The Challenge
Millimeters and inches collide hardest in precision work. A drill bit set is labeled in fractional inches; the technical drawing calls out 6.35mm. A bolt is M8 — the nearest inch equivalent is 5/16", not a direct conversion. Camera sensor sizes are given in millimeters; lens filter threads too. 3D printing files from European designers use mm; US makers think in inches. The exact conversion: 1 inch = 25.4mm, so divide millimeters by 25.4 to get inches. For mental math, divide by 25 — error is under 1.6%. Common references: 25.4mm = 1 inch exactly, 12.7mm = 0.5 inch, 6.35mm = 0.25 inch, 304.8mm = 12 inches (1 foot).
Millimeters to Inches Conversion Chart
| Millimeters (mm) | Decimal Inches | Fractional Inches |
|---|---|---|
| 0.5 mm | 0.0197 in | 1/64 in |
| 1 mm | 0.0394 in | 3/64 in |
| 1.5 mm | 0.0591 in | 1/16 in (approx) |
| 2 mm | 0.0787 in | 5/64 in |
| 2.5 mm | 0.0984 in | 3/32 in (approx) |
| 3 mm | 0.1181 in | 7/64 in |
| 4 mm | 0.1575 in | 5/32 in |
| 5 mm | 0.1969 in | 13/64 in |
| 6 mm | 0.2362 in | 15/64 in |
| 6.35 mm | 0.2500 in | 1/4 in (exact) |
| 7 mm | 0.2756 in | 9/32 in |
| 8 mm | 0.3150 in | 5/16 in |
| 9 mm | 0.3543 in | 23/64 in |
| 10 mm | 0.3937 in | 25/64 in |
| 12 mm | 0.4724 in | 15/32 in |
| 12.7 mm | 0.5000 in | 1/2 in (exact) |
| 15 mm | 0.5906 in | 19/32 in |
| 16 mm | 0.6299 in | 5/8 in |
| 19.05 mm | 0.7500 in | 3/4 in (exact) |
| 20 mm | 0.7874 in | 25/32 in |
| 25 mm | 0.9843 in | 63/64 in |
| 25.4 mm | 1.0000 in | 1 in (exact) |
| 30 mm | 1.1811 in | 1 3/16 in |
| 40 mm | 1.5748 in | 1 37/64 in |
| 50 mm | 1.9685 in | 1 31/32 in |
| 75 mm | 2.9528 in | 2 61/64 in |
| 100 mm | 3.9370 in | 3 15/16 in |
| 150 mm | 5.9055 in | 5 29/32 in |
| 200 mm | 7.8740 in | 7 7/8 in |
| 300 mm | 11.8110 in | 11 13/16 in |
| 304.8 mm | 12.0000 in | 12 in (1 ft, exact) |
Standard Drill Bit Sizes: mm to Fractional Inches
| Metric (mm) | Decimal Inches | Nearest Fractional Inch | Difference |
|---|---|---|---|
| 1.0 mm | 0.0394 in | 3/64 in (0.0469 in) | +0.0075 in |
| 1.5 mm | 0.0591 in | 1/16 in (0.0625 in) | +0.0034 in |
| 2.0 mm | 0.0787 in | 5/64 in (0.0781 in) | -0.0006 in |
| 2.5 mm | 0.0984 in | 3/32 in (0.0938 in) | -0.0047 in |
| 3.0 mm | 0.1181 in | 1/8 in (0.1250 in) | +0.0069 in |
| 3.5 mm | 0.1378 in | 9/64 in (0.1406 in) | +0.0028 in |
| 4.0 mm | 0.1575 in | 5/32 in (0.1563 in) | -0.0013 in |
| 4.5 mm | 0.1772 in | 11/64 in (0.1719 in) | -0.0053 in |
| 5.0 mm | 0.1969 in | 13/64 in (0.2031 in) | +0.0063 in |
| 6.0 mm | 0.2362 in | 15/64 in (0.2344 in) | -0.0018 in |
| 8.0 mm | 0.3150 in | 5/16 in (0.3125 in) | -0.0025 in |
| 10.0 mm | 0.3937 in | 25/64 in (0.3906 in) | -0.0031 in |
Why Millimeters, Not Centimeters, Dominate Engineering
In technical drawings, manufacturing, and machining, millimeters are the preferred metric unit — not centimeters. The reason is practical: using millimeters eliminates decimal points in most everyday dimensions. A shaft diameter written as 25mm is cleaner than 2.5cm and less prone to transcription error than 0.025m. ISO 129, the international standard for technical drawing notation, recommends millimeters as the default linear unit. This is why CNC machine files, CAD software defaults, and precision tool catalogs all use mm even in metric countries. When converting for engineering use, always convert to mm — not cm — to stay compatible with technical documentation.
Converting mm to Fractional Inches for Machining
- Divide the mm value by 25.4 to get decimal inches — example: 9.525mm ÷ 25.4 = 0.375 inches
- Multiply the decimal portion by 64 to find the nearest 64th — 0.375 × 64 = 24, so 24/64 = 3/8 inch
- Check the difference: 3/8 inch = 9.525mm exactly in this case — a perfect match
- For tolerances, keep the decimal value — rounding to a fraction introduces up to 0.2mm error at 64ths resolution
When to Use Decimal vs Fractional Inch Equivalents
- Use decimal inches (0.3937 in) for CNC programming, metrology, and any tolerance-critical work — fractions round and compound error
- Use fractional inches (25/64 in) when selecting standard off-the-shelf tooling, fasteners, or hardware where nominal sizes are fractional
- Use millimeters directly when working with metric-standard components — converting to inches and back introduces unnecessary rounding
- Always specify the unit explicitly on shared drawings — a dimension of '10' is ambiguous; '10mm' or '0.394 in' is not
Step-by-Step Workflow
Enter your millimeter value in the input field
Inches result appears instantly — decimal and fractional
Click swap to reverse and convert inches back to mm
Specifications
- Formula
- inches = millimeters ÷ 25.4
- 1 mm equals
- 0.039370 inches
- 1 inch equals
- 25.4 mm (exact, defined)
- Quick estimate
- divide by 25 (1.6% error)
- 10 mm
- 0.3937 inches
- 25.4 mm
- 1.000 inch (exact)
- 304.8 mm
- 12 inches (1 foot)
Best Practices
- Divide by 25.4 for exact result — 25.4 is a defined constant, not a rounded figure
- Fractional inch equivalents: 1/16in=1.5875mm, 1/8in=3.175mm, 1/4in=6.35mm, 1/2in=12.7mm
- Drill sizes: 3mm≈7/64in, 4mm≈5/32in, 5mm≈13/64in, 6mm≈15/64in, 8mm≈5/16in
- Pipe threads: M10=10mm OD, nearest NPT is 1/4in pipe (13.72mm OD) — not interchangeable
- Photography: full-frame sensor is 36×24mm = 1.417×0.945 inches
Frequently Asked Questions
How many inches is 1 millimeter?
1 millimeter equals exactly 0.0393700787 inches. Since 1 inch is defined as exactly 25.4mm, 1mm = 1/25.4 inches. For practical use, 1mm ≈ 0.0394 inches, or roughly 3/64 of an inch.
How do I convert mm to inches without a calculator?
Divide by 25.4 for exact results, or divide by 25 for a quick estimate (1.6% error). Examples: 50mm ÷ 25 = 2 inches (exact: 1.9685in). 100mm ÷ 25 = 4 inches (exact: 3.937in). Another option: multiply by 0.04 — this gives 2% error but is easy to do mentally.
What is 10mm in inches?
10mm = 10 ÷ 25.4 = 0.3937 inches, which is just under 13/32 inch. In fractional terms, the nearest standard fractions are 3/8 inch (9.525mm) and 13/32 inch (10.319mm). For machining tolerances, use the decimal 0.3937 rather than rounding to a fraction.
What is 25mm in inches?
25mm = 25 ÷ 25.4 = 0.9843 inches. This is just under 1 inch — 1 inch is 25.4mm, so 25mm falls 0.4mm short. In fractional inches it is 63/64 inch. When shopping for US inch-size fittings, a nominal 1-inch fitting will be slightly larger than 25mm metric.
How do metric bolt sizes convert to fractional inches?
Metric bolt OD: M6=6mm=0.236in≈1/4in, M8=8mm=0.315in≈5/16in, M10=10mm=0.394in≈3/8in, M12=12mm=0.472in≈1/2in, M14=14mm=0.551in≈9/16in, M16=16mm=0.630in≈5/8in. These are outer diameter references only — thread pitch differs from UNC/UNF standards, so inch and metric bolts are not interchangeable even when OD is close.
What common tool sizes are in both mm and inches?
Hex keys: 1/16in=1.587mm, 5/64in=1.984mm, 3/32in=2.381mm, 1/8in=3.175mm, 5/32in=3.969mm, 3/16in=4.762mm, 1/4in=6.35mm. Spanner/socket: 8mm≈5/16in, 10mm≈3/8in, 13mm≈1/2in, 17mm≈11/16in, 19mm=3/4in (exact for automotive). 19mm and 3/4in are one of the few near-exact crossovers at 0.4% difference.
How are camera lens filter sizes in mm converted to inches?
Lens filter threads are always in millimeters and do not map to inch sizes. Common thread diameters: 49mm=1.929in, 52mm=2.047in, 58mm=2.283in, 67mm=2.638in, 77mm=3.031in, 82mm=3.228in. These are thread diameters — you cannot substitute an inch-threaded filter on a metric lens mount.