millimetres: 3 mm, 4 mm, 5 mm. A knitted piece measures 20 stitches in 10 cm, which is 2 stitches per centimetre, 0.2 stitches per millimetre.
Language and symbols
Symbols
The symbols are mm and cm, lower case, no full stop, no plural s. Put a space between the number and the unit: 25 mm. In some languages a comma replaces the decimal point: 2,5 cm.
Words
Milli is from Latin mille, a thousand; centi from centum, a hundred. A millimetre is thus a thousandth of a metre and a centimetre a hundredth. The common word for ten times is deci.
A worked example: fitting a picture frame
Measuring a photograph
A photograph is 178 by 127 mm. To convert mm to cm, divide each by ten: 17.8 by 12.7 cm. Frames come in 13 by 18 cm and 15 by 20 cm. The 13 by 18 cm frame is too small, since 17.8 is under 18 but the picture needs a mat of at least 2 mm each side; the 15 by 20 cm frame fits with a mat of 1.1 cm on the short side and 1.1 cm on the long.
Choosing
With a 15 by 20 cm frame the border around the photo is 1.1 cm all round, a pleasing proportion. The decision follows directly from the converted dimensions.
Final checks
Round trip
Multiply your answer by 10. If you return to the number of millimetres you started with, the conversion is right. If you get ten times too much, you divided by 100.
Notes
Write both figures in notes or drawings. A later reader can verify them in seconds.
Furniture drawings and flat-pack plans
Which unit to trust
Flat-pack instructions often give the size of each panel in millimetres, while the product page gives overall dimensions in centimetres. A wardrobe of 1,000 by 580 by 2,010 mm is listed as 100 by 58 by 201 cm. Before you buy, convert mm to cm for every dimension of your alcove, then check the clearance at the top, where a 2,010 mm wardrobe needs at least 20 mm, 2 cm, to be tilted into place.
Tolerances in the wall
Walls are rarely straight. Measure the width at floor, middle and top, and take the smallest. If the narrowest point is 1,012 mm, 101.2 cm, a 1,000 mm unit fits with 6 mm each side. A skirting board of 12 mm reduces the clearance to 0.
Bicycles, tyres and fittings
Everything in millimetres
Bicycle parts are specified in millimetres: a seat post of 27.2 mm, a handlebar of 31.8 mm, a tyre of 622 mm bead seat diameter and 40 mm width. Frame sizes are in centimetres: 54 cm. A rider who reads an online description must know both. To compare a 40 mm tyre, 4 cm, with the 3.8 cm clearance of a fork, a quick convert mm to cm reveals that it will not fit.
Mudguards and racks
Mudguards come in widths of 45, 50 or 60 mm. A rack for a 28 inch wheel spans 622 mm plus tyre, about 71 cm. Cyclists planning a tour measure the clearance in millimetres and the length of the bike in centimetres.
Cooking and kitchenware
Sizes in the kitchen
A baking tin is 240 mm, 24 cm, across. A pizza stone of 330 mm is 33 cm. Pasta is rolled to 1 mm, or 0.1 cm. Knife blades are 200 mm, 20 cm, for a chef's knife. Recipes seldom use millimetres, but cookware does, and equipment manuals are in millimetres.
Slicing
Slices for crisps are 1.5 mm, 0.15 cm, thick. Vegetables for a stew are 10 mm, 1 cm. In cookbooks, the slicing is in centimetres, in machine manuals in millimetres.
Gardening
Seed depth and spacing
Small seeds are sown 5 mm, 0.5 cm, deep; beans 30 mm, 3 cm. Spacing is in centimetres: 25 cm for lettuces, 40 cm for peppers. A seed packet may use both units. Growth rates are quoted in millimetres per day: bamboo can grow 30 mm a day, 3 cm.
Rain gauges
A garden gauge reads millimetres. A week of 20 mm rain is 2 cm of water on the soil. If you add 1 cm with a watering can, you give it 10 mm, half a week's rain.
Medicine and the body
Small measurements
A doctor may describe a wound as 25 mm long, 2.5 cm. A growth of 8 mm is 0.8 cm. A baby's head circumference is 350 mm, 35 cm. In reports both units appear; the choice often depends on the specialty. Radiologists prefer millimetres, general practitioners centimetres.
Reading a report
When a report gives a size in millimetres and you want a feel for it, divide by 10. A lesion of 12 mm is 1.2 cm, about the width of a fingernail.
Paper, board and packaging
Standard sizes
An A4 sheet is 210 by 297 mm, 21 by 29.7 cm. An A5 sheet is 148 by 210 mm, 14.8 by 21 cm. A business card is 85 by 55 mm, 8.5 by 5.5 cm. A cardboard box of 400 by 300 by 200 mm is 40 by 30 by 20 cm.
Thickness
Paper is 0.1 mm thick, 0.01 cm. A ream of 500 sheets is 50 mm, 5 cm. Card is 0.3 to 0.5 mm. Corrugated board is 3 to 5 mm.
Electronics and cables
Connectors and boards
A printed circuit board is 1.6 mm, 0.16 cm, thick. A USB-C plug is 8.4 mm wide. A pin header has 2.54 mm spacing, 0.254 cm. A microcontroller board of 51 by 21 mm is 5.1 by 2.1 cm. Datasheets use millimetres.
Cable thickness
A network cable is 6 mm across, 0.6 cm. A power cable of 10 mm is 1 cm. Conduit for cables is 20 mm, 2 cm.
Spreadsheet formulas
Formula
To convert mm to cm for a list, put millimetres in column A and enter =A2/10 in column B. Format with one decimal. To go back, multiply by 10. Apply to a whole column of dimensions to prepare a table in centimetres.
Check values
Enter 10: result 1. Enter 35: 3.5. Enter 2400: 240. If the result looks a hundred times too small, you divided by 100.
Rounding
How many decimals
A measurement of 37 mm made to the nearest millimetre is 3.7 cm. Writing 3.70 cm adds a false zero. For a measurement to the nearest 0.5 mm, one decimal place in centimetres is still enough.
Tolerance
A tolerance of plus or minus 0.2 mm is plus or minus 0.02 cm. Convert the tolerance together with the size so that the drawing stays consistent.
Teaching the idea
With a ruler
To teach the “convert mm to cm” calculation, give every pupil a 15 cm ruler. They count the small marks between two numbered ones: ten. They then measure a pencil and record the length in millimetres and centimetres. The two numbers differ by one decimal place.
A drill
Say a length in millimetres and ask pupils to write it in centimetres before the teacher counts to five. Start with 30, 45, 120, 7, 85. Repeat in the other direction.
More values for convert mm to cm
A longer list of conversions
If the value you need is not in the first list, these further conversions may be close enough to read off directly:
- 0.1 mm = 0.01 cm
- 0.5 mm = 0.05 cm
- 3 mm = 0.3 cm
- 4 mm = 0.4 cm
- 7 mm = 0.7 cm
- 15 mm = 1.5 cm
- 20 mm = 2 cm
- 75 mm = 7.5 cm
- 200 mm = 20 cm
- 500 mm = 50 cm
- 2,000 mm = 200 cm
- 10,000 mm = 1,000 cm
Mistakes when you convert mm to cm
Moving the point the wrong way
A centimetre is larger, so the number gets smaller. If 25 mm becomes 250, you multiplied.
Dividing by 100
That gives decimetres. For centimetres, divide by 10.
Forgetting the unit
A bare 40 could be centimetres or millimetres. Write mm or cm after the number.
Quick answers
How many millimetres in a centimetre?
Ten.
How do I convert mm to cm?
Divide by 10.
What is 45 mm in centimetres?
4.5 cm.
Is the conversion exact?
Yes, exact.
Does my number leave my device?
No. The calculation runs in your browser.
Related pages and a last word
If you work with lengths, these conversions are close at hand: Convert mm to dm, Convert mm to µm, Convert dm to cm, Convert m to cm, Convert cm to in, Convert °C to °F, Convert °F to °C, Convert L to gal, Convert gal to L. The reverse of this page is Convert cm to mm.
In short, to convert mm to cm divide by ten, move the decimal point one place to the left and keep only the digits your measurement supports. Your number stays on your device: nothing is uploaded and no account is needed.