Hardware Reference
Screw Size Conversion (mm to inches and gauge equivalents)
Convert screw diameters and lengths between metric and imperial, map gauge numbers to millimeters, and see M-size references. Use the examples to pick practical fractional lengths.
Conversion basics
inches = mm ÷ 25.4. For gauge screws, use the typical major diameter shown below as an approximation and always confirm thread pitch.
For fractional lengths, convert the mm value and round to the nearest available length (e.g., 1/2", 3/4", 1"). Note whether you prefer undersize or oversize rounding based on clearance or engagement depth.
Check fractions fast
Use the fraction converter to evaluate rounding error on lengths or diameters before committing to a fastener size.
Screw Size Quick Calculator
Major diameter
Based on typical standard values; manufacturer charts may vary.
Recommended tap drill
Clearance hole (close fit)
Heuristic allowance by size band; verify against process/spec for critical assemblies.
Length conversion & fraction
Screw size chart
Adjust precision to match your measuring tools and tolerance needs.
Imperial Gauge Screw Sizes
| Gauge | Inches | Fraction | Millimeters | Clearance holes (close/normal/loose) | Tap drill (UNC/UNF) |
|---|---|---|---|---|---|
| #4 | 0.1120" | 0" | 2.84 mm | Close: 0.1220" (3.10 mm) Normal: 0.1320" (3.35 mm) Loose: 0.1420" (3.61 mm) | UNC: 0.0870" (2.21 mm) UNF: 0.0912" (2.32 mm) |
| #6 | 0.1380" | 0" | 3.51 mm | Close: 0.1480" (3.76 mm) Normal: 0.1580" (4.01 mm) Loose: 0.1680" (4.27 mm) | UNC: 0.1068" (2.71 mm) UNF: 0.1130" (2.87 mm) |
| #8 | 0.1640" | 0" | 4.17 mm | Close: 0.1740" (4.42 mm) Normal: 0.1840" (4.67 mm) Loose: 0.1940" (4.93 mm) | UNC: 0.1328" (3.37 mm) UNF: 0.1362" (3.46 mm) |
| #10 | 0.1900" | 0" | 4.83 mm | Close: 0.2000" (5.08 mm) Normal: 0.2100" (5.33 mm) Loose: 0.2200" (5.59 mm) | UNC: 0.1483" (3.77 mm) UNF: 0.1588" (4.03 mm) |
| #12 | 0.2160" | 1/64" | 5.49 mm | Close: 0.2260" (5.74 mm) Normal: 0.2360" (5.99 mm) Loose: 0.2460" (6.25 mm) | UNC: 0.1743" (4.43 mm) UNF: 0.1803" (4.58 mm) |
| #14 | 0.2420" | 1/64" | 6.15 mm | Close: 0.2520" (6.40 mm) Normal: 0.2620" (6.65 mm) Loose: 0.2720" (6.91 mm) | UNC: 0.1920" (4.88 mm) UNF: 0.2003" (5.09 mm) |
Values are typical; verify with manufacturer data. Tap drill (imperial) uses a rule of thumb: major − (1/TPI).
Metric Screw Sizes
| Metric size | Millimeters | Inches | Fraction | Clearance holes (close/normal/loose) | Tap drill (coarse/fine) |
|---|---|---|---|---|---|
| M2 | 2.00 mm | 0.0787" | 0" | Close: 0.0866" (2.20 mm) Normal: 0.0945" (2.40 mm) Loose: 0.1024" (2.60 mm) | Coarse: 0.0630" (1.60 mm) Fine: 0.0689" (1.75 mm) |
| M3 | 3.00 mm | 0.1181" | 0" | Close: 0.1260" (3.20 mm) Normal: 0.1339" (3.40 mm) Loose: 0.1417" (3.60 mm) | Coarse: 0.0984" (2.50 mm) Fine: 0.1043" (2.65 mm) |
| M4 | 4.00 mm | 0.1575" | 0" | Close: 0.1654" (4.20 mm) Normal: 0.1732" (4.40 mm) Loose: 0.1811" (4.60 mm) | Coarse: 0.1299" (3.30 mm) Fine: 0.1378" (3.50 mm) |
| M5 | 5.00 mm | 0.1969" | 0" | Close: 0.2047" (5.20 mm) Normal: 0.2126" (5.40 mm) Loose: 0.2205" (5.60 mm) | Coarse: 0.1654" (4.20 mm) Fine: 0.1772" (4.50 mm) |
| M6 | 6.00 mm | 0.2362" | 1/64" | Close: 0.2441" (6.20 mm) Normal: 0.2520" (6.40 mm) Loose: 0.2598" (6.60 mm) | Coarse: 0.1969" (5.00 mm) Fine: 0.2067" (5.25 mm) |
| M8 | 8.00 mm | 0.3150" | 1/64" | Close: 0.3228" (8.20 mm) Normal: 0.3307" (8.40 mm) Loose: 0.3386" (8.60 mm) | Coarse: 0.2657" (6.75 mm) Fine: 0.2756" (7.00 mm) |
| M10 | 10.00 mm | 0.3937" | 1/64" | Close: 0.4016" (10.20 mm) Normal: 0.4094" (10.40 mm) Loose: 0.4173" (10.60 mm) | Coarse: 0.3346" (8.50 mm) Fine: 0.3445" (8.75 mm) |
| M12 | 12.00 mm | 0.4724" | 1/64" | Close: 0.4803" (12.20 mm) Normal: 0.4882" (12.40 mm) Loose: 0.4961" (12.60 mm) | Coarse: 0.4035" (10.25 mm) Fine: 0.4134" (10.50 mm) |
| M14 | 14.00 mm | 0.5512" | 1/64" | Close: 0.5591" (14.20 mm) Normal: 0.5669" (14.40 mm) Loose: 0.5748" (14.60 mm) | Coarse: 0.4724" (12.00 mm) Fine: 0.4921" (12.50 mm) |
| M16 | 16.00 mm | 0.6299" | 1/32" | Close: 0.6378" (16.20 mm) Normal: 0.6457" (16.40 mm) Loose: 0.6535" (16.60 mm) | Coarse: 0.5512" (14.00 mm) Fine: 0.5709" (14.50 mm) |
| M18 | 18.00 mm | 0.7087" | 1/32" | Close: 0.7165" (18.20 mm) Normal: 0.7244" (18.40 mm) Loose: 0.7323" (18.60 mm) | Coarse: 0.6102" (15.50 mm) Fine: 0.6299" (16.00 mm) |
| M20 | 20.00 mm | 0.7874" | 1/32" | Close: 0.7953" (20.20 mm) Normal: 0.8031" (20.40 mm) Loose: 0.8110" (20.60 mm) | Coarse: 0.6890" (17.50 mm) Fine: 0.7087" (18.00 mm) |
| M24 | 24.00 mm | 0.9449" | 1/32" | Close: 0.9528" (24.20 mm) Normal: 0.9606" (24.40 mm) Loose: 0.9685" (24.60 mm) | Coarse: 0.8268" (21.00 mm) Fine: 0.8661" (22.00 mm) |
Tap drill (metric) uses a common rule of thumb: major − pitch. For critical tolerances, consult ISO thread tables.
Thread series and pitch reference
Typical coarse/fine threads for common sizes. Verify against drawings and supplier charts for critical fits.
Imperial gauge (UNC/UNF)
| Gauge | UNC (TPI) | UNF (TPI) |
|---|---|---|
| #4 | 40 | 48 |
| #6 | 32 | 40 |
| #8 | 32 | 36 |
| #10 | 24 | 32 |
| #12 | 24 | 28 |
| #14 | 20 | 24 |
Metric (coarse/fine pitch)
| Size | Coarse (mm) | Fine (mm) |
|---|---|---|
| M2 | 0.40 | 0.25 |
| M3 | 0.50 | 0.35 |
| M4 | 0.70 | 0.50 |
| M5 | 0.80 | 0.50 |
| M6 | 1.00 | 0.75 |
| M8 | 1.25 | 1.00 |
| M10 | 1.50 | 1.25 |
| M12 | 1.75 | 1.50 |
| M14 | 2.00 | 1.50 |
| M16 | 2.00 | 1.50 |
| M18 | 2.50 | 2.00 |
| M20 | 2.50 | 2.00 |
| M24 | 3.00 | 2.00 |
For more tapping and drill selection, open the Tap Drill Calculator and Drill Size Conversion.
Length examples and rounding
Convert screw lengths from mm to inches and pick the nearest fractional length that is commonly stocked. Note whether a slightly shorter or longer screw is acceptable for your joint.
Fractions are approximations. Confirm actual stocked lengths and consider head height when space is limited.
Material and hole type tips
- Softwood and plastics: prefer loose clearance; tap drills slightly oversize to reduce splitting and stress.
- Aluminum: normal clearance is typically sufficient; coarse pitch speeds assembly and reduces risk of galling.
- Steel: close clearance improves alignment; fine pitch is useful for thin walls or higher thread engagement.
- Sheet metal: use close clearance or pre-punched holes; consider thread-forming screws instead of tapping thin sections.
- Countersunk vs. pan head: countersunk heads reduce protrusion; ensure countersink angle matches the fastener spec.
- Blind holes: favor undersize rounding on length to avoid bottoming; verify minimum thread engagement depth.
- Coatings/plating: add 0.1–0.2 mm (≈0.004–0.008") clearance for powder coat or zinc plating.
Guidance is general. Always validate against drawings, standards (ISO/ANSI), and supplier charts for critical tolerances.
Related tools & guides
Frequently Asked Questions
Specify screws with clarity
Convert diameters and lengths, then state the unit system, thread series, and tolerance on your drawings to prevent mix-ups in the field.