Square millimeters to square inches (mm² to in²) converter

What is a square millimeters to square inches converter?

A square millimeters to square inches converter is an online tool that translates an area measured in square millimeters (mm²) into the equivalent area in square inches (in²), and back again. The square millimeter is a metric unit equal to the area of a square that is one millimeter on each side, while the square inch is an imperial and US customary unit equal to the area of a square that is one inch on each side. Because much of the world specifies small areas in millimeters while drawings, datasheets, and tooling in the United States and the United Kingdom still use inches, a quick way to move between the two is invaluable.

This converter is bidirectional: type a value into either field and the other updates automatically. It is well suited to gasket sizes, printed circuit board pads, cross-sections of small components, and any other situation where a tiny area needs to be expressed in the other unit system.

How it works

The conversion rests on a single fixed relationship between the inch and the millimeter. One inch is defined as exactly 25.4 millimeters. Squaring both sides gives the area relationship:

• 1 inch = 25.4 mm, so 1 in² = 25.4 × 25.4 = 645.16 mm².

That means every square inch contains exactly 645.16 square millimeters. To go from square millimeters to square inches you divide by 645.16; to go the other way you multiply by 645.16. Because the inch-to-millimeter definition is exact, this area factor is exact too, so there is no rounding hidden inside the conversion itself.

Formula

$A_{in^2} = \frac{A_{mm^2}}{645.16}$

$A_{mm^2} = A_{in^2} \times 645.16$

Square millimeters to square inches conversion table

The table below lists common square-millimeter values alongside their square-inch equivalents, rounded to five decimal places.

Examples

Example 1: One square inch in square millimeters

A datasheet lists a contact pad as 1 in². To express it metrically, multiply by 645.16:

$1 \text{ in}^2 \times 645.16 = 645.16 \text{ mm}^2$

So a single square inch equals exactly 645.16 square millimeters.

Example 2: 1000 mm² to square inches

A laser-cut metal tab has an area of 1000 mm². Divide by 645.16 to find its area in square inches:

$\frac{1000 \text{ mm}^2}{645.16} = 1.55000 \text{ in}^2$

The tab is about 1.55 square inches.

Example 3: 10 square inches to square millimeters

A label measures 10 in². Multiply by 645.16 to convert to square millimeters:

$10 \text{ in}^2 \times 645.16 = 6451.6 \text{ mm}^2$

The label covers 6451.6 square millimeters.

Example 4: Back to square millimeters

Take the 645.16 mm² from Example 1 and convert it back to confirm the round trip:

$\frac{645.16 \text{ mm}^2}{645.16} = 1 \text{ in}^2$

The value returns exactly to 1 in², showing the conversion is reversible without loss.

Notes

• The factor 1 in² = 645.16 mm² is exact because the inch is defined as exactly 25.4 mm.
• Square millimeters are convenient for very small areas; once a value grows past a few thousand square millimeters, square centimeters (cm²) often read more cleanly.
• Always square both the unit and any linear scale factor when converting areas — doubling a length quadruples the area.
• Entering 0 returns 0, and clearing both fields leaves them empty rather than showing a result.

Frequently asked questions

How many square millimeters are in one square inch?

There are exactly 645.16 square millimeters in one square inch, because one inch equals 25.4 millimeters and 25.4 × 25.4 = 645.16.

How do I convert square millimeters to square inches by hand?

Divide the number of square millimeters by 645.16. For example, 1000 mm² ÷ 645.16 ≈ 1.55 in².

Why is the area factor 645.16 and not 25.4?

25.4 is the factor for converting a single length from inches to millimeters. Area is two-dimensional, so you square that factor: 25.4² = 645.16.

Is the conversion exact or approximate?

It is exact. The inch-to-millimeter definition (25.4 mm) is a defined constant, so the area factor 645.16 carries no rounding error. Any rounding you see comes only from displaying the final result.

How many square inches is 100 mm²?

Divide 100 by 645.16 to get about 0.155 in², roughly one sixth of a square inch.

When would I use this converter?

It is handy for engineering drawings, machine parts, PCB layout, gaskets, and product specifications that mix metric and imperial units for small areas.