Convert cm² to mm² square centimetres ↔ square millimetres Free Online Secure
Type a value in square centimetres and read it in square 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 cm² to mm² (square centimetres ↔ square millimetres)
| cm² | mm² |
|---|---|
| 1 | 100 |
| 2 | 200 |
| 3 | 300 |
| 5 | 500 |
| 10 | 1000 |
| 20 | 2000 |
| 50 | 5000 |
| 100 | 10000 |
Where these units come from
Square units need no definition of their own: a square metre is a metre by a metre, and everything follows from the length. The land measures are older and stranger. The acre began as the land a yoke of oxen could plough in a day, which is why it was long and thin rather than square — one chain by one furlong, 4840 square yards. The hectare and the are came out of the French Revolution, when land had to be measured in something decimal; neither belongs to the International System, but both were kept for land because a hectare is a comfortable size to picture and a square metre is not. Since the 1959 agreement on the yard, the acre is exactly 4046.8564224 square metres, so the conversions on this page are exact rather than approximate.
Where you meet them
Square metres price flats, size carpets and decide how much paint a wall needs; square feet do the same work in the United States and in much of the property market worldwide. Hectares belong to farmland, forests and planning permissions; acres to land deeds in the English-speaking world. Square kilometres and square miles are for countries, lakes and fires. Square centimetres and square millimetres live in workshops, on datasheets and anywhere a cross-section matters.
Questions people ask
Why does a square metre hold 10,000 square centimetres?
A metre is 100 centimetres, and area multiplies length by length, so a square metre is 100 × 100 = 10,000 square centimetres. It is a common slip to use 100, which is the factor for length. Whenever you convert an area, square the factor you would use for the length.
Why is a square metre a million square millimetres and not a thousand?
Because area is length multiplied by length. A metre is 1000 millimetres, so a square metre is 1000 × 1000, which is 1,000,000 square millimetres. The same trap catches people with every squared unit: the conversion factor of the length is squared along with the unit.
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.
A hundred little squares in every square centimetre
When you look closely at a chip, a wire, a seal or a needle, the square centimetre is already too coarse a unit. Engineers, electricians and laboratory staff switch to square millimetres, a hundred of which fit into one square centimetre. To convert cm² to mm² you multiply by 100, so 3 cm² is 300 mm² and 0.25 cm² is 25 mm². The rule is almost too simple, but it is easy to confuse with the linear factor of ten between centimetres and millimetres. This page multiplies as you type, and the text below shows where these small areas matter, from processor dies to cable cross sections.
Below you will find the rule with worked examples, short profiles of both units, a comparison, a long tour of real situations and a list of the most frequent mistakes. The calculation runs in your browser and nothing you type is sent anywhere.
How to convert cm² to mm²: multiply by 100
The rule
One square centimetre is exactly 100 square millimetres. To convert cm² to mm², multiply by 100, or shift the decimal point two places to the right. So 2 cm² is 200 mm², 7.5 cm² is 750 mm² and 0.04 cm² is 4 mm². Going back, divide by 100.
Worked examples
An SD card of 24 by 32 mm has 768 mm², which is 7.68 cm². A microSD card of 11 by 15 mm has 165 mm², 1.65 cm². A nano SIM card of 12.3 by 8.8 mm has 108 mm², 1.08 cm². In each case you move the decimal point two places, and you know the result is right when the number in mm² is a hundred times that in cm².
Why the factor is not ten
A centimetre is ten millimetres long, but a square centimetre has ten rows of ten squares, which makes a hundred. Using ten for an area gives a result ten times too small. The error is common when people copy the linear factor from a ruler.
A quick check
The square millimetre is the small unit, so the number must grow by two digits. If your “convert cm² to mm²” calculation produced fewer digits than you began with, you divided instead of multiplying.
The square centimetre
Ten millimetres on each side
The square centimetre is the area of a square whose sides measure 1 cm, or 10 mm. It is used for small surfaces: packaging, labels, craft, textiles and biology. A fingernail is about 1 to 1.5 cm², a coin 4 to 6 cm².
Where you see it
Product data sheets, medical dressings, school geometry and small printed items. For anything under a few square centimetres, many manufacturers prefer square millimetres.
The square millimetre
The area of a square of one millimetre
The square millimetre is one hundredth of a square centimetre and one millionth of a square metre. It measures wire cross sections, chip sizes, gaskets, small holes and the contact area of tiny parts. A pinhead is about 1 mm², a grain of table salt a fraction of that.
Where it appears
Electrical standards for cables, semiconductor industry data sheets, mechanical engineering and optics all use square millimetres, because the numbers are comfortable: 1.5 mm², 2.5 mm², 119 mm².
Square centimetre and square millimetre compared
A square centimetre holds a hundred square millimetres. The two units suit different scales, and in practice the choice depends on the audience: a package designer uses cm², a chip designer mm². Moving between them is just a shift of two decimal places.
The danger is not the arithmetic but the exponent. A reader who sees 300 mm² and 3 cm² must recognise that these are the same area, and a reader who sees 300 mm and 3 cm must know they are lengths. Writing the units clearly prevents misreadings.
Where the conversion appears
Memory cards and chips
An SD card has 768 mm², 7.68 cm². A microSD has 165 mm². The processor die in a modern laptop has an area of 100 to 200 mm², 1 to 2 cm². A large graphics chip of 628 mm² measures 6.28 cm². Data sheets use mm², while articles about chips are often in cm², so a “convert cm² to mm²” calculation appears in most reviews.
Electrical cables
Cable conductors are described by their cross section in square millimetres. A lighting cable of 1.5 mm² is 0.015 cm², a socket circuit of 2.5 mm² is 0.025 cm², and a 16 mm² feeder is 0.16 cm². The larger the section, the more current the cable carries safely.
Seals and gaskets
A rubber O ring of 3 mm cross section has an area of 7.07 mm², 0.0707 cm². Seals are described by their section diameter, while the contact area is calculated in square millimetres. Engineers who convert between the two get a feel for pressure on the seal.
Processor dies
The Apple M1 die is about 119 mm², 1.19 cm². A typical laptop chip has 100 to 150 mm², while a server processor may use 600 mm² or more. Manufacturers care about every square millimetre, because each wafer yields fewer chips as the die grows. A “convert cm² to mm²” calculation puts the quoted figure in the unit you prefer.
Rubber stamps and seals
A rubber stamp of 40 by 20 mm has 800 mm², 8 cm². A hand stamp for a cashier may be 30 by 12 mm. The printing area determines how much ink the pad holds, so craft suppliers list sizes in both millimetres and square centimetres.
Skin and tiny wounds
A small cut of 10 by 2 mm has an area of 20 mm², 0.2 cm². A blister of 1 cm² is 100 mm². Dermatologists measure lesions in millimetres for diameter and square millimetres for area, and they compare the change over time to follow healing.
Insects, seeds and tiny things
A mosquito wing is about 12 mm² in area. A seed of a poppy is 1 mm across, so a surface of 0.8 mm². A honey bee wing has about 80 mm², 0.8 cm². Natural scientists record such areas to study flight and dispersal, and use convert cm² to mm² when they compare results published in different units.
Gaskets and flat seals
A flat gasket of 50 by 50 mm with a central hole of 30 mm diameter has an area of 1,793 mm², 17.9 cm². Pressure is force divided by area, so the area in square millimetres lets you use pressure in megapascals directly. An engineer who has the area in cm² only needs to multiply by 100.
Rounding
The factor 100 is exact, so there is nothing to round. If the area came from a measurement with two significant figures, report the result with two: 0.35 cm² becomes 35 mm², not 35.0. Write the unit clearly after each number.
Spreadsheet use
Write cm² in column A and =A2*100 in column B. For the reverse, use =B2/100. Add a column showing the ratio between the two so you can spot rows where you entered the wrong unit.
Graph paper demonstration
On millimetre graph paper, draw a square of 10 by 10 millimetres. Count the small squares inside: exactly 100. That is a square centimetre in square millimetres, and the picture makes it obvious why the factor is a hundred and not ten.
Optics: lenses and apertures
A lens opening of 25 mm diameter has an area of 491 mm², 4.9 cm². A smaller aperture of 12 mm has 113 mm². Light gathering is proportional to area, so doubling the diameter quadruples the light. Optical specifications use mm², while datasheets for cameras list apertures in f numbers.
Adhesives and glued joints
The strength of a glued joint depends on its bonding area. A joint of 25 by 12 mm has 300 mm², 3 cm². If the glue holds 5 N per mm², the joint carries 1,500 N. Adhesive data sheets use N/mm², so engineers work in mm² and then convert to cm² when they compare with other joint areas.
Why the factor is 100
The reason is geometric. Along one side, the centimetre holds 10 millimetres. Along the other side, again 10. A square has two sides, so 10 times 10 equals 100. The same logic gives 10,000 for square metres in square centimetres and 1,000,000 for square metres in square millimetres. Once you understand this, you can derive any area factor from the length factor.
Graphics processors
A large graphics processor such as the GA102 has a die of 628 mm², 6.28 cm². Compared with an M1, it is more than five times larger. Prices rise steeply with area because one defect can ruin the whole chip. Reports about chip sizes mix both units, and a quick conversion makes comparisons fair.
Camera pixels and sensor sites
A modern phone camera sensor of 30 mm² carries 50 million pixels, so each photosite occupies about 0.0000006 mm², a figure that makes most people's heads spin. A full frame sensor of 864 mm² with 24 million pixels gives each pixel 36 square micrometres, or 0.000036 mm². Larger photosites collect more light, which is why bigger sensors cope better in dim rooms. Reviews quote sensor area in mm², while buyers think in cm², so a quick convert cm² to mm² is a useful habit.
Dental and medical devices
The head of a toothbrush has an area of about 3 cm², 300 mm², and the tip of a dental probe a fraction of a square millimetre. A stent of 3 mm diameter and 18 mm length has a surface area of about 170 mm², 1.7 cm². Medical engineers specify device surfaces in mm² because coatings, drug release and contact with tissue all depend on small areas, and they convert to cm² when they report totals to regulators.
Small print and fine detail
The letters in a newspaper are about 2 mm high, so a single character occupies roughly 2 to 3 mm². A page of text with 3,000 characters thus uses about 7.5 cm² of ink coverage, 750 mm². Typographers think in points and picas, printers in millimetres, and paper makers in square centimetres, which is why conversions between them show up in every production schedule.
Reading a datasheet without mistakes
A datasheet may list a die area of 1.19 cm² in one paragraph and a pad area of 120 mm² in the next. To compare them, convert both to the same unit with a “convert cm² to mm²” calculation before you reach any conclusion. Check that every exponent is written, that the decimal separator matches the language of the document, and that the figure belongs to the right component. Many errors come from copying a number from one table into another and forgetting that the heading changed from mm² to cm² somewhere in between.
SIM cards and memory cards
A full size SIM card of 25 by 15 mm has 375 mm², 3.75 cm². A micro SIM, 15 by 12 mm, has 180 mm², and a nano SIM 108 mm². Each generation reduced the area by about 40 per cent. Memory cards are similar: SD 768 mm², microSD 165 mm². The area of the card determines how many contacts and how much silicon it can carry.
Wire cross sections
A wire of 1 mm diameter has a cross section of 0.785 mm², 0.00785 cm². A wire of 2 mm has 3.14 mm². Resistance depends on the cross section: doubling the diameter quadruples the area and cuts the resistance to a quarter. Standard sizes in mm² make the choice of cable easy.
Mechanical stress and MPa
One newton per square millimetre is one megapascal. A force of 5,000 N on a bolt with a cross section of 20 mm² gives 250 MPa. In older documents stress may appear in N/cm², where 100 N/cm² is 1 N/mm². Engineers must convert the area before calculating stress, and a “convert cm² to mm²” calculation gives the first number.
Fuses, connectors and contacts
A USB A plug has a contact area of about 54 mm², 0.54 cm². The pins inside are a few square millimetres each. Contact area affects heat and resistance at the connection, so manufacturers specify these tiny surfaces in mm².
LED chips and sensors
A small LED die of 1 by 1 mm has 1 mm², 0.01 cm². A camera sensor of 6.4 by 4.8 mm has 30.7 mm², 0.307 cm². A full frame sensor of 36 by 24 mm has 864 mm², 8.64 cm². Photographers compare sensor areas to estimate light gathering ability.
Needles and pins
The eye of a sewing needle is about 1 to 2 mm². A syringe needle of 0.8 mm inner diameter has a bore of 0.5 mm², 0.005 cm². Medical devices are described by gauge and by cross section, and a conversion explains why a fine needle passes so little fluid.
Printed circuit tracks
A track of 0.2 mm width and 0.035 mm thickness has a cross section of 0.007 mm². A power track of 2 mm by 0.07 mm has 0.14 mm², 0.0014 cm². The capacity of a track to carry current depends on its section, so board designers calculate it in square millimetres.
More values for convert cm² to mm²
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 cm² = 10 mm²
- 0.5 cm² = 50 mm²
- 3 cm² = 300 mm²
- 4 cm² = 400 mm²
- 7 cm² = 700 mm²
- 15 cm² = 1,500 mm²
- 20 cm² = 2,000 mm²
- 75 cm² = 7,500 mm²
- 200 cm² = 20,000 mm²
- 500 cm² = 50,000 mm²
- 2,000 cm² = 200,000 mm²
- 10,000 cm² = 1,000,000 mm²
Mistakes when you convert cm² to mm²
Multiplying by ten
That is the linear factor. For areas, multiply by 100.
Shifting one decimal place
The decimal point moves two places. If you moved it only one, the answer is ten times too small.
Mixing units in one calculation
Do not add 5 cm² to 30 mm² without converting one of them first.
Quick answers
How many square millimetres in a square centimetre?
100.
How do I convert cm² to mm²?
Multiply by 100.
What is 2.5 cm² in mm²?
250 mm².
How do I go back from mm² to cm²?
Divide by 100.
Does my number leave my device?
No. The calculation runs in your browser.
Related pages and a last word
If you work with small areas, these conversions are close at hand: Convert cm² to in², Convert cm² to m², Convert in² to mm², Convert m² to mm², Convert mm² to m², Convert km to mi, Convert mi to km, Convert °C to °F, Convert °F to °C. The reverse of this page is Convert mm² to cm².
In short, to convert cm² to mm² multiply by 100, remember that the factor is the square of ten, and keep the digits your measurement supports. Your number stays on your device: nothing is uploaded and no account is needed.