Inches to Millimeters Converter
A darkroom print photographer hands a technician a rolled sheet and asks for an 11 by 14 reprint sized to fit a European matting frame. Within seconds the lab translates that 11 by 14 into 279.4 by 355.6 millimeters and pulls the correct matboard blank from the metric stockroom. That kind of hand-off runs through photographic printing, firearm parts sourcing, watch case machining, and any trade where American fractional-inch convention collides with European metric suppliers. Inches-to-millimeters is the precision conversion — the one where a third decimal place matters because tolerances stack up and a 0.2 mm error turns into a seized bearing or a lens that won't thread.
Calculator
1 × 25.4 = 25.4000
Formula
Multiply inches by 25.4 to land in millimeters — the relationship is exact since the 1959 treaty that defined the inch to four significant figures. For mental estimates, shift the decimal by 25 (a quarter of 100) and round up. A 5-inch feature scales to 5 × 25 = 125, then bump to 127 mm — within 1 percent of the 127.0 exact answer. Fraction conversions also follow cleanly: 1/4 inch is 6.35 mm, 1/8 is 3.175 mm, 1/16 is 1.5875 mm. The sixteen-per-inch common ruler thus ticks off in convenient 1.5875 mm steps.
Where You'll Use This
Photo print sizing lives and dies on this conversion. An 8x10-inch gallery print becomes 203.2 by 254 mm for mat-cutting, and a 16x20 jumps to 406.4 by 508 mm. Firearm handloaders buying European reamers calibrated in millimeters cross-reference their favorite 0.30-caliber chambers (7.62 mm) and 0.45 bore rounds (11.43 mm) in that direction routinely. Camera lens mount work converts 39 mm Leica thread as it relates to the M39 legacy (1.535 inch) and modern M42 Pentax (1.65 inch) mounts when adapter rings come up. Watch restoration dealing with an American-market Hamilton 982 caliber measures crown tube diameters in fractional inches and must order a replacement tube cut in millimeter stock. Woodworkers sourcing European router bits to drop into a 1/4-inch American collet note the exact 6.35 mm spec to avoid the 6 mm sloppy-fit trap. Even archery cross-compares 70-inch recurve limbs to the metric 1778 mm bow length stamped on a Korean-made target rig.
Reference Table
| From (Inches) | To (Millimeters) |
|---|---|
| 0.0625 | 1.5875 |
| 0.125 | 3.175 |
| 0.25 | 6.35 |
| 0.375 | 9.525 |
| 0.5 | 12.7 |
| 0.625 | 15.875 |
| 0.75 | 19.05 |
| 0.875 | 22.225 |
| 1 | 25.4 |
| 1.25 | 31.75 |
| 1.5 | 38.1 |
| 1.75 | 44.45 |
| 2 | 50.8 |
| 2.5 | 63.5 |
| 3 | 76.2 |
| 4 | 101.6 |
| 5 | 127 |
| 6 | 152.4 |
| 8 | 203.2 |
| 10 | 254 |
| 11 | 279.4 |
| 14 | 355.6 |
| 16 | 406.4 |
| 20 | 508 |
| 24 | 609.6 |
A Bit of History
Photographic paper sizes crystallized the inches-to-millimeter pipeline during the Kodak era when American sheet-film stock came in 8x10, 11x14, and 16x20 inch formats while European enlarger easels arrived pre-marked in millimeters. The resulting 25.4 cross-walk became second nature to professional labs long before the 1959 yard-and-pound accord made it a legal definition. Gunsmiths lived in the same bilingual world even earlier, working with 0.45-inch Colt rounds (11.43 mm) and 7.62 mm NATO (0.300 inch) on the same bench. The modern CAD era retired the fractional-inch ruler from most engineering offices, but North American hobby and restoration trades still list parts in inches and convert when a metric fastener is the only option.
FAQ
How many mm is a quarter inch?
A quarter inch equals 6.35 millimeters exactly. This figure shows up constantly in shop work because quarter-inch bits, quarter-inch shanks, and quarter-inch clearance holes are the American defaults, while European toolmakers more often produce 6 mm or 6.5 mm alternatives. The 0.35 mm gap between 6 mm and quarter-inch is large enough that a quarter-inch shank won't seat properly in a 6 mm collet.
What is 1/8 inch in mm?
One-eighth of an inch equals 3.175 millimeters. Common nearby metric sizes are 3 mm (0.1181 inch) and 3.5 mm (0.1378 inch) — neither lines up precisely with 1/8. Hobby model builders substituting metric plastic rod for American 1/8-inch stock need to trim or sand to fit when the part tolerance is tight, otherwise structural joints rock loose.
Why do camera lenses list filter threads in mm?
Photographic industry standards settled on millimeters decades before digital. A 52 mm, 58 mm, or 77 mm thread describes the front filter ring diameter, and the metric figure is authoritative. When adapters or step-up rings list an inch equivalent for American retail markets, the underlying engineered dimension is still metric, so always trust the millimeter number on the lens barrel itself.
What's a 1 inch thread in mm for plumbing?
A one-inch nominal pipe thread is not actually 25.4 mm outer diameter — nominal pipe sizing is a legacy naming convention. The true outside diameter of a 1-inch NPT pipe is 33.4 mm, and the thread pitch is 11.5 threads per inch. European pipe standards use BSP threads at slightly different dimensions, so cross-continental plumbing jobs often need adapter fittings rather than direct thread-matching.