Metric vs. Imperial Fastener Conversion Chart

Metric vs Imperial ConversionMETRIC · INCH
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Reference only. Conversions use 1 in = 25.4 mm. Metric and inch fasteners are not interchangeable. Verify critical dimensions against the applicable ISO, DIN or ASME standard. © Mutual Screw & Supply. Printed from mutualscrew.com/pages/metric-vs-imperial-fastener-conversion-chart

Convert between metric and inch fasteners. Use the calculator for length, diameter and torque, then check the closest bolt sizes, thread pitches and wrench sizes.

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.

Metric to Imperial Fastener Calculator

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
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

Metric vs Imperial Bolt Size Equivalents

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 (imperial − metric)
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 1-1/8" (1.125") 28.58 mm −1.42 mm
M36 36.00 mm 1-3/8" (1.375") 34.93 mm −1.07 mm

Thread Pitch Conversion: TPI to 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 12
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 Conversion Chart

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. Metric sizes shown are DIN 933/934 across-flats. ISO 4017/4032 parts use 16, 18, 21 and 34 mm for M10, M12, M14 and M22.

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. Serving customers since 1947.

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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.