Thread Size Converter (Metric ↔ UNC/UNF)
Look up a metric or inch thread (M10, M8x1, 1/4-20, #10-32) — pitch, threads per inch, tap drill and the nearest size in the other system.
Thread
M10 × 1.5 (coarse)
- Major diameter
- 10 mm (0.3937 in)
- Pitch
- 1.5 mm = 16.93 TPI
- Tap drill
- 8.5 mm (≈ 0.3346 in)
- ISO 262 pitches for this size
- coarse 1.5 mm; fine 1.25 or 1 mm
Tap drills give about 75 % thread depth. Metric and inch threads of similar size do not interchange — the pitches differ and the threads strip.
Nearest inch (UNC/UNF/UNEF) threads — close in size, NOT interchangeable
| Thread | Major Ø | Pitch | Ø difference | Pitch difference |
|---|---|---|---|---|
| 3/8-16 UNC | 9.525 mm | 1.587 mm | −0.475 mm | +0.087 mm |
| 3/8-24 UNF | 9.525 mm | 1.058 mm | −0.475 mm | −0.442 mm |
| 3/8-32 UNEF | 9.525 mm | 0.794 mm | −0.475 mm | −0.706 mm |
| 7/16-20 UNF | 11.112 mm | 1.27 mm | +1.112 mm | −0.23 mm |
How it works
Pitch (mm) = 25.4 ÷ TPI. Metric tap drill ≈ major diameter − pitch (about 75 % thread). Inch tap drills are the preferred sizes from Machinery's Handbook; sizes it doesn't list use major diameter − 1/TPI. Nearest equivalents are ranked by diameter, then pitch.
Metric sizes and pitches: ISO 262 selected sizes (subset of ISO 261). Inch: Unified Thread Standard (ASME B1.1) UNC/UNF/UNEF threads per inch and preferred tap drills, Machinery's Handbook 29th ed. p. 1816 (as tabulated on Wikipedia). Numbered sizes: #N = 0.060 in + N × 0.013 in. Nearest equivalents are close in size only — metric and inch threads are not interchangeable.
Questions
- How does the thread size converter work?
- Pitch (mm) = 25.4 ÷ TPI. Metric tap drill ≈ major diameter − pitch (about 75 % thread). Inch tap drills are the preferred sizes from Machinery's Handbook; sizes it doesn't list use major diameter − 1/TPI. Nearest equivalents are ranked by diameter, then pitch.
- What does it assume?
- Metric sizes and pitches: ISO 262 selected sizes (subset of ISO 261). Inch: Unified Thread Standard (ASME B1.1) UNC/UNF/UNEF threads per inch and preferred tap drills, Machinery's Handbook 29th ed. p. 1816 (as tabulated on Wikipedia). Numbered sizes: #N = 0.060 in + N × 0.013 in. Nearest equivalents are close in size only — metric and inch threads are not interchangeable.
- Can I use a 1/4-20 bolt in an M6 hole?
- No. They are close in diameter (6.35 vs 6 mm) but the pitch differs (1.27 vs 1 mm), so the threads cross and strip.
- What does M10 mean without a pitch?
- The coarse pitch — 1.5 mm for M10. Fine pitches are written out, like M10x1.25.
Related tools
- Angular Velocity ConverterConvert rotational speed between radians per second, degrees per second and RPM.
- Acceleration ConverterConvert m/s², ft/s², standard gravity (g) and gal.
- Angular Acceleration ConverterConvert rad/s², degrees per second squared and revolutions per minute squared.
- Density ConverterConvert kg/m³, g/cm³, pounds per cubic foot, pounds per gallon and more.
- Specific Volume ConverterConvert volume per unit mass — m³/kg, L/kg, ft³/lb and more.
- Moment of Inertia ConverterConvert rotational inertia between kg·m², g·cm², lb·ft² and slug·ft².
Want this on your site? Embed the thread size converter (metric ↔ unc/unf).