Convert in to mm instantly using a precise conversion factor.
Multiply inches by 25.4. The calculator applies this relationship immediately and supports positive, negative, and decimal values where appropriate.
This conversion is commonly useful for fasteners, manufacturing, woodworking, and technical drawings.
| in | mm |
|---|---|
| 1 in | 25.4 mm |
| 5 in | 127 mm |
| 10 in | 254 mm |
| 25 in | 635 mm |
| 100 in | 2540 mm |
For 10 in, multiply 10 by 25.4. The result is 254 mm.
Multiply inches by 25.4.
Yes. Enter any decimal value and the result updates automatically.
Length units describe a single dimension. Metric length units scale by powers of ten, while U.S. customary and imperial units use relationships such as 12 inches per foot and 5,280 feet per mile. Convertly keeps the full conversion factor during calculation and rounds only the displayed answer. This avoids compounding rounding error when you enter a decimal or a large value.
For construction or manufacturing, match the displayed precision to the precision of the original measurement. A tape reading recorded to the nearest eighth of an inch does not become more accurate simply because a calculator displays many decimal places. For science and engineering work, retain extra digits during intermediate calculations and round the final result to the appropriate number of significant figures.
The calculation runs locally in your browser: the number you enter is multiplied or divided using the stated factor, and the result is formatted for readability. Convertly does not send calculator entries to a server. We preserve more precision internally than is normally useful on screen, so you can enter decimals without the calculator repeatedly rounding intermediate values.
Reviewed: September 3, 2026. Conversion relationships are checked against recognized measurement definitions, including the International System of Units and U.S. National Institute of Standards and Technology measurement guidance. Read our calculation methodology and source policy for definitions, rounding rules, limitations, and correction procedures.
Before converting, confirm that the input unit matches the value you have and that the output unit is the one required. Enter the unrounded source value whenever possible. After calculating, choose a sensible number of decimal places for the task rather than copying every digit automatically.
A useful cross-check is to estimate the direction and approximate size of the answer. Converting to a smaller unit usually produces a larger number; converting to a larger unit usually produces a smaller number. If the result differs greatly from that expectation, check for a misplaced decimal, a confused unit abbreviation, or the reverse conversion. The formula and worked example on this page are provided so the result can be independently checked.