Convert Inches to Millimeters
Convert inches into millimeter precision for drilling charts, fastener sizing, machining tolerances or specifying parts in metric.
1 in = 25.4 mm
One inch equals exactly 25.4 millimetres. To convert inches to millimetres, multiply by 25.4 — so 0.5 in = 12.7 mm and 4 in = 101.6 mm. Because the relationship is definitional, converted values are exact and can safely be used as machining dimensions rather than approximations. The thousandth of an inch, or thou, is the working unit in machining: one thou is exactly 0.0254 mm, so a tolerance of ±5 thou is ±0.127 mm. Screen and paper sizes convert the same way — a 15.6 in laptop display is 396.2 mm diagonally, and US Letter at 8.5 × 11 in is 215.9 × 279.4 mm. The rounding convention to watch is that engineering drawings usually specify one decimal place in millimetres, which corresponds to about four thou and is coarser than many imperial tolerances.
Quick reference: inches to millimeters
| Inches | Millimeters |
|---|---|
| 0.0625 | 1.5875 mm (1/16 in) |
| 0.125 | 3.175 mm (1/8 in) |
| 0.25 | 6.35 mm (1/4 in) |
| 0.375 | 9.525 mm (3/8 in) |
| 0.5 | 12.7 mm (1/2 in) |
| 1 | 25.4 mm (exact) |
| 1.75 | 44.45 mm (1U rack) |
| 4 | 101.6 mm |
| 19 | 482.6 mm (rack width) |
How do you convert inches to millimeters?
Formula: millimetres = inches × 25.4 • A 1/4 in hole → 0.25 × 25.4 = 6.35 mm, which is why a 6 mm bit leaves a quarter-inch bolt slightly tight and a 6.5 mm bit leaves it loose. • A 3/8 in hydraulic fitting → 0.375 × 25.4 = 9.525 mm, close to but not the same as a 9.5 mm metric equivalent. • A 19 in rack-mount width → 19 × 25.4 = 482.6 mm, the standard figure quoted in metric datacentre specifications.
When and why people use inches to millimeters
Converting drawings is the primary use. Legacy engineering drawings, US-sourced parts and imperial-standard fittings all need translating into millimetres before they can be modelled in CAD software that works metric by default, and the conversion has to be exact because downstream tolerances are tight. Rack and enclosure work is a specific and common case. The 19-inch rack standard governs servers, audio equipment and network gear worldwide, and every metric specification for cabinets, shelves and blanking plates is derived from converting inch dimensions. 1U of rack height is 1.75 in, which is 44.45 mm. The third case is fasteners and plumbing. Imperial pipe threads, wrench sizes and bolt diameters remain widespread even in metric countries, particularly in older buildings and in equipment imported from North America, so maintenance work routinely requires both systems side by side.
Common mistakes to watch for when converting inches to millimeters
Fraction handling causes most errors. 7/16 in must become 0.4375 before multiplying, and rounding it to 0.44 produces 11.18 mm instead of the correct 11.1125 mm. On a press fit that difference is the whole tolerance band. A second problem is assuming the nearest whole millimetre is an acceptable substitute — 6.35 mm rounded to 6 mm is a 5.5% reduction in hole diameter, which will not accept the bolt. A third is nominal pipe sizes, where a 'half-inch' pipe is not 12.7 mm in any measurable dimension; nominal pipe size is a label, not a measurement, and converting it produces a meaningless number. Finally, keep full precision through multi-step calculations and round only at the end.
How to estimate inches to millimeters in your head
Multiply by 25 and add 1.5%. For 4 inches: 4 × 25 = 100, plus 1.5 gives 101.5 mm against a true 101.6. For fractions, the fastest route is to memorise the sixteenths: 1/16 is 1.6 mm, 1/8 is 3.2 mm, 1/4 is 6.35 mm, 3/8 is 9.5 mm, 1/2 is 12.7 mm, 5/8 is 15.9 mm, 3/4 is 19.05 mm. Everything else is a sum of these — 7/16 is 1/4 plus 3/16, or 6.35 + 4.76 = 11.11 mm. A useful sanity check is that any inch figure multiplied by 25 gives a number about 1.5% too small, so if your mental answer and your calculator disagree by more than a couple of percent, you have made an arithmetic error rather than a rounding one.
Inches to Millimeters FAQ
How many mm is an inch?
- Exactly 25.4 mm. The figure is definitional, so converted values are exact.
How do I convert a fraction like 7/16?
- Convert to decimal first — 7 ÷ 16 = 0.4375 — then multiply by 25.4 to get 11.1125 mm. Rounding the fraction early loses the tolerance.
Can I use a 6 mm bit for a 1/4 inch bolt?
- A quarter inch is 6.35 mm, so a 6 mm hole is undersized. Use 6.5 mm for clearance or ream to 6.35 mm for a close fit.
How wide is a 19-inch rack in millimetres?
- 482.6 mm. One rack unit of height is 1.75 in, or 44.45 mm.
Is a half-inch pipe 12.7 mm?
- No. Nominal pipe size is a label rather than a measurement — no dimension of a half-inch pipe actually measures 12.7 mm.
Should I round converted dimensions to whole millimetres?
- Not on fits or holes. Rounding 6.35 mm to 6 mm removes 5.5% of the diameter and the fastener will not enter.
How thick is 1/8 inch material?
- 3.175 mm, which is why 3 mm sheet is a common but not identical substitute.
Is my input uploaded?
- No. Everything is computed inside your browser.
Last updated:
Sources: 1959 international yard and pound agreement; EIA-310 rack unit standard defining 1U as 1.75 in; ASME B1.20.1 pipe threads; ISO 261 metric screw threads.