Metric vs. Imperial Fastener Conversion Chart
Metric vs. Imperial Fastener Conversion Chart
Mutual Screw & Supply · mutualscrew.com · (800) 222-0324
Important: Metric and imperial fasteners are not interchangeable even when diameters are close. Thread pitch, thread angle, and fit tolerance all differ. Never mix metric and imperial fasteners in the same assembly.
Live Converter: Enter a value in any field and the conversion updates instantly.
Length / Diameter — mm → in
→
— in
Torque — N·m → ft·lbf / in·lbf
→
— ft·lbf
→
— in·lbf
1 inch = 25.4 mm
1 mm = 0.03937 in
1 ft·lbf = 1.35582 N·m
1 in·lbf = 0.11299 N·m
1 mm = 0.03937 in
1 ft·lbf = 1.35582 N·m
1 in·lbf = 0.11299 N·m
Quick fractions:
1/4" = 6.35 mm · 5/16" = 7.94 mm
3/8" = 9.53 mm · 1/2" = 12.70 mm
5/8" = 15.88 mm · 3/4" = 19.05 mm
1/4" = 6.35 mm · 5/16" = 7.94 mm
3/8" = 9.53 mm · 1/2" = 12.70 mm
5/8" = 15.88 mm · 3/4" = 19.05 mm
Metric bolt sizing uses the format M[diameter] × [pitch] — e.g. M8 × 1.25 is an 8 mm diameter bolt with 1.25 mm thread pitch. The table below shows the nearest imperial equivalent and the actual size difference.
| Metric Size | Diameter (mm) | Nearest Imperial | Imperial (mm) | Difference |
|---|---|---|---|---|
| Small / Machine Screw Range | ||||
| M2 | 2.00 mm | #2 (0.086") | 2.18 mm | +0.18 mm |
| M2.5 | 2.50 mm | #3 (0.099") | 2.51 mm | +0.01 mm |
| M3 | 3.00 mm | #4 (0.112") | 2.84 mm | −0.16 mm |
| M3.5 | 3.50 mm | #6 (0.138") | 3.51 mm | +0.01 mm |
| M4 | 4.00 mm | #8 (0.164") | 4.17 mm | +0.17 mm |
| M5 | 5.00 mm | #10 (0.190") | 4.83 mm | −0.17 mm |
| M6 | 6.00 mm | 1/4" (0.250") | 6.35 mm | +0.35 mm |
| Standard Bolt Range | ||||
| M8 | 8.00 mm | 5/16" (0.3125") | 7.94 mm | −0.06 mm |
| M10 | 10.00 mm | 3/8" (0.375") | 9.53 mm | −0.47 mm |
| M12 | 12.00 mm | 1/2" (0.500") | 12.70 mm | +0.70 mm |
| M14 | 14.00 mm | 9/16" (0.5625") | 14.29 mm | +0.29 mm |
| M16 | 16.00 mm | 5/8" (0.625") | 15.88 mm | −0.12 mm |
| M18 | 18.00 mm | 3/4" (0.750") | 19.05 mm | +1.05 mm |
| M20 | 20.00 mm | 3/4" (0.750") | 19.05 mm | −0.95 mm |
| Large Bolt Range | ||||
| M22 | 22.00 mm | 7/8" (0.875") | 22.23 mm | +0.23 mm |
| M24 | 24.00 mm | 1" (1.000") | 25.40 mm | +1.40 mm |
| M27 | 27.00 mm | 1-1/16" (1.0625") | 26.99 mm | −0.01 mm |
| M30 | 30.00 mm | 1-1/4" (1.250") | 31.75 mm | +1.75 mm |
| M36 | 36.00 mm | 1-3/8" (1.375") | 34.93 mm | −1.07 mm |
Metric thread pitch is the distance between threads in millimeters. Coarse pitch is standard for most applications; fine pitch provides greater strength and vibration resistance.
| Metric Size | Coarse Pitch (mm) | Fine Pitch (mm) | Imperial Equiv. | UNC TPI | UNF TPI |
|---|---|---|---|---|---|
| M2 | 0.40 | — | #2 | 56 | 64 |
| M2.5 | 0.45 | — | #3 | 48 | 56 |
| M3 | 0.50 | — | #4 | 40 | 48 |
| M3.5 | 0.60 | — | #6 | 32 | 40 |
| M4 | 0.70 | — | #8 | 32 | 36 |
| M5 | 0.80 | — | #10 | 24 | 32 |
| M6 | 1.00 | 0.75 | 1/4" | 20 | 28 |
| M8 | 1.25 | 1.00 | 5/16" | 18 | 24 |
| M10 | 1.50 | 1.25 | 3/8" | 16 | 24 |
| M12 | 1.75 | 1.25 | 1/2" | 13 | 20 |
| M14 | 2.00 | 1.50 | 9/16" | 12 | 18 |
| M16 | 2.00 | 1.50 | 5/8" | 11 | 18 |
| M20 | 2.50 | 1.50 | 3/4" | 10 | 16 |
| M24 | 3.00 | 2.00 | 1" | 8 | 14 |
| M30 | 3.50 | 2.00 | 1-1/4" | 7 | 12 |
| M36 | 4.00 | 3.00 | 1-3/8" | 6 | 12 |
Converting pitch: TPI → mm pitch: divide 25.4 by TPI (e.g. 20 TPI = 1.27 mm). mm pitch → TPI: divide 25.4 by pitch (e.g. 1.25 mm = 20.3 TPI).
Wrench size refers to the across-flats (AF) dimension of the bolt head or nut. Metric and imperial wrench sizes are close but not interchangeable — using the wrong size risks rounding the flats.
| Bolt Size | Metric Wrench (mm) | Nearest Imperial Wrench | Imperial (mm) | Gap (mm) |
|---|---|---|---|---|
| Machine Screw / Small Bolt Hex Sockets | ||||
| M3 | 5.5 mm | 7/32" | 5.56 mm | +0.06 |
| M4 | 7 mm | 9/32" | 7.14 mm | +0.14 |
| M5 | 8 mm | 5/16" | 7.94 mm | −0.06 |
| M6 | 10 mm | 3/8" | 9.53 mm | −0.47 |
| Standard Bolt Range | ||||
| M8 | 13 mm | 1/2" | 12.70 mm | −0.30 |
| M10 | 17 mm | 11/16" | 17.46 mm | +0.46 |
| M12 | 19 mm | 3/4" | 19.05 mm | +0.05 |
| M14 | 22 mm | 7/8" | 22.23 mm | +0.23 |
| M16 | 24 mm | 15/16" | 23.81 mm | −0.19 |
| M18 | 27 mm | 1-1/16" | 26.99 mm | −0.01 |
| M20 | 30 mm | 1-3/16" | 30.16 mm | +0.16 |
| Large Bolt Range | ||||
| M22 | 32 mm | 1-1/4" | 31.75 mm | −0.25 |
| M24 | 36 mm | 1-7/16" | 36.51 mm | +0.51 |
| M27 | 41 mm | 1-5/8" | 41.28 mm | +0.28 |
| M30 | 46 mm | 1-13/16" | 46.04 mm | +0.04 |
| M36 | 55 mm | 2-3/16" | 55.56 mm | +0.56 |
Tip: Gaps over 0.5 mm risk rounding the flats — especially on hardened fasteners. Always use the correct size wrench for torque-critical connections.
Shop Metric & Imperial Fasteners at Mutual Screw & Supply
We stock both metric and imperial fasteners across all grades and materials — from standard hardware to military and aerospace specifications. Family owned since 1947.
© 2026 Mutual Screw & Supply · 68 West Passaic Street, Rochelle Park, NJ 07662 · mutualscrew.com
All conversions based on 1 in = 25.4 mm exactly (NIST). Thread pitch data per ISO 261/262 and ASME B1.1. Wrench AF dimensions per ISO 272 and ASME B18.2.2. Verify with product datasheets for critical applications.