The Fix / Sizing tools
Metric / Imperial Fastener Converter
Convert mm to inches or inches to mm, compare metric coarse threads from M3 to M24 with UNC, UNF and BSW sizes, and look up suggested tap drills. One inch is exactly 25.4 mm.
Compare sizes, then check the thread. The closest diameter is not an interchangeable fastener. Metric and imperial threads have different pitches or forms.
Find your size
Thread and tap drill reference
Nominal major diameters, pitches and suggested cutting tap drills. Metric threads here are coarse pitch only. UNC and UNF have a 60° form; BSW has a 55° form. Closest sizes are ranked by diameter within these tables, not by fit or pitch. Non-preferred Whitworth-form sizes and UNS are excluded.
Tap drills are suggested starting sizes for cutting taps. Material, tap design and required thread engagement can change the drill needed. Do not use these sizes for forming taps, clearance holes or wood screw pilot holes.
| Thread | Diameter (mm) | Pitch (mm) | Tap drill (mm) | Closest diameter only |
|---|---|---|---|---|
| M3 × 0.5 | 3 | 0.5 | 2.5 | #4-40 UNC / #4-48 UNF / 1/8-40 BSW |
| M3.5 × 0.6 | 3.5 | 0.6 | 2.9 | #6-32 UNC / #6-40 UNF / 1/8-40 BSW |
| M4 × 0.7 | 4 | 0.7 | 3.3 | #8-32 UNC / #8-36 UNF / 3/16-24 BSW |
| M4.5 × 0.75 | 4.5 | 0.75 | 3.7 | #10-24 UNC / #10-32 UNF / 3/16-24 BSW |
| M5 × 0.8 | 5 | 0.8 | 4.2 | #10-24 UNC / #10-32 UNF / 3/16-24 BSW |
| M6 × 1 | 6 | 1 | 5 | 1/4-20 UNC / 1/4-28 UNF / 1/4-20 BSW |
| M8 × 1.25 | 8 | 1.25 | 6.8 | 5/16-18 UNC / 5/16-24 UNF / 5/16-18 BSW |
| M10 × 1.5 | 10 | 1.5 | 8.5 | 3/8-16 UNC / 3/8-24 UNF / 3/8-16 BSW |
| M12 × 1.75 | 12 | 1.75 | 10.2 | 1/2-13 UNC / 1/2-20 UNF / 1/2-12 BSW |
| M14 × 2 | 14 | 2 | 12 | 9/16-12 UNC / 9/16-18 UNF / 9/16-12 BSW |
| M16 × 2 | 16 | 2 | 14 | 5/8-11 UNC / 5/8-18 UNF / 5/8-11 BSW |
| M18 × 2.5 | 18 | 2.5 | 15.5 | 3/4-10 UNC / 3/4-16 UNF / 3/4-10 BSW |
| M20 × 2.5 | 20 | 2.5 | 17.5 | 3/4-10 UNC / 3/4-16 UNF / 3/4-10 BSW |
| M22 × 2.5 | 22 | 2.5 | 19.5 | 7/8-9 UNC / 7/8-14 UNF / 7/8-9 BSW |
| M24 × 3 | 24 | 3 | 21 | 1-8 UNC / 1-12 UNF / 1-8 BSW |
| Thread | Diameter (mm) | Pitch (mm) | Tap drill (mm) | Closest diameter only |
|---|---|---|---|---|
| #2-56 UNC | 2.1844 | 0.453571 | 1.85 | M3 (outside M3–M24 range) |
| #3-48 UNC | 2.5146 | 0.529167 | 2.1 | M3 (outside M3–M24 range) |
| #4-40 UNC | 2.8448 | 0.635 | 2.3 | M3 (outside M3–M24 range) |
| #5-40 UNC | 3.175 | 0.635 | 2.6 | M3 |
| #6-32 UNC | 3.5052 | 0.79375 | 2.8 | M3.5 |
| #8-32 UNC | 4.1656 | 0.79375 | 3.4 | M4 |
| #10-24 UNC | 4.826 | 1.058333 | 3.8 | M5 |
| #12-24 UNC | 5.4864 | 1.058333 | 4.5 | M5 |
| 1/4-20 UNC | 6.35 | 1.27 | 5.1 | M6 |
| 5/16-18 UNC | 7.9375 | 1.411111 | 6.6 | M8 |
| 3/8-16 UNC | 9.525 | 1.5875 | 8 | M10 |
| 7/16-14 UNC | 11.1125 | 1.814286 | 9.4 | M12 |
| 1/2-13 UNC | 12.7 | 1.953846 | 10.8 | M12 |
| 9/16-12 UNC | 14.2875 | 2.116667 | 12.2 | M14 |
| 5/8-11 UNC | 15.875 | 2.309091 | 13.5 | M16 |
| 3/4-10 UNC | 19.05 | 2.54 | 16.5 | M20 |
| 7/8-9 UNC | 22.225 | 2.822222 | 19.5 | M22 |
| 1-8 UNC | 25.4 | 3.175 | 22.2 | M24 (outside M3–M24 range) |
| Thread | Diameter (mm) | Pitch (mm) | Tap drill (mm) | Closest diameter only |
|---|---|---|---|---|
| #3-56 UNF | 2.5146 | 0.453571 | 2.1 | M3 (outside M3–M24 range) |
| #4-48 UNF | 2.8448 | 0.529167 | 2.35 | M3 (outside M3–M24 range) |
| #5-44 UNF | 3.175 | 0.577273 | 2.65 | M3 |
| #6-40 UNF | 3.5052 | 0.635 | 2.9 | M3.5 |
| #8-36 UNF | 4.1656 | 0.705556 | 3.5 | M4 |
| #10-32 UNF | 4.826 | 0.79375 | 4.1 | M5 |
| #12-28 UNF | 5.4864 | 0.907143 | 4.6 | M5 |
| 1/4-28 UNF | 6.35 | 0.907143 | 5.5 | M6 |
| 5/16-24 UNF | 7.9375 | 1.058333 | 6.9 | M8 |
| 3/8-24 UNF | 9.525 | 1.058333 | 8.5 | M10 |
| 7/16-20 UNF | 11.1125 | 1.27 | 9.8 | M12 |
| 1/2-20 UNF | 12.7 | 1.27 | 11.5 | M12 |
| 9/16-18 UNF | 14.2875 | 1.411111 | 12.8 | M14 |
| 5/8-18 UNF | 15.875 | 1.411111 | 14.5 | M16 |
| 3/4-16 UNF | 19.05 | 1.5875 | 17.5 | M20 |
| 7/8-14 UNF | 22.225 | 1.814286 | 20.5 | M22 |
| 1-12 UNF | 25.4 | 2.116667 | 23.5 | M24 (outside M3–M24 range) |
| Thread | Diameter (mm) | Pitch (mm) | Tap drill (mm) | Closest diameter only |
|---|---|---|---|---|
| 1/8-40 BSW | 3.175 | 0.635 | 2.55 | M3 |
| 3/16-24 BSW | 4.7625 | 1.058333 | 3.7 | M5 |
| 1/4-20 BSW | 6.35 | 1.27 | 5.1 | M6 |
| 5/16-18 BSW | 7.9375 | 1.411111 | 6.5 | M8 |
| 3/8-16 BSW | 9.525 | 1.5875 | 7.9 | M10 |
| 7/16-14 BSW | 11.1125 | 1.814286 | 9.3 | M12 |
| 1/2-12 BSW | 12.7 | 2.116667 | 10.5 | M12 |
| 9/16-12 BSW | 14.2875 | 2.116667 | 12.1 | M14 |
| 5/8-11 BSW | 15.875 | 2.309091 | 13.5 | M16 |
| 3/4-10 BSW | 19.05 | 2.54 | 16.25 | M20 |
| 7/8-9 BSW | 22.225 | 2.822222 | 19.25 | M22 |
| 1-8 BSW | 25.4 | 3.175 | 22 | M24 (outside M3–M24 range) |
Common length conversions
| Inches | Millimetres |
|---|---|
| 1/8 | 3.175 |
| 1/4 | 6.35 |
| 3/8 | 9.525 |
| 1/2 | 12.7 |
| 3/4 | 19.05 |
| 1 | 25.4 |
| 1 1/2 | 38.1 |
| 2 | 50.8 |
| 3 | 76.2 |
| 4 | 101.6 |
For a searchable, printable reference with pitch, TPI and suggested cutting-tap drills, use the Simfix tap drill size chart: metric, UNC, UNF and BSW. Pitch in millimetres for imperial threads is 25.4 ÷ TPI, rounded to six decimal places. Data checked 5 October 2026; source and scope are listed on the chart.
Measure bolt length under the head for hex, pan and socket heads; include the head for countersunk screws. Read the Simfix fastener size guide for measuring help.
Common questions
Is M6 the same as 1/4 inch?
No. M6 has a nominal diameter of 6 mm; 1/4 inch is 6.35 mm. Metric coarse M6 has a 1 mm pitch; 1/4-20 UNC has a 1.27 mm pitch. They are not interchangeable.
How do I convert inches to millimetres?
Multiply inches by exactly 25.4. For example, 1 1/2 inches is 38.1 mm. To convert millimetres to inches, divide by 25.4.
Can I replace a metric bolt with the closest imperial size?
A similar diameter is only a comparison. A replacement must match the thread standard, diameter and pitch, as well as the required length, head type, material grade and strength.
Are UNC and BSW threads interchangeable?
Do not assume compatibility. UNC has a 60-degree thread form and BSW has a 55-degree form. Some diameters share a TPI, but the forms differ. Use the specified standard.
What drill should I use before tapping an M8 coarse thread?
The suggested cutting tap drill for M8 x 1.25 is 6.8 mm. Confirm with the tap manufacturer for the material and required thread engagement. Forming taps require different drill sizes.
Does this converter cover metric fine, pipe or wood screw threads?
The metric thread chart covers coarse M3 to M24. It also covers the listed UNC, UNF and BSW threads. Metric fine, BSP, NPT, BA, BSF and wood screw pilot holes need their own reference data.