Universal Screw & Bolt Size Chart
One reference for metric and imperial screw sizes, thread pitch, tap drill sizes, and bolt head dimensions. Use the tables below to compare fastener sizes and jump to our tools for instant conversions.
Metric vs Imperial Screw Sizes
Common metric sizes and their closest imperial (SAE) equivalents by diameter. Use for ordering or substituting in mixed systems.
| Metric | Diameter (mm) | Closest Imperial |
|---|---|---|
| M3 | 3.0 | #4 |
| M4 | 4.0 | #8 |
| M5 | 5.0 | #10 |
| M6 | 6.0 | 1/4" |
| M8 | 8.0 | 5/16" |
| M10 | 10.0 | 3/8" |
| M12 | 12.0 | 1/2" |
Thread Pitch vs TPI Chart
Metric pitch (mm) and equivalent threads per inch (TPI) for common coarse threads.
| Thread | Pitch (mm) | TPI |
|---|---|---|
| M6 | 1.0 | 25.4 |
| M8 | 1.25 | 20.3 |
| M10 | 1.5 | 16.9 |
| M12 | 1.75 | 14.5 |
Tap Drill Size Chart
Recommended drill diameter for cutting internal metric threads (approx. 75% thread depth).
| Thread | Pitch | Recommended drill |
|---|---|---|
| M3 | 0.5 | 2.5 mm |
| M4 | 0.7 | 3.3 mm |
| M5 | 0.8 | 4.2 mm |
| M6 | 1.0 | 5.0 mm |
| M8 | 1.25 | 6.8 mm |
| M10 | 1.5 | 8.5 mm |
Bolt Head Size Chart
Hex key or wrench size for standard metric bolt heads.
| Bolt size | Hex head size |
|---|---|
| M6 | 10 mm |
| M8 | 13 mm |
| M10 | 17 mm |
| M12 | 19 mm |
How to Read Fastener Size Charts
Metric sizes are written as diameter × pitch (e.g. M8 × 1.25). Imperial sizes often show nominal diameter and threads per inch (e.g. 1/4-20). Diameter is measured across the crests of the threads; pitch is the distance between one crest and the next. Use calipers and a pitch gauge to measure an unknown bolt, then match it to this chart or our Thread Identifier Tool.
FAQ
What is the difference between metric and imperial screws?
Metric screws use diameter and pitch in millimeters (e.g. M6 x 1.0). Imperial screws use diameter in inches and threads per inch (e.g. 1/4-20). They are not interchangeable without conversion.
How do you convert metric bolt sizes to imperial?
Use a conversion chart or tool to match diameter and thread pitch. Common equivalents include M6 to 1/4", M8 to 5/16", and M10 to 3/8". Use our Metric to Imperial Converter for instant results.
What drill size is used for M6 threads?
For M6 x 1.0 coarse thread, the recommended tap drill size is 5.0 mm. For other pitches, use major diameter minus pitch. Use the Tap Drill Calculator for exact values.
What does thread pitch mean?
Thread pitch is the distance between adjacent thread crests. In metric it is given in millimeters; in imperial it is often expressed as threads per inch (TPI). Fine threads have smaller pitch than coarse.
Are metric and SAE bolts interchangeable?
No. Metric and SAE (imperial) bolts have different thread angles and pitch. Using the wrong type can strip threads or cause failure. Always match the thread system to the nut or hole.