The Quick Answer
Divide by 25.4. That's the entire formula:
inches = millimeters ÷ 25.4
One inch is defined as exactly 25.4 mm. This is not a rounded number — it's the international standard since 1959.
Quick examples:
- 10 mm = 0.3937" ≈ 25/64"
- 25 mm = 0.9843" ≈ 63/64" (close to 1 inch, but not exact)
- 50 mm = 1.9685" ≈ 1 31/32"
For instant conversions, use the millimeters to inches converter — it shows decimal, fractional, and nearest standard fraction.
Why 25.4?
The number 25.4 is not arbitrary. In 1959, six countries (US, UK, Canada, Australia, New Zealand, South Africa) signed the International Yard and Pound Agreement, which defined:
- 1 yard = 0.9144 meters (exactly)
- 1 inch = 25.4 millimeters (exactly)
Before this agreement, "one inch" had slightly different definitions in different countries. The 25.4 mm standard eliminated that confusion.
This means the conversion factor is exact — not an approximation. When you divide by 25.4, the only rounding happens in how many decimal places you display.
The Formula in Both Directions
| From → To | Formula | Example |
|---|---|---|
| mm → inches | inches = mm ÷ 25.4 | 10 mm ÷ 25.4 = 0.3937" |
| inches → mm | mm = inches × 25.4 | 0.5" × 25.4 = 12.7 mm |
Common Millimeters to Inches Reference
These are the values people look up most often:
| mm | Decimal Inches | Nearest Fraction | Practical Context |
|---|---|---|---|
| 1 | 0.0394" | ~1/32" | Paper, thin sheet metal |
| 2 | 0.0787" | ~5/64" | Pencil lead, wire gauge |
| 3 | 0.1181" | ~1/8" | Small screws, 3D printer nozzle |
| 5 | 0.1969" | ~13/64" | Common bolt/Allen key size |
| 6.35 | 0.2500" | ¼" (exact) | Quarter-inch reference |
| 8 | 0.3150" | ~5/16" | Common wrench/socket size |
| 10 | 0.3937" | ~25/64" | Common bolt size |
| 12.7 | 0.5000" | ½" (exact) | Half-inch reference |
| 15 | 0.5906" | ~19/32" | Pipe fittings |
| 18 | 0.7087" | ~45/64" | Plywood, MDF panels |
| 19.05 | 0.7500" | ¾" (exact) | Three-quarter inch reference |
| 20 | 0.7874" | ~25/32" | Pipe, bolts |
| 25 | 0.9843" | ~63/64" | Close to 1 inch |
| 25.4 | 1.0000" | 1" (exact) | Exact inch definition |
| 50 | 1.9685" | ~1 31/32" | Close to 2 inches |
| 100 | 3.9370" | ~3 15/16" | Close to 4 inches |
Notice a pattern: exact fractional inches only happen at specific mm values — 6.35 mm (¼"), 12.7 mm (½"), 19.05 mm (¾"), 25.4 mm (1"). Everything else requires rounding to the nearest fraction.
How Fractional Inches Work
Imperial measurements use fractions based on powers of 2. The standard increments are:
| Precision | Steps per inch | Smallest unit | Typical use |
|---|---|---|---|
| 1/8" | 8 | 3.175 mm | Rough carpentry |
| 1/16" | 16 | 1.588 mm | General DIY, hardware |
| 1/32" | 32 | 0.794 mm | Fine woodworking |
| 1/64" | 64 | 0.397 mm | Machining, precision work |
When you convert a metric value to inches, you choose a precision level and round to the nearest fraction at that level.
Example: Converting 7 mm
- Decimal: 7 ÷ 25.4 = 0.2756"
- To 1/16": 0.2756 × 16 = 4.41 → round to 4/16 = ¼" (error: 0.65 mm)
- To 1/32": 0.2756 × 32 = 8.82 → round to 9/32 = 9/32" (error: 0.15 mm)
- To 1/64": 0.2756 × 64 = 17.64 → round to 18/64 = 9/32" (same, error: 0.15 mm)
Higher precision doesn't always change the result. In this case, 1/32" and 1/64" give the same answer.
Practical Scenarios
Woodworking: Metric Plywood in Imperial Projects
Metric plywood comes in standard thicknesses of 3 mm, 6 mm, 9 mm, 12 mm, 15 mm, and 18 mm. These do not exactly match imperial sizes:
| Metric Plywood | Decimal Inches | Closest Imperial | Difference |
|---|---|---|---|
| 3 mm | 0.118" | 1/8" (0.125") | -0.18 mm |
| 6 mm | 0.236" | 1/4" (0.250") | -0.35 mm |
| 12 mm | 0.472" | 1/2" (0.500") | -0.70 mm |
| 18 mm | 0.709" | 3/4" (0.750") | -1.05 mm |
The differences matter for joinery. A dado cut sized for ¾" stock won't hold 18 mm plywood snugly — you'll have 1 mm of play. For tight joints, measure the actual sheet and cut to fit.
3D Printing: Nozzle and Filament Sizes
3D printing uses metric exclusively, but if you're designing parts that interface with imperial hardware:
| 3D Printing Term | mm | Inches |
|---|---|---|
| Standard nozzle | 0.4 mm | 0.016" |
| Layer height (fine) | 0.1 mm | 0.004" |
| Filament (standard) | 1.75 mm | 0.069" |
| Filament (thick) | 2.85 mm | 0.112" |
Lesson: keep 3D printing dimensions in metric. Convert only when mating with imperial parts.
Hardware: Metric Bolts vs. Imperial Drill Bits
When you need to drill a clearance hole for a metric bolt using an imperial drill bit:
| Bolt | Bolt Diameter | Clearance Hole | Nearest Imperial Drill |
|---|---|---|---|
| M3 | 3.0 mm | 3.4 mm | 9/64" (3.57 mm) |
| M4 | 4.0 mm | 4.5 mm | 3/16" (4.76 mm) |
| M5 | 5.0 mm | 5.5 mm | 7/32" (5.56 mm) |
| M6 | 6.0 mm | 6.6 mm | 17/64" (6.75 mm) |
| M8 | 8.0 mm | 9.0 mm | 23/64" (9.13 mm) |
These imperial drill sizes provide slightly oversized clearance holes, which is generally fine for through-bolting.
Common Mistakes
Mistake 1: Assuming 25 mm = 1 inch
25 mm = 0.9843 inches. You're short by 0.4 mm. For rough estimates this is fine. For anything involving fit or alignment, use 25.4 mm.
Mistake 2: Rounding too aggressively
Saying "10 mm ≈ 3/8 inch" loses 0.47 mm of accuracy. That's noticeable in hardware sizing. 10 mm is closer to 25/64" than to 3/8".
Mistake 3: Mixing nominal and actual sizes
Imperial lumber sizes are nominal: a "2×4" is actually 1.5" × 3.5" (38 mm × 89 mm). Don't convert the nominal name — convert the actual dimension.
Frequently Asked Questions
What is the formula to convert mm to inches?
Divide the millimeter value by 25.4. One inch equals exactly 25.4 millimeters. For example, 10 mm ÷ 25.4 = 0.3937 inches.
How many mm are in 1 inch?
Exactly 25.4 millimeters. This is a defined constant, not a rounded number, established by international agreement in 1959.
Is 10 mm the same as 3/8 inch?
Close but not exact. 10 mm = 0.3937", while 3/8" = 0.375". The difference is about 0.47 mm. 10 mm is slightly larger than 3/8 inch.
What is 25 mm in inches?
25 mm equals approximately 0.9843 inches. It's very close to 1 inch but not exact — you need 25.4 mm for exactly 1 inch.
How do I convert mm to fractional inches for woodworking?
First divide the mm value by 25.4 to get decimal inches. Then find the nearest fraction at your needed precision (1/16", 1/32", or 1/64"). For example, 6 mm = 0.2362" ≈ 15/64". Use the mm to inches converter for instant fractional results.
Why do metric and imperial sizes not match exactly?
Imperial fractions are based on powers of 2 (halves, quarters, eighths), while metric values are based on powers of 10. These systems have different mathematical foundations, so most metric values fall between imperial fractions rather than landing on one.
When should I use mm vs inches?
Use whichever system your project, supplier, or local standard specifies. Most of the world uses metric. The US, UK, and Canada commonly use inches for construction, hardware, and consumer products. When in doubt, check the specification or packaging.
What mm values convert to exact fractions?
Only multiples of 25.4 divided by a power of 2 produce exact imperial fractions. The most common: 6.35 mm = ¼", 12.7 mm = ½", 19.05 mm = ¾", 25.4 mm = 1". All other mm values require rounding to the nearest fraction.
Is this conversion different in different countries?
No. The 25.4 mm per inch definition is an international standard. The conversion is the same everywhere. However, note that the US uses a slightly different "survey inch" (≈ 25.400051 mm) for legacy land survey purposes — the difference is negligible for all practical work.
How accurate is a fractional inch conversion?
It depends on the fraction precision. At 1/16" precision, the maximum error is about 0.8 mm. At 1/32", about 0.4 mm. At 1/64", about 0.2 mm. For anything requiring tighter tolerance, use decimal inches or stay in metric.