Convert mm² to in² square millimetres ↔ square inches Free Online Secure
Type a value in square millimetres and read it in square inches 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 mm² to in² (square millimetres ↔ square inches)
| mm² | in² |
|---|---|
| 1 | 0.00155000310001 |
| 2 | 0.00310000620001 |
| 3 | 0.00465000930002 |
| 5 | 0.00775001550003 |
| 10 | 0.0155000310001 |
| 20 | 0.0310000620001 |
| 50 | 0.0775001550003 |
| 100 | 0.155000310001 |
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 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.
How many square millimetres is a square inch?
Exactly 645.16, or 6.4516 square centimetres. The figure follows from the inch being exactly 25.4 millimetres: squaring 25.4 gives 645.16. Square inches turn up in the pressure unit psi, in the sizes of screens and prints, and in small component specifications.
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.
Bolts, rebar and heel tips: small cross-sections in square inches
Engineers describe the cross-section of a bolt or a steel bar in square millimetres, yet a specification written in the United States gives the same part in square inches. To convert mm² to in² you multiply by 0.00155. An M10 bolt with a stress area of 58 mm² therefore has 0.0899 in², and a 12 mm reinforcing bar, 113 mm², has 0.175 in². This page multiplies as you type, and the text below walks through the places where tiny areas meet inch-based paperwork: fasteners, bars, tyres, connectors and a few household surprises.
You will find the rule with examples, a note on each unit, a comparison, a tour of workshop and site situations, and the slips that cause wrong orders. All arithmetic happens in your own browser; nothing is sent anywhere.
How to convert mm² to in²: multiply by 0.00155
The rule
One square millimetre equals 0.0015500031 square inches. To convert mm² to in², multiply the square millimetres by 0.00155. So 200 mm² is 0.31 in², 500 mm² is 0.775 in², 1,000 mm² is 1.55 in² and 2,500 mm² is 3.875 in². Going back, multiply square inches by 645.16, because a square inch holds exactly 645.16 square millimetres.
Worked examples
A 3 mm by 20 mm copper strip has 60 mm², or 0.093 in². A 6 mm drilled hole has 28.3 mm², or 0.0438 in². A 150 mm by 100 mm panel has 15,000 mm², or 23.25 in². If you only need a rough figure, divide by 650: 6,500 mm² comes to about 10 in², against 10.08 exactly.
Why not 0.0394
A millimetre is 0.0394 inches, but an area multiplies two lengths, so the factor is 0.0394 squared, which is 0.00155. Using the length factor on an area leaves you with an answer about 25 times too large.
A fast sanity test
A square inch is the bigger unit, so the figure should drop by roughly a factor of 650. If it only dropped by 25, you used the linear factor.
The square millimetre
One millimetre by one millimetre
The square millimetre is a millionth of a square metre and a hundredth of a square centimetre. It is the working unit for cross-sections, contacts, nozzle openings and small seals. A pinhead covers about 1 mm².
Where it appears
Fastener tables, cable and bar standards, mechanical drawings, tensile test reports and component data sheets.
The square inch
One inch by one inch
The square inch is 6.4516 cm², exactly 645.16 mm². It is the everyday small area unit in the United States, and it is hidden inside the pound per square inch used for tyres, hydraulics and gas cylinders.
Where it appears
American drawings and bolt tables, steel handbooks, tool catalogues and pressure ratings.
Square millimetre and square inch compared
A square inch holds about 645 square millimetres, so a figure in the metric unit looks hundreds of times bigger than the same area in the imperial one. A 58 mm² bolt and a 0.0899 in² bolt are the same bolt. Anyone reading a European data sheet next to an American handbook has to bridge that gap.
The factor is not round, so mental estimates are only approximate: dividing by 650 is good to within a fraction of a percent. For exact work, use 0.0015500031 in one direction and 645.16 in the other.
Where the conversion appears
Bolts and threaded fasteners
Strength tables for bolts use the tensile stress area, not the plain shank. An M8 has 36.6 mm², 0.0567 in². An M10 has 58 mm², 0.0899 in². An M12 has 84.3 mm², 0.131 in². An M16 has 157 mm², 0.243 in². On the inch side, a 3/8-16 bolt has 0.0775 in², 50 mm², and a 1/2-13 has 0.1419 in², 91.5 mm². When you convert mm² to in² for a substitute part, this is the figure that matters.
Reinforcing bars
A 10 mm bar has 78.5 mm², 0.122 in². A 12 mm bar has 113 mm², 0.175 in². A 16 mm bar has 201 mm², 0.312 in². American bar sizes run the other way: a No. 4 bar has about 0.20 in², roughly 129 mm², and a No. 5 has 0.31 in², around 200 mm². Designers use the area to pick the nearest equivalent size.
Everyday items
A USB-C opening of 8.4 by 2.6 mm is about 21.8 mm², 0.034 in². A 19 by 72 mm adhesive strip has 1,368 mm², 2.12 in². A sheet of sandpaper 230 by 280 mm has 64,400 mm², 99.8 in², almost exactly a 9 by 11 inch sheet.
Why fastener tables use stress area
A threaded bolt is weaker than its plain shank, because the thread cuts material away. Standards therefore define a tensile stress area somewhere between the minor and the major diameter. Strength is then force divided by that area. A grade 8.8 M10 bolt, with a nominal tensile strength of 800 MPa and 58 mm², breaks at about 46.4 kN, roughly 10,400 pounds. A catalogue that quotes 0.0899 in² for the same bolt expects you to apply the same logic, and if you convert mm² to in² first, the two tables agree.
Processor heat spreaders
The metal lid on a desktop processor is often about 35 by 35 mm, giving 1,225 mm² or 1.9 in². Cooler makers publish the contact plate area of their heat sinks, and manuals from American vendors do so in square inches. Heat flow through the interface grows with area, so a plate of 2.5 in², 1,613 mm², covers the whole lid with a margin, while a 1.5 in² plate, 968 mm², leaves the corners uncovered. A quick conversion decides which cooler fits.
Abrasives and sanding sheets
A European sandpaper sheet is 230 by 280 mm, which is 64,400 mm², or 99.8 in². An American sheet of 9 by 11 inches has 99 in², 63,871 mm². The sizes are nearly identical, which is no accident, and a sheet of one fits the holder of the other. Per-sheet prices can be compared once you convert mm² to in² for each size: a pack of 25 European sheets covers about 1.6 m², and a pack of 25 American sheets about 1.6 m² too.
Sticky notes and labels
The American three by three inch note covers exactly 9 in², or 5,806 mm². The European 76 by 76 mm note covers 5,776 mm², 8.95 in². The difference is half a percent, so you can swap one for the other. Labels are less forgiving: a 50 by 25 mm shipping label has 1,250 mm², 1.94 in², and a 2 by 1 inch label has 2 in², 1,290 mm². If a printer template is set for one, the other is clipped or loose, and area comparison helps you pick the closest stock.
Busbars and flat conductors
A copper busbar 20 mm wide and 3 mm thick has a cross-section of 60 mm², 0.093 in². A bar 1 inch by 1/8 inch has 0.125 in², 80.6 mm². Current ratings depend on area, material, cooling and ambient temperature, so the figure has to be read from the manufacturer's table, not worked out from area alone. Still, the area is the first thing an installer compares when swapping a metric bar for an inch-sized one, and it is the number to check after you convert mm² to in² and before you rely on any table.
In a spreadsheet
Enter the square millimetres in column A and use =A2*0.0015500031 in column B. For the reverse direction, B2*645.16 gives the area in square millimetres. A third column with =B2/C2, where C2 is the area of one part, gives how many parts fit in a given space. Format the result with three or four decimals for small areas, because 0.0438 in² and 0.04 in² are very different when you are checking a hole size.
How many digits to keep
Match the precision of the source. A stress area quoted as 58 mm² has two significant figures, so 0.090 in² is enough, while a catalogue value of 58.0 mm² supports 0.0899 in². Do not round the factor itself: 0.00155 is fine for estimates, but a long chain of operations is better done with 0.0015500031. In a report, state the unit after every number and mention the factor you used, so a reader can retrace the work.
Dual-dimensioned drawings
Some drawings print both metric and inch values on every dimension. Areas are rarely dimensioned directly, but they appear in notes such as minimum flow area, cross-section of a weld or contact patch. If the two printed values disagree, check which one is the master. A good practice is to write the master value in the system the part is made in and treat the second as reference. Whoever converts a figure should say which one they started from.
Tolerances and rounding
When a tolerance of plus or minus 5 percent is allowed on a cross-section, rounding errors of 0.1 percent do not matter, but they pile up if you convert, round, convert back and round again. Keep the original figure in its source unit, derive the other unit once and never use the derived value as a new input. For a 36.6 mm² thread, that means keeping 36.6, not 0.0567, as the number you work from, and converting again whenever the other unit is needed.
A mental shortcut
Because 645 is close to 650, you can estimate in your head by dividing by 650, or by multiplying by 1.55 per thousand. Take 4,000 mm²: four thousands give 6.2 in². Take 12,000: 12 thousands give 18.6 in². The error is below one percent, which is enough for sizing a bracket or a label. For anything that goes into a specification, convert mm² to in² with the converter above, which carries the full factor of 0.0015500031 and shows enough decimals.
Reference values
50 mm² is 0.0775 in², 100 mm² is 0.155 in², 250 mm² is 0.3875 in², 500 mm² is 0.775 in², 645 mm² is about 1 in², 1,000 mm² is 1.55 in², 2,500 mm² is 3.875 in² and 5,000 mm² is 7.75 in². These anchors let you spot a wrong digit in seconds. When you convert mm² to in² and land far from this ladder, repeat the multiplication before trusting the result.
From newtons per square millimetre to psi
A stress of 1 N/mm² equals 1 MPa, which is 145 psi. Because pressure is force over area, the area conversion and the pressure conversion go hand in hand. A 20 kN pull on a 37.5 mm² test strip gives 533 MPa, about 77,000 psi. If you instead convert the area first, 37.5 mm² is 0.058 in², and 4,496 pounds of force over 0.058 in² gives the same 77,000 psi. Checking both routes catches errors, which is a habit worth keeping whenever you convert mm² to in².
Procurement and substitution
Buyers who source from both sides of the Atlantic compare catalogue pages with different units. A European supplier lists a spring with a wire cross-section of 4.5 mm², while an American one lists 0.007 in². Converting the first gives 0.00698 in², essentially the same part. Building a small conversion sheet for the parts you buy most saves time, and the sheet remains valid for years because the factor never changes.
Tensile test coupons
A flat test coupon 12.5 mm wide and 3 mm thick has a cross-section of 37.5 mm², or 0.058 in². The laboratory pulls it in a machine and reports the stress by dividing the load by that original area. American standards define coupons in inches, so a half-inch wide, 0.12 inch thick sample has 0.06 in², close to 38.7 mm². Labs that swap reports with partners abroad keep both columns in the results table, and the area conversion is where the two sets of numbers meet.
The weight on a stiletto heel
A heel tip of 10 by 10 mm covers 100 mm², which is 0.155 in². Put 60 kg of body weight on one tip and the force is about 589 N, so the pressure is 5.9 N/mm², about 5.9 MPa or 854 psi. This is higher than the pressure inside many car tyres by a factor of more than twenty. The comparison always draws a gasp, and it works because the contact area is tiny. Once you convert mm² to in² for the tip, you can compare it with any imperial pressure figure.
The patch under a bicycle tyre
Pressure equals force divided by area, so area equals force divided by pressure. A bicycle wheel carrying 40 kg presses down with about 392 N. At 100 psi, which is 690 kPa, the tyre touches the road over roughly 570 mm², or 0.88 in². A mountain bike tyre at 25 psi under the same load touches over about four times that area. Cyclists in Europe quote pressure in bar and the patch in square millimetres; Americans quote psi and square inches, and the conversion ties the two accounts together.
Drilled holes and clearance
A 6 mm hole has an area of 28.3 mm², 0.0438 in². A quarter-inch hole has 31.7 mm², 0.0491 in². The two are close but not equal, and the quarter-inch hole passes about 12 percent more air, water or wire. For ventilation openings, cable glands and vents, designers sometimes specify total open area, not diameter. When a drawing is transferred between metric and imperial shops, comparing the hole areas shows whether the substituted size is acceptable.
O-rings and seals
An O-ring with a 3 mm cord has a cross-section of 7.07 mm², 0.0110 in². The common American cord of 0.139 inch has 0.0152 in², about 9.8 mm². Squeeze, fill ratio and gland volume are all calculated from the cross-sectional area. A seal designer who has to replace a metric ring with an inch one compares the two areas first, then the diameter, because a ring with the wrong cord section will leak even when the bore fits.
Nozzles and small orifices
A 3D printer nozzle with a 0.4 mm bore has an opening of 0.126 mm², which is 0.000195 in². A 4 mm sprinkler orifice has 12.6 mm², 0.0195 in². Flow through an opening grows with its area, so a small change in diameter has a large effect: a nozzle only 10 percent wider passes about 21 percent more. When you read a vendor sheet in inches, the quick way to convert mm² to in² for comparison is to convert the area, not the diameter.
Connector openings
A USB-C receptacle opening is about 8.4 by 2.6 mm, roughly 21.8 mm² or 0.034 in². A USB-A plug measures about 12 by 4.5 mm, 54 mm², 0.084 in². Panel cut-outs for these ports are drawn in millimetres, while enclosure suppliers in North America list punch sizes in inches. Area is only a rough guide, since the shape matters as well, but a quick comparison of the two figures shows at a glance whether a standard punch will clear the connector.
More values for convert mm² to in²
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 mm² = 0.000155 in²
- 0.5 mm² = 0.000775002 in²
- 3 mm² = 0.00465001 in²
- 4 mm² = 0.00620001 in²
- 7 mm² = 0.01085 in²
- 15 mm² = 0.02325 in²
- 20 mm² = 0.0310001 in²
- 75 mm² = 0.11625 in²
- 200 mm² = 0.310001 in²
- 500 mm² = 0.775002 in²
- 2,000 mm² = 3.10001 in²
- 10,000 mm² = 15.5 in²
Mistakes when you convert mm² to in²
Using 0.0394
That is the linear factor. For areas, multiply by 0.00155.
Multiplying where you should divide
The square inch is the large unit, so the number shrinks. If 1,000 mm² came out as 645,160, you used the reverse factor.
Losing the exponent
in and in² are different units. Check the exponent on every figure copied from a table, because a lost square changes the meaning completely.
Quick answers
How many square inches in one square millimetre?
0.00155.
How do I convert mm² to in²?
Multiply by 0.00155.
How many square millimetres in a square inch?
645.16.
What is 1,000 mm² in square inches?
1.55 in².
Is the number I type uploaded?
No, it stays in your browser.
Related pages and a final word
If you work with small areas, these conversions sit nearby: Convert mm² to m², Convert mm² to cm², Convert ft² to in², Convert cm² to in², Convert in² to cm², Convert km to mi, Convert mi to km, Convert °C to °F, Convert °F to °C. The opposite direction of this page is Convert in² to mm².
In short: to convert mm² to in², multiply by 0.00155, remember that this factor is the square of 0.0394, and write the unit next to the result. The value you type is handled on your own device, with no account and no upload.