What does the mm to Inches Converter do?
The mm to Inches Converter turns any millimetre value into inches and back, instantly. Type a number in either field; the other updates as you type. The relationship is exact and was set in stone in 1959: 1 inch equals exactly 25.4 millimetres. Every conversion the tool produces is mathematically precise, not rounded.
Most useful when you're cross-referencing metric and imperial — TVs and monitors are quoted in inches but their spec sheets list mm; jewelry and watches mix both; drill-bit sets sometimes label one only; international shipping declares in mm; US construction uses inches. The conversion happens dozens of times a day in any cross-border manufacturing or engineering workflow.
When you'll use this conversion
The mm-to-inches conversion shows up in surprisingly many specific contexts. The most common:
Screen sizes. Monitors, TVs, phones, tablets, and laptops all quote their diagonal in inches (a "27-inch monitor"), but spec sheets often list dimensions in millimetres. A 27-inch screen is 685.8 mm diagonal. A 6-inch phone is 152.4 mm. Quick conversions help when comparing physical product dimensions to your desk or pocket space.
Drill bits and fasteners. Imperial drill bits (1/4", 3/8", etc.) and metric drill bits (6 mm, 8 mm) are interchangeable in many uses, but never quite identical. A 1/4-inch bit is 6.35 mm; a 6 mm bit is 0.236 inches. Knowing the conversion lets you sub one for the other when a hardware store is out of the size you wanted.
Jewelry and watches. Ring sizes, bracelet lengths, and watch case dimensions are often quoted in millimetres internationally. US jewelry sizing is more often in inches. A 16-inch necklace is 406 mm; a 42 mm watch case is 1.65 inches.
Print and paper. A4 paper is 210 × 297 mm; US Letter is 8.5 × 11 inches (216 × 279 mm). Designing layouts that work in both contexts requires knowing how millimetre design specs translate to inch print specs.
Construction and woodworking. US construction uses inches almost universally; European/Asian projects use millimetres. Cross-border architecture, furniture making, and DIY projects from international tutorials all need quick conversions.
3D printing. 3D printers spec in millimetres (most slicer software defaults to mm), but model files from US designers are often in inches. A 1-inch (25.4 mm) cube comes out wrong if you forget to scale.
The math (it's exact)
1 inch = 25.4 millimetres (exactly, by international agreement)
To convert mm → inches: divide by 25.4 (or multiply by 0.0394)
To convert inches → mm: multiply by 25.4
Worked example: a 6-inch phone screen.
- 6 in × 25.4 mm/in = 152.4 mm diagonal
- Reverse: 152.4 mm ÷ 25.4 = 6.000 in (exact)
The 25.4 mm/inch ratio is exact, set by the 1959 international yard and pound agreement and reaffirmed in 1996 by the International Bureau of Weights and Measures. Before 1959, the US and UK used slightly different inch definitions (~0.0001 mm apart), but the modern international inch is one number used everywhere.
Common drill bit conversions
The single most common mm-to-inch lookup. Imperial bits don't have exact metric equivalents — the closest metric size is usually within 0.5 mm:
| Imperial | Exact mm | Closest metric bit |
|---|---|---|
| 1/16 in | 1.59 mm | 1.5 mm or 2 mm |
| 1/8 in | 3.18 mm | 3 mm or 3.5 mm |
| 3/16 in | 4.76 mm | 5 mm |
| 1/4 in | 6.35 mm | 6 mm or 6.5 mm |
| 5/16 in | 7.94 mm | 8 mm |
| 3/8 in | 9.53 mm | 9.5 mm or 10 mm |
| 7/16 in | 11.11 mm | 11 mm |
| 1/2 in | 12.70 mm | 12.5 mm or 13 mm |
| 5/8 in | 15.88 mm | 16 mm |
| 3/4 in | 19.05 mm | 19 mm |
| 1 in | 25.40 mm | 25 mm or 26 mm |
For wood, the 0.5 mm difference between sizes is usually invisible — pick whichever bit you have. For metal or precision work, use the exact bit; the error compounds across multiple holes.
Common screen sizes
| Screen type | Diagonal (in) | Diagonal (mm) |
|---|---|---|
| Phone (compact) | 5.5 | 140 |
| Phone (standard) | 6.1 | 155 |
| Phone (large) | 6.7 | 170 |
| Tablet (mini) | 8.3 | 211 |
| Tablet (standard) | 10.9 | 277 |
| Tablet (pro) | 12.9 | 328 |
| Laptop (small) | 13 | 330 |
| Laptop (standard) | 15.6 | 396 |
| Monitor (24") | 24 | 610 |
| Monitor (27") | 27 | 686 |
| Monitor (32") | 32 | 813 |
| TV (55") | 55 | 1397 |
| TV (65") | 65 | 1651 |
All these dimensions are diagonal — the actual width and height are smaller, with the ratio depending on aspect ratio (16:9 for most modern screens). For a 27-inch 16:9 monitor, the actual width is about 597 mm and height 336 mm.
Tips
Tip: a quick mental conversion — 1 inch ≈ 25 mm. So 4 inches ≈ 100 mm, 8 inches ≈ 200 mm, 12 inches ≈ 300 mm. The 0.4 mm error per inch (1.6%) is well within rounding tolerance for most casual measurements. For precision work, use the converter.
Watch out for fractional inches. US measurements often quote fractional inches (3/8", 7/16", 1 1/4") rather than decimals. To convert a fraction to mm, first convert to a decimal: 3/8 = 0.375, then multiply by 25.4 = 9.525 mm. The converter accepts decimal inches but not fraction notation.
For paper sizes, mm is the international standard. A4 (210 × 297 mm), A5 (148 × 210 mm), Legal (216 × 356 mm). When designing print layouts, working in mm avoids the rounding issues that come from converting back and forth.
3D printing tip: if your model file is in inches and you need to print in mm, multiply every dimension by 25.4. A 4-inch model becomes a 101.6 mm print. Most slicer software has a unit-conversion option in the import dialog — use it rather than scaling after the fact.
Related tools
- For inch-to-foot conversions (heights, lumber lengths, room dimensions), use the Inches to Feet Converter.
- For broader length conversions (m, km, yd, mi), the Length Converter covers all common units.
- For aspect-ratio math when working with screens (e.g. "what's the height of a 27-inch 21:9 monitor?"), the Aspect Ratio Calculator handles diagonal/width/height relationships.
- For web/CSS unit conversions (rem, px, em), use the REM to Pixel Converter.
Frequently asked questions
How many mm in an inch?
Exactly 25.4 mm. The relationship was internationally standardized in 1959 and hasn't changed. So 2 in = 50.8 mm, 10 in = 254 mm, and 1 metre (1,000 mm) = 39.37 inches. The 25.4 number is exact, derived from defining 1 yard as exactly 0.9144 metres.
How many inches is 100 mm?
100 mm = 3.937 inches (about 4 inches). A useful rough conversion: every 25 mm is roughly 1 inch, so 100 mm ≈ 4 inches. The exact value is 3.9370 in. For most purposes, "about 4 inches" is precise enough.
What's a 7-inch screen in mm?
7 inches = 177.8 mm (diagonal). Common screen sizes for reference: phone 6-6.5 in (152-165 mm), tablet 9-12 in (229-305 mm), laptop 13-16 in (330-406 mm), monitor 24-32 in (610-813 mm), TV 55-65 in (1397-1651 mm). All measurements are diagonal — the actual width and height depend on aspect ratio.
What size in mm is a 1/4 inch drill bit?
1/4 inch = 6.35 mm exactly. Closest metric bits are 6 mm (slightly smaller) or 6.5 mm (slightly larger). For wood the 0.35 mm difference is usually invisible; for metal or precision work, use the exact 6.35 mm equivalent. Common imperial drill bit conversions: 1/8 in = 3.18 mm, 3/8 in = 9.53 mm, 1/2 in = 12.70 mm, 3/4 in = 19.05 mm.
How do I convert a fractional inch like 3/8" to mm?
Convert the fraction to decimal first: 3/8 = 0.375. Then multiply by 25.4: 0.375 × 25.4 = 9.525 mm. Other common fractions: 1/16 = 0.0625 (1.59 mm), 1/8 = 0.125 (3.18 mm), 1/4 = 0.25 (6.35 mm), 1/2 = 0.5 (12.70 mm), 3/4 = 0.75 (19.05 mm), 1 1/4 = 1.25 (31.75 mm).
Are millimetres and centimetres the same thing?
No — 1 cm = 10 mm. So 100 mm = 10 cm = ~3.94 inches. People sometimes mix the two up. If a recipe or spec lists "10 cm", that's 100 mm or about 4 inches. If it lists "10 mm", that's only 1 cm or about 0.4 inches. Easy mistake to make if you're rushing.
Is my input saved or sent anywhere?
No. The conversion runs entirely in your browser using JavaScript — nothing is sent to a server, logged, or stored.