Convert inches to mm
Convert inches to millimetres. 1 inch = 25.4 mm by definition.
inch
mm
25.4mm
History
Press Enter in any field to keep a snapshot here.
Formula
Formula: mm = inch × 25.4
Design notes
Turn component dimensions and panel cut-outs quoted in inches into millimetres. It applies to rack equipment ("1U = 1.75 inch"), display diagonals and plenty of other everyday cases.
When you need this
For bringing overseas datasheets and mechanical drawings given in inches into metric design data — connector mounting holes, chassis machining, rack dimensions.
Reading fractions
| Fraction | Decimal | mm |
|---|---|---|
| 1/32" | 0.03125" | 0.794 mm |
| 1/16" | 0.0625" | 1.588 mm |
| 1/8" | 0.125" | 3.175 mm |
| 3/16" | 0.1875" | 4.763 mm |
| 1/4" | 0.25" | 6.35 mm |
| 3/8" | 0.375" | 9.525 mm |
| 1/2" | 0.5" | 12.7 mm |
| 3/4" | 0.75" | 19.05 mm |
American drawings sometimes go down to 1/64" in fractions. Divide out the fraction, then multiply by 25.4.
Bringing drawings across
- Watch cumulative error on hole patterns. Rounding an inch-based pitch to millimetres accumulates along a row of holes. Convert absolute coordinates from a datum instead.
- Check thread specifications separately. Inch threads such as "#4-40" or "1/4-20" do not become M threads by conversion. Either source the inch fastener or change the mechanical design.
- Sheet gauge is not a direct dimension. American sheet metal is often specified by gauge number, which requires a lookup table rather than conversion.
Frequently asked questions
What does "#4-40" mean?
An inch machine screw designation: "#4" is the nominal diameter (about 2.84mm) and "40" is threads per inch. It is close to M3 in size but not interchangeable. #4-40 and #6-32 are common as connector jackscrews in electronics.
How do I read "1/4-20"?
A 1/4 inch (6.35mm) nominal diameter with 20 threads per inch, corresponding to UNC (coarse). This is the standard tripod thread.
Can I put converted values straight on a drawing?
If the part is defined in inches, the converted value will have trailing digits. Writing it as 6.35 (1/4") keeps the original intent visible so anyone reading later can judge the rounding.
Standards and references
- ANSI/ASME B1.1 — Unified inch screw threads.
- ISO 2768 — General tolerances.
Last updated: 2026-08-29