Fastener Reference Guide (Metric-Imperial Equivalents)
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When it comes to identifying fastener sizes, thread is key. You have to know when you are dealing with metric or imperial fasteners. From there, you can cross-reference from the table below, which contains data diligently compiled from verified sources.
Metric fasteners use thread pitch
Thread pitch is the distance from one thread to the next (adjacent thread). It is expressed in millimetres.
Metric fasteners are typically prefixed by an M, which makes them easily distinguishable from imperial fasteners.
Thread patterns are classified as either coarse (longer) or fine (shorter) pitch.
American and British fasteners use threads per inch
Threads Per Inch (TPI) is the number of threads in an inch.
When dealing with threaded fasteners from North America, you'll refer to them in Unified Thread Standard (UTS) that use:
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Unified National Coarse (UNC) - Coarse threads*, lower TPI values
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United National Fine (UNF) - Fine threads*, higher TPI values
You may also encounter threaded fasteners from the UK that still use the British Standard:
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British Standard Whitworth (BSW) - Coarse threads*
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British Standard Fine (BSF) - Fine threads*
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British Association (BA) - Typically suffixed with BA
*Labeled in fractional inches (eg. 1/8”) based on nominal size
Metric vs Imperial equivalents
Refer to this table when cross-referencing your bolt, nut or screw. Not all metric fasteners have imperial equivalents, and vice versa.
| Metric (Pitch in mm) | Unified Thread Standard (Threads Per Inch) | British Standard (Threads Per Inch) | ||||||||
| Size | Coarse (mm) | Fine (mm) | Size | Coarse (UNC) | Fine (UNF) | Size | Coarse (BSW) | Fine (BSF) | BA | |
| -- | -- | -- | -- | #0000 | -- | 160 | -- | -- | -- | -- |
| -- | -- | -- | -- | #000 | -- | 120 | -- | -- | -- | -- |
| -- | -- | -- | -- | #00 | -- | 90 | -- | -- | -- | -- |
| M1.6 | 0.35 | 0.20 | -- | #0 | -- | 80 | -- | -- | -- | -- |
| M2 | 0.40 | 0.25 | -- | #1 | 64 | 72 | -- | -- | -- | -- |
| -- | -- | -- | -- | -- | -- | -- | 1/16" | 60 | -- | -- |
| -- | -- | -- | -- | #2 | 56 | 64 | 8BA | -- | -- | 59.1 |
| M2.5 | 0.45 | 0.35 | -- | #3 | 48 | 56 | -- | -- | -- | -- |
| -- | -- | -- | -- | -- | -- | -- | 3/32" | 48 | -- | -- |
| -- | -- | -- | -- | #4 | 40 | 48 | 6BA | -- | -- | 47.9 |
| M3 | 0.50 | 0.35 | 1/8” | #5 | 40 | 44 | 1/8" | 40 | -- | -- |
| M3.5 | 0.60 | 0.35 | -- | #6 | 32 | 40 | 4BA | -- | -- | 38.5 |
| -- | -- | -- | -- | -- | -- | -- | 5BA | -- | -- | 43 |
| M4 | 0.70 | 0.50 | -- | #8 | 32 | 36 | 3BA | -- | -- | 34.8 |
| M4.5 | 0.75 | 0.50 | -- | -- | -- | -- | -- | -- | -- | -- |
| -- | -- | -- | -- | -- | -- | -- | 2BA | -- | -- | 31.4 |
| M5 | 0.80 | 0.50 | 3/16” | #10 | 24 | 32 | 3/16" | 24 | 32 | 32 |
| M5.5 | -- | 0.50 | -- | -- | -- | -- | -- | -- | -- | -- |
| -- | -- | -- | -- | #12 | 24 | 28 | 1BA | -- | -- | 28.2 |
| M6 | 1.00 | 0.75 | -- | -- | -- | -- | 0BA | -- | -- | 25.4 |
| -- | -- | -- | 1/4" | -- | 20 | 28 | 1/4" | 20 | 26 | -- |
| M7 | 1.00 | 0.75 | -- | -- | -- | -- | 9/32" | -- | 26 | -- |
| M8 | 1.25 | 1.00 | 5/16” | -- | 18 | 24 | 5/16" | 18 | 22 | -- |
| M9 | 1.25 | 1.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M10 | 1.50 | 1.25 | 3/8” | -- | 16 | 24 | 3/8" | 16 | 20 | -- |
| M11 | 1.50 | 1.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| -- | -- | -- | 7/16” | -- | 14 | 20 | 7/16” | 14 | 18 | -- |
| M12 | 1.75 | 1.25 | 1/2" | -- | 13 | 20 | 1/2" | 12 | 16 | -- |
| M14 | 2.00 | 1.50 | 9/16” | -- | 12 | 18 | 9/16" | 12 | 16 | -- |
| M15 | -- | 1.50 | -- | -- | -- | -- | -- | -- | -- | -- |
| M16 | 2.00 | 1.50 | 5/8” | -- | 11 | 18 | 5/8” | 11 | 14 | -- |
| -- | -- | -- | -- | -- | -- | -- | 11/16" | 11 | 14 | -- |
| M17 | -- | 1.50 | -- | -- | -- | -- | -- | -- | -- | -- |
| M18 | 2.50 | 1.50 | -- | -- | -- | -- | -- | -- | -- | -- |
| M20 | -- | 1.50 | ¾" | -- | 10 | 16 | 3/4" | 10 | 12 | -- |
| M22 | 2.50 | 1.50 | 7/8” | -- | 9 | 14 | 7/8” | 9 | 11 | -- |
| M24 | 3.00 | 2.00 | 1” | -- | 8 | 14 / 12 | 1" | 8 | 10 | -- |
| M25 | -- | 2.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M27 | -- | 2.00 | 1 1/8” | -- | 7 | 12 | 1 1/8” | 7 | 9 | -- |
| M28 | -- | 2.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M30 | 3.50 | 2.00 | 1 ¼" | -- | 7 | 12 | 1 ¼" | 7 | 9 | -- |
| M32 | 00 | 2.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M33 | 3.50 | 2.00 | 1 3/8” | -- | 6 | 12 | 1 3/8” | 6 / 7 | 8 | -- |
| M35 | -- | 1.50 | -- | -- | -- | -- | -- | -- | -- | -- |
| M36 | 4.00 | 3.00 | 1 ½" | -- | 6 | 12 | 1 ½" | 6 | 8 | -- |
| M38 | -- | 1.50 | -- | -- | -- | -- | -- | -- | -- | -- |
| M39 | 4.00 | 3.00 | 1 5/8” | -- | -- | -- | 1 5/8” | 5 | 8 | -- |
| M40 | -- | 3.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M42 | 4.50 | 4.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M45 | 4.50 | 4.00 | 1 ¾" | -- | 5 | -- | 1 ¾" | 5 | 7 | -- |
| M48 | 5.00 | 4.00 | 1 7/8” | -- | 5 | -- | -- | -- | -- | -- |
| M50 | -- | 3.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M52 | 5.00 | 4.00 | 2” | -- | 4.5 | -- | 2” | 4.5 | 7 | -- |
| M55 | -- | 4.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M56 | 5.50 | 4.00 | 2 ¼" | -- | 4.5 | -- | 2 ¼" | 4 | 6 | -- |
| M58 | -- | 4.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M60 | 5.50 | 4.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M62 | -- | 4.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M64 | 6.00 | 4.00 | 2 ½" | -- | 4 | -- | 2 ½" | 4 | 6 | -- |
| M65 | -- | 4.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M68 | 6.00 | 4.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M70 | 6.00 | 4.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M72 | 6.00 | 4.00 | 2 3/4" | -- | 4 | -- | 2 3/4" | 3.5 | 6 | -- |
| M75 | -- | 4.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M76 | 6.00 | 4.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M78 | -- | 2.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M80 | 6.00 | 4.00 | 3” | -- | 4 | -- | 3” | 3.5 | 5 | -- |
| -- | -- | -- | 3 ¼" | -- | 4 | -- | 3 ¼" | 3.25 | 5 | -- |
| M85 | 6.00 | 4.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M90 | 6.00 | 4.00 | 3 ½" | -- | 4 | -- | 3 ½" | 3.25 | 4.5 | -- |
| -- | -- | -- | 3 ¾" | -- | 4 | -- | 3 ¾" | 3 | 4.5 | -- |
| M100 | 6.00 | -- | 4” | -- | 4 | -- | 4” | 3 | 4.5 | -- |
| -- | -- | -- | -- | -- | -- | -- | 4 1/4" | 2.875 | 4 | -- |
| -- | -- | -- | -- | -- | -- | -- | 4 1/2" | 2.875 | -- | -- |
| -- | -- | -- | -- | -- | -- | -- | 4 3/4" | 2.75 | -- | -- |
| -- | -- | -- | -- | -- | -- | -- | 5" | 2.75 | -- | -- |
| -- | -- | -- | -- | -- | -- | -- | 5 1/4" | 2.625 | -- | -- |
| -- | -- | -- | -- | -- | -- | -- | 5 1/2" | 2.625 | -- | -- |
| -- | -- | -- | -- | -- | -- | -- | 5 3/4" | 2.5 | -- | -- |
| -- | -- | -- | -- | -- | -- | -- | 6 | 2.5 | -- | -- |
Table values compiled from:
Aside from the thread pitch (metric) or count (imperial), other factors to consider when buying fasteners include:
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Type (bolt, nut or screw)
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Head
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Strength (classified by grade/class)
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Material and finish (coating)
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Tensile properties (low/high tensile)
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Thread length (full/partial)
(We’ll cover them in detail in future articles.)
It’s always great to have a sample of the fastener you’re shopping for, but that won’t always be the case. We have a wide range of fastener listings with pictures that you can use for reference.
If in doubt, reach out to us via chat or email and we’ll assist you promptly.
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