The closest imperial equivalent to M8 is 5/16", M10 is 3/8", and M12 is 1/2". Metric (M-series) and imperial (UNC/UNF/BSW/BSF) threads share only nominal diameter — pitch and TPI differ, so they are not interchangeable. The compact reference below covers the most-used conversions; the full chart with thread pitch, TPI, BA and large sizes follows.
Quick answer — metric to imperial
M3 ≈ 4-40 / #5 · M4 ≈ 8-32 / #8 · M5 ≈ #10 / 10-32 · M6 ≈ 1/4" · M8 ≈ 5/16" · M10 ≈ 3/8" · M12 ≈ 1/2" · M14 ≈ 9/16" · M16 ≈ 5/8" · M20 ≈ 3/4" · M22 ≈ 7/8" · M24 ≈ 15/16" · M27 ≈ 1-1/16" · M30 ≈ 1-3/16"
⚠️ Diameter only. Thread pitch / TPI differs — metric and imperial fasteners are not interchangeable. Full pitch and TPI chart below.
Metric to Imperial Fastener Quick Reference
The most common metric fastener sizes and their closest imperial equivalents:
| Metric | UNC / UNF (US) | BSW / BSF (UK) |
| M3 | 1/8" | 1/8" |
| M5 | 3/16" | 3/16" |
| M6 | 1/4" | 1/4" |
| M8 | 5/16" | 5/16" |
| M10 | 3/8" | 3/8" |
| M12 | 1/2" | 1/2" |
| M14 | 9/16" | 9/16" |
| M16 | 5/8" | 5/8" |
| M20 | 3/4" | 3/4" |
| M24 | 1" | 1" |
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
Drill Bit Size Chart — metric, imperial and gauge drill bit sizes matched in a single reference table.
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 M3 in imperial?
M3 has the closest imperial equivalent of 4-40 UNC or #5 gauge. M3 is 3mm nominal diameter with 0.5mm coarse pitch. The exact match is 0.118 inches — there is no exact imperial bolt at this size, so #5 is the closest standard.
What is M4 in imperial?
M4 has the closest imperial equivalent of 8-32 UNC or #8 gauge. M4 is 4mm nominal diameter with 0.7mm coarse pitch (1/8 inch is close but not identical at 0.157"). They are not interchangeable — thread pitch differs.
What is M5 in imperial?
M5 has the closest imperial equivalent of #10 (10-24 UNC or 10-32 UNF). M5 is 5mm nominal diameter with 0.8mm coarse pitch. The match is approximate — diameter is close but thread pitches do not match.
What is M6 in imperial?
M6 has the closest imperial equivalent of 1/4 inch. M6 is 6mm diameter with 1.0mm coarse pitch; 1/4" UNC is 20 TPI (1.27mm pitch) and 1/4" UNF is 28 TPI (0.91mm pitch). They are close in diameter only — not interchangeable.
What is M8 in imperial?
M8 has the closest imperial equivalent of 5/16 inch (UNC: 18 TPI, UNF: 24 TPI). M8 is 8mm diameter with 1.25mm coarse pitch and 1.0mm fine pitch. The diameter is close (5/16" = 7.94mm) but threads do not match.
What is M10 in imperial?
M10 has the closest imperial equivalent of 3/8 inch. M10 is 10mm diameter with 1.5mm coarse pitch; 3/8" UNC is 16 TPI (1.59mm pitch) and 3/8" UNF is 24 TPI (1.06mm pitch). M10 = 10mm, 3/8" = 9.525mm — close but not identical.
What is M12 in imperial?
M12 has the closest imperial equivalent of 1/2 inch. M12 is 12mm diameter with 1.75mm coarse pitch; 1/2" UNC is 13 TPI (1.95mm pitch) and 1/2" UNF is 20 TPI (1.27mm pitch). M12 = 12mm, 1/2" = 12.7mm.
What is M14 in imperial?
M14 has the closest imperial equivalent of 9/16 inch (14.29mm). M14 is 14mm diameter with 2.0mm coarse pitch; 9/16" UNC is 12 TPI (2.12mm pitch). Diameter is close, pitch differs.
What is M16 in imperial?
M16 has the closest imperial equivalent of 5/8 inch (15.88mm). M16 is 16mm diameter with 2.0mm coarse pitch; 5/8" UNC is 11 TPI (2.31mm pitch) and 5/8" UNF is 18 TPI (1.41mm pitch).
What is M20 in imperial?
M20 has the closest imperial equivalent of 3/4 inch (19.05mm). M20 is 20mm diameter with 2.5mm coarse pitch; 3/4" UNC is 10 TPI (2.54mm pitch).
What is M30 in imperial?
M30 has the closest imperial equivalent of 1-3/16 inch (30.16mm). M30 is 30mm diameter with 3.5mm coarse pitch.
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 used in general construction and engineering. UNF (Unified National Fine) threads have more threads per inch, providing finer thread form for greater tensile strength or finer adjustability. 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. 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 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.
Are metric and imperial fasteners interchangeable?
No. Even when the diameter looks close (M8 vs 5/16", M10 vs 3/8", M12 vs 1/2"), the thread pitch differs and forcing a metric bolt into an imperial thread (or vice versa) will strip or cross-thread it. Use the correct standard for the receiving thread — always.
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