Convert cm to mm centimetres ↔ millimetres Free Online Secure
Type a value in centimetres and read it in millimetres 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 cm to mm (centimetres ↔ millimetres)
| cm | mm |
|---|---|
| 1 | 10 |
| 2 | 20 |
| 3 | 30 |
| 5 | 50 |
| 10 | 100 |
| 20 | 200 |
| 50 | 500 |
| 100 | 1000 |
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
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.
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.
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.
Down to the last tenth of a centimetre: why you convert cm to mm
Some jobs are done in centimetres and others demand millimetres. A tailor marks a hem at 3 cm, but the zip supplier lists the tape width as 25 mm; a woodworker plans a shelf 90 cm long, yet the drill bit chart is in millimetres; a nurse reads a wound as 2.5 cm, while the dressing pack says 25 mm. The two units sit next to each other on every school ruler, and still it is surprisingly easy to slip between them. This page exists to convert cm to mm quickly and without error: enter a length in centimetres and read it in millimetres at once.
The rule is a multiplication by ten, which is the simplest sum there is, but the simple sums are where tired people make mistakes. A misplaced point turns a 4 mm screw into a 40 mm one and a 6 cm gap into 6 mm. Below you will find the factor, worked examples, short profiles of the two units, a comparison, the trades that rely on the conversion and the pitfalls to avoid.
The rule: times ten
The factor
One centimetre is exactly 10 millimetres, and one millimetre is 0.1 cm. To convert cm to mm, multiply by ten, which is the same as moving the decimal point one place to the right. 7 cm is 70 mm; 2.5 cm is 25 mm; 0.8 cm is 8 mm; 12.35 cm is 123.5 mm. Because both units come from the metre by powers of ten, the conversion is exact.
Examples to copy
A credit card is 8.56 cm by 5.4 cm, which is 85.6 mm by 54 mm. A coin of 2.4 cm is 24 mm across. A standard A4 sheet is 21 cm by 29.7 cm, or 210 mm by 297 mm. A pencil lead of 0.2 cm is 2 mm thick. Each answer has the same digits as the question, shifted by one place.
In your head
Add a zero if the number is a whole number: 15 cm is 150 mm. If it has a decimal part, move the point one place: 3.7 cm is 37 mm. Check that the answer looks ten times bigger; if a screw of 3 cm comes out as 3 mm, you have gone the wrong way.
How many digits to keep
The millimetre invites more precision than the centimetre. A reading of 2.5 cm from a ruler is really 25 mm, not 25.0 mm, unless you measured with a tool that reads tenths of a millimetre.
The centimetre
A hundredth of a metre
The centimetre is the working unit of rulers, fabrics, furniture and people. It is small enough to describe a hand or a book without decimals and large enough that most household measurements fit in two or three digits. Its prefix, centi, means one hundredth.
Where it is used
Height charts, pattern pieces, shelf widths and paper formats are given in centimetres. Many school exercises set a first task of measuring a pencil, a book or a desk in centimetres.
The millimetre
A thousandth of a metre
The millimetre is a thousandth of a metre, roughly the thickness of a credit card or a coin. The prefix milli means a thousandth, from the Latin mille. It is the standard unit of technical drawings, machined parts, optics, rainfall and small tolerances, and the unit in which engineers prefer to give dimensions to avoid decimals.
Where it is used
Screws, drill bits, pipe walls, cable diameters, camera lenses, film widths and watch cases are specified in millimetres. Weather reports measure rain in millimetres, which equals litres per square metre. The symbol is mm.
Centimetre and millimetre compared
A centimetre is ten times a millimetre, so a number in millimetres is ten times the same length in centimetres. 4 cm is 40 mm. A tip: the cm rulings on a school ruler are numbered; the mm ones are the small lines between them, ten to each centimetre.
Use centimetres when people talk about objects, millimetres when parts have to fit. In technical documents, millimetres are the norm for any length under a metre, with no decimals if possible. In shops, centimetres reassure customers. When you convert cm to mm for a document, write the unit clearly.
Where the conversion helps
Workshops and DIY
Drill bits, screws and bolts are sized in millimetres, but wood is often marked in centimetres. A 6 cm screw, 60 mm long, needs a pilot hole of about 3 mm, and a shelf pin hole of 0.5 cm is 5 mm. Plan a project in one unit: if you buy fittings in millimetres, convert cm to mm on your sketch before you cut. A tape that shows both scales avoids the conversion altogether.
Sewing and craft
Pattern pieces give seam allowances in centimetres, while haberdashery counters sell lace, ribbon and tape by millimetre widths. A 1.5 cm seam is 15 mm, a bias tape of 12 mm is 1.2 cm, a button of 18 mm is 1.8 cm. Embroidery hoops and needle sizes follow the same mixture, so keep a conversion table next to your sewing machine.
Health and science
Doctors measure wounds and lesions in millimetres and centimetres, depending on the size: a 7 mm mole is 0.7 cm, a 2 cm ulcer is 20 mm. Ultrasound reports give a fetus length in millimetres in early pregnancy and in centimetres later. In the lab, slides are 75 × 25 mm, or 7.5 × 2.5 cm. When you convert cm to mm for a medical figure, copy the unit exactly as written.
Photography and printing
A full-frame camera sensor is 36 × 24 mm, or 3.6 × 2.4 cm. A film negative is the same. A passport photo is 35 × 45 mm, 3.5 × 4.5 cm, and a business card 85 × 55 mm, 8.5 × 5.5 cm. Printers specify margins in millimetres, so a margin of 1.5 cm is 15 mm.
A short history of the small units
The metric system takes shape
When France's scientists built the metric system in the 1790s, they gave every unit a decimal relative: a tenth, a hundredth, a thousandth. The names came from Latin for the small steps, deci, centi, milli, and from Greek for the big ones, deca, hecto, kilo. The idea was that anyone who could shift a decimal point could change units. The centimetre and the millimetre became the everyday and the technical members of the family.
Tolerances and industry
The industrial revolution made the millimetre indispensable. Interchangeable parts, which let a broken component be replaced from a stock, required tolerances of a fraction of a millimetre. Gauge blocks, invented in the early twentieth century, allowed workshops everywhere to calibrate their tools against reference blocks accurate to a millionth of a metre. Today a car engine is built to tolerances of 0.01 mm, a tenth of a hair's thickness.
The everyday millimetre
Rain, snow, fish and insects are measured in millimetres in weather and nature reports. A hailstone of 25 mm is the size of a coin; a sleet of 5 mm is a pea. When you convert cm to mm, you move into the scale in which engineers and scientists describe the physical world.
Reading the small lines on a ruler
Ten little lines
Every centimetre on a ruler is divided into ten millimetres, and the fifth line is usually a little longer to help your eye. To read a length such as 7.3 cm, find the 7, count three small lines past it and you have 73 mm. Beginners often count the lines from the wrong end or include the zero line, so start your count at the first line after the number. Once you can read a ruler at a glance, there is little left to convert cm to mm on paper.
Rulers for different jobs
Engineering rulers have a bevelled edge and fine lines that are easier to read at 0.5 mm. School rulers are thick and wide, good for beginners. Flexible rulers adapt to curves but are less accurate. Choose the ruler that fits the job, and keep it clean and straight: a bent or worn end introduces an error that no conversion can correct.
How to measure to the millimetre
The right ruler
A good steel ruler with engraved markings is accurate to half a millimetre. Wooden and plastic rulers can be a millimetre out, and tailors' tapes stretch. Read the scale from directly above, with your eye over the mark, to avoid parallax, the apparent shift caused by looking from the side. Start measuring from the 1 cm mark if the end of the ruler is worn.
Callipers and micrometers
For parts that must fit, use callipers, which read to 0.02 mm, or a micrometer, which reads to 0.001 mm. A digital calliper can switch between millimetres, centimetres and inches at a button. Zero it before each reading, close the jaws gently and take two readings to confirm.
Accuracy versus precision
Precision is how many digits you can read; accuracy is how close they are to the truth. A cheap digital calliper may show 0.01 mm but be wrong by 0.05. Calibrate against a known block or the shank of a drill bit.
Paper and screens
Paper sizes in millimetres
International paper sizes are defined in millimetres: A4 is 210 × 297, A5 148 × 210, A3 297 × 420, B5 176 × 250. Each size is half of the next larger one, a design that preserves the proportion 1 : √2. Envelopes, folders and frames follow the same logic. If you design a leaflet in centimetres, convert cm to mm for every dimension before sending it to print, so that the printer's template matches.
Screens and pixel pitch
A phone screen about 6.1 inches across the diagonal is 155 mm; the pixel pitch, the distance between pixels, is around 0.05 mm. Laptop bezels, keyboard keys and port spacings are given in millimetres, and a standard key is 19 mm, 1.9 cm, from centre to centre.
Quick reference
A small table
0.1 cm = 1 mm; 0.5 cm = 5 mm; 1 cm = 10 mm; 2.5 cm = 25 mm; 5 cm = 50 mm; 10 cm = 100 mm; 30 cm = 300 mm; 100 cm = 1,000 mm. With these anchors you can convert cm to mm for any value by scaling.
Anchors in the hand
A fingernail is about 10 mm wide, a little-finger width 15 to 18 mm, a thumb 20 to 25 mm, a hand span 180 to 200 mm. These help you judge a length by eye without a ruler.
Using both units well
Choosing the unit
Use millimetres when the quantity is small enough that centimetres would need decimals, or when a standard calls for it; use centimetres when it makes numbers short and familiar. A table 74 cm high is better than 740 mm for a shopper; a drawing of a bracket 74 mm wide is better than 7.4 cm for a machinist.
Consistency in documents
Do not mix units in one list. If a plan gives most dimensions in millimetres, give all of them in millimetres, even the long ones. Mixed units are the source of the commonest errors in construction and manufacturing.
Examples from daily life
Around the house
A light switch plate is about 86 mm square. A plug pin is 4 mm thick. A smartphone is 7 to 9 mm thick. A bath tap tail is 15 mm. In centimetres: 8.6, 0.4, 0.7 to 0.9 and 1.5. If you must convert cm to mm for a repair, bring the old part to the shop and measure it together.
Outdoors
A fishing hook of 20 mm is 2 cm. A rainfall of 25 mm in a day is a heavy shower, 2.5 cm. A hailstone of 50 mm is 5 cm and can dent a car. In nature, millimetres give a sense of scale to insects, seeds and shells.
Putting it to work: a small project
A shelf, step by step
Suppose you are building a bookshelf from boards 18 mm thick. The plan says the shelf is 80 cm wide and 25 cm deep, with a gap of 30 cm between shelves. Convert the plan once: 800 mm wide, 250 mm deep, 300 mm between. The side panels must be long enough for three gaps and four boards, which is 3 × 300 + 4 × 18 = 972 mm, or 97.2 cm. Work in millimetres throughout, and you will never meet a decimal. At the end, if you need to convert cm to mm again for the shopping list, buy 100 cm boards and trim them.
Checking the result
Lay the pieces on the floor and check them against the plan. Measure diagonals: if the two diagonals of a rectangle are equal to the millimetre, the corners are square. Mark the drill points with a pencil at the millimetre, and drill a test hole in scrap first. A day of planning in millimetres often saves a weekend of corrections.
More values for convert cm to mm
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 cm = 1 mm
- 0.5 cm = 5 mm
- 3 cm = 30 mm
- 4 cm = 40 mm
- 7 cm = 70 mm
- 15 cm = 150 mm
- 20 cm = 200 mm
- 75 cm = 750 mm
- 200 cm = 2,000 mm
- 500 cm = 5,000 mm
- 2,000 cm = 20,000 mm
- 10,000 cm = 100,000 mm
Mistakes when you convert cm to mm
Dividing instead of multiplying
Going from the bigger unit to the smaller means a bigger number. Multiply by ten. A result smaller than the input is a warning.
Confusing mm with cm on a drawing
An engineering drawing in millimetres marked "25" is not 25 cm. Always read the unit note in the title block.
Counting zeros
When numbers have many zeros, like 1,000 mm for a metre, errors creep in. Use the page or a calculator and check the order of magnitude.
Quick answers
How many millimetres are in a centimetre?
Ten.
How do I convert cm to mm?
Multiply by ten, or move the decimal point one place to the right.
What is 3.5 cm in mm?
35 mm.
Is the conversion exact?
Yes, because both units are defined from the metre by powers of ten.
Is my number sent anywhere?
No. Everything runs in your browser.
Related pages and a last word
If you measure small lengths often, these conversions are close by: Convert cm to dm, Convert cm to in, Convert dm to mm, Convert µm to mm, Convert mm to dm, Convert °C to °F, Convert °F to °C, Convert L to gal, Convert gal to L. The reverse of this page is Convert mm to cm.
In short, to convert cm to mm multiply by ten, check that the answer is ten times larger and keep only the digits your tool can read. Your number stays on your device: nothing is uploaded, and there is no account.