Hardware Reference
Screw Size Conversion (mm to inches and gauge equivalents)
Convert screw dimensions for comparison, then check pitch, thread family and fit. A nearby metric diameter is not an interchangeable imperial fastener.
Maintained by mmtoinches.im · Updated
NIST describes the standard inch as exactly equivalent to 25.4 mm
. NIST length definition. ASME — unified inch screw threads. Method and limitations.
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
No. Gauge numbers (#4, #6, #8) are imperial conventions with approximate diameters, while metric screws use the actual nominal diameter (e.g., M5 ≈ 5 mm). Pitch, head style, and threads differ.
Convert mm to inches (divide by 25.4) and round to the nearest stocked fractional length in your market. For tight fits, favor rounding down to avoid bottoming out; for clearance holes, rounding up is fine.
Avoid mixing systems. Even small pitch or diameter differences can strip threads or loosen joints. Match the thread form (metric vs UNC/UNF) and length to the specification.
For clearance holes, choose close, normal, or loose fit based on assembly tolerance and coating. For tap drills, use the tap drill chart: imperial rule of thumb is tap drill ≈ major diameter − (1/TPI), metric is ≈ major − pitch. Account for the specified coating thickness and hole tolerances, and verify against the applicable standard and supplier specifications. See our metric clearance hole chart and tap drill chart for specific sizes.
UNC (Unified Coarse) uses fewer threads per inch and a larger pitch—good for softer materials and quick assembly. UNF (Unified Fine) has more TPI and a smaller pitch for higher engagement in thin walls. When converting or specifying, match the thread series and consult a UNC/UNF thread pitch chart to avoid mismatches.
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.
Sources and calculation notes
Convert screw dimensions for comparison, then check pitch, thread family and fit. A nearby metric diameter is not an interchangeable imperial fastener.
Method: Divide millimeters by 25.4 to obtain inches. The factor is exact; displayed decimal values are rounded. Fractional inches are rounded to the selected denominator.
Scope: Tables are reference selections. Use the original thread designation and manufacturer documentation when choosing a replacement.
Content maintained by mmtoinches.im. Updated and reference notes reviewed . How we calculate and present measurements. Sources support the stated definitions; examples and calculators are not certification of a product or compliance with a standard.