Use this fastener conversion chart to cross-reference metric (M-series), imperial (UNC/UNF) and British Standard (BSW/BSF/BA) thread sizes in one table. Whether you're identifying an unmarked bolt, sourcing a replacement fastener, or verifying thread compatibility across systems, find your size and read across the row for the equivalent.
Thread Pitch vs. Threads Per Inch
Metric and imperial fasteners use different systems to describe thread spacing, and understanding the difference is essential before cross-referencing sizes.
Metric fasteners use thread pitch. Thread pitch is the distance in millimetres between adjacent threads. A lower pitch number means finer threads. Metric fasteners are identified by the prefix M followed by the nominal diameter — for example, M8. Coarse thread (standard) has a larger pitch; fine thread has a smaller pitch.
American fasteners use threads per inch (TPI). TPI counts how many threads fit in one inch. A higher TPI means finer threads. The Unified Thread Standard covers two main series: Unified National Coarse (UNC) for general use, and Unified National Fine (UNF) for applications requiring higher tensile strength or finer adjustment.
British fasteners use threads per inch too. British Standard Whitworth (BSW) is the coarse series and British Standard Fine (BSF) is the fine series — both expressed in TPI using fractional inch nominal sizes. British Association (BA) threads are a smaller-diameter series, identified by a number suffix (0BA being the largest) with their own TPI values.
Metric to Imperial Fastener Conversion Chart
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) | BA | |||||||
| Size | Coarse (mm) | Fine (mm) | Size | Coarse (UNC) | Fine (UNF) | Size | Coarse (BSW) | Fine (BSF) | ||
| -- | -- | -- | -- | #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 | -- |
| M17 | -- | 1.50 | -- | -- | -- | -- | -- | -- | -- | -- |
| -- | -- | -- | -- | -- | -- | -- | 11/16" | 11 | 14 | -- |
| M18 | 2.50 | 1.50 | -- | -- | -- | -- | -- | -- | -- | -- |
| M20 | -- | 1.50 | 3/4" | -- | 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 1/4" | -- | 7 | 12 | 1 1/4" | 7 | 9 | -- |
| M32 | -- | 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 1/2" | -- | 6 | 12 | 1 1/2" | 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 3/4" | -- | 5 | -- | 1 3/4" | 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 1/4" | -- | 4.5 | -- | 2 1/4" | 4 | 6 | -- |
| M58 | -- | 4.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M60 | 5.50 | 4.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M62 | -- | 4.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M64 | 6.00 | 4.00 | 2 1/2" | -- | 4 | -- | 2 1/2" | 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 1/4" | -- | 4 | -- | 3 1/4" | 3.25 | 5 | -- |
| M85 | 6.00 | 4.00 | -- | -- | -- | -- | -- | -- | -- | -- |
| M90 | 6.00 | 4.00 | 3 1/2" | -- | 4 | -- | 3 1/2" | 3.25 | 4.5 | -- |
| -- | -- | -- | 3 3/4" | -- | 4 | -- | 3 3/4" | 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 | -- | -- |
What Else to Consider When Selecting Fasteners
Aside from thread pitch or TPI, the following factors affect whether a fastener is right for your application: the fastener type (bolt, nut, screw or stud); head style; strength grade or property class; material and surface finish (zinc, stainless, hot-dip galvanised); tensile rating; and thread engagement length. Where possible, have a sample fastener on hand to verify diameter, pitch and thread form before ordering.
Related Size Charts
Anchor Bolt Size Chart — metric anchor bolt types including sleeve anchors, drop-in anchors and stud anchors, with common metric diameters.
Drill Bit Size Chart — metric, imperial and gauge drill bit sizes matched in a single reference table.
Loctite Application Chart — which Loctite threadlocker or adhesive product to use for different materials and surfaces.
Socket Size Chart — metric and imperial socket sizes with drive equivalents.
Spanner Size Chart — spanner sizes matched to bolt and nut hex sizes across metric and imperial.
Tapping Drill Size Chart — drill sizes for cutting metric and imperial threads with hand taps.
Metric vs Imperial Fasteners Guide — which thread system is standard in Australia, how UNC, UNF, BSW and BSF compare to metric, and when the two are (and aren't) interchangeable.
Frequently Asked Questions
What is the imperial equivalent of M8?
The closest imperial equivalent to M8 is 5/16" (UNC: 18 TPI, UNF: 24 TPI). In British Standard, 5/16" BSW uses 18 TPI coarse and 22 TPI fine. M8 has a metric coarse pitch of 1.25mm and fine pitch of 1.00mm.
How do I convert metric bolt sizes to imperial?
Use the conversion table above to cross-reference your metric size (M-series) with the nearest UTS (UNC/UNF) or British Standard (BSW/BSF) equivalent. Note that metric and imperial threads are not interchangeable — only the nominal diameter is comparable. Always verify thread pitch or TPI before substituting fasteners.
What is the difference between UNC and UNF threads?
UNC (Unified National Coarse) threads have fewer threads per inch and are more commonly used in general construction and engineering applications. UNF (Unified National Fine) threads have more threads per inch, providing a finer thread form suited to applications requiring greater tensile strength or finer adjustability. For example, 1/4" UNC is 20 TPI while 1/4" UNF is 28 TPI.
Is M10 the same as 3/8 inch?
M10 and 3/8" are close in diameter (M10 = 10mm, 3/8" = 9.525mm) but they are not interchangeable. Their thread pitches differ: M10 coarse is 1.50mm pitch, while 3/8" UNC is 16 TPI (1.588mm pitch) and 3/8" UNF is 24 TPI. Always match both diameter and thread pitch when selecting a fastener.
What does the M in metric fastener sizes mean?
The M stands for metric, and the number that follows is the nominal outer diameter of the fastener in millimetres. So M8 has a nominal diameter of 8mm, M10 is 10mm, and so on. Metric fasteners also specify thread pitch in millimetres — for example, M8 x 1.25 means 8mm diameter with a 1.25mm thread pitch.
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