Inches to MM — Convert Inches to Millimeters

To convert inches to millimeters, multiply by $25.4$:

Here $\text{in}$ is the length in inches and $\text{mm}$ is the same length in millimeters. The factor $25.4$ is exact because $1$ inch is defined as exactly $25.4$ millimeters, so $2.5$ inches is $63.5$ mm.

Why The Factor Is Exactly $25.4$

An inch is a U.S. customary unit and a millimeter is a metric unit, and the millimeter is much smaller, so the number grows when you rewrite the same length in millimeters. The factor traces back through centimeters. By definition,

and since $1\ \text{cm} = 10\ \text{mm}$, the millimeter factor is ten times larger:

That relation is fixed, so every inches-to-mm problem reuses the same multiplication; only the starting number changes. This is also why an answer that comes out too small by a factor of $10$ usually means $2.54$ was used by mistake.

Worked Example: $2.5$ Inches To MM

Start with the rule:

So

This case is worth noting because the answer is not a whole number. Decimal millimeter values are normal in measurements, drawings, and product dimensions.

Try It Yourself

Convert $3.75$ inches and $12$ inches to millimeters. Multiply each by $25.4$, then run the sanity check: the millimeter figure should be larger than the inch figure, since a millimeter is the smaller unit. (For reference, $3.75 \times 25.4 = 95.25$ and $12 \times 25.4 = 304.8$.)

Calculation Pitfalls

• Using $2.54$ instead of $25.4$. That factor converts inches to centimeters, not millimeters, and leaves the answer ten times too small.
• Dividing when you should multiply. Division by $25.4$ converts millimeters back to inches; a problem that starts in inches calls for multiplication.
• Expecting the number to shrink. Because a millimeter is smaller than an inch, the millimeter count should usually be larger than the inch count.
• Rounding too early. For machining, 3D printing, or specifications, keep the exact factor and round only at the end, to the precision the job requires.

Where Inches To Millimeters Is Used

This conversion turns up in hardware sizing, machining, 3D printing, construction drawings, and product specifications, anywhere one source gives inches and another expects metric units. The rule never changes across these cases; only the rounding does. A quick estimate may need one decimal place, while technical work may need the full decimal result.