Fastener sizes
Measure the diameter across the thread crests and the pitch, and this identifies the thread and gives you the drill sizes. Everything here comes from the published thread standards, so it does not change.
How to identify a thread
- Measure across the thread crests with calipers. Read a little low — crests are truncated by the standard and then worn, so a used M8 typically measures about 7.8 mm.
- Measure the pitch. Metric threads are quoted as the distance between crests in millimetres; inch threads as the number of threads per inch.
- If the diameter is a round number of millimetres it is almost certainly metric. If it is a round fraction of an inch, it is not.
- Coarse and fine threads share a diameter and differ only in pitch. This is the distinction worth getting right — forcing the wrong one strips the thread in a turn or two.
Charts
- Metric thread chart M1.6 to M48, coarse and fine
- Unified inch thread chart UNC and UNF, #2 to 1"
Common sizes
| Thread | Tap drill | Clearance | Where you meet it |
|---|---|---|---|
| 1/4"-20 UNC | #7 | 6.76 mm | Camera tripod sockets and quick-release plates |
| M8×1.25 | 6.8 mm | 8.4 mm | Bed frame and flat-pack furniture bolts |
| M6×1 | 5 mm | 6.4 mm | Cage nuts in 19-inch server racks outside North America |
| M10×1.5 | 8.5 mm | 10.5 mm | Brake caliper and suspension bolts on cars |
| 3/8"-16 UNC | 5/16" | 10.08 mm | Studio tripod heads and light stand spigots |
| M5×0.8 | 4.2 mm | 5.3 mm | Bicycle bottle cage bosses |
| M12×1.75 | 10.2 mm | 13 mm | Structural steel connections in metric practice |
| 1/2"-13 UNC | 27/64" | 13.49 mm | Concrete wedge anchors and sill plate bolts |
| M4×0.7 | 3.3 mm | 4.3 mm | VESA 75×75 and 100×100 monitor mounts |
| M3×0.5 | 2.5 mm | 3.2 mm | 2.5-inch SATA drive and SSD mounting screws |
| #6-32 UNC | #36 | 3.8 mm | US electrical outlet and switch cover plates |
| #10-32 UNF | #21 | 5.11 mm | Grounding screws in US electrical boxes |
| M12×1.5 | 10.5 mm | 13 mm | Wheel studs and lug nuts on Toyota, Honda, Ford and GM |
| 5/16"-18 UNC | F | 8.43 mm | US automotive body and bracket bolts |
| M12×1.25 | 10.8 mm | 13 mm | Wheel studs on Nissan, Subaru, Mitsubishi and older Toyota |
| #4-40 UNC | #43 | 3.26 mm | D-subminiature connector jackscrews on serial and VGA sockets |
- 1/4"-20 UNC #7 tap drill
- M8×1.25 6.8 mm tap drill
- M6×1 5 mm tap drill
- M10×1.5 8.5 mm tap drill
- 3/8"-16 UNC 5/16" tap drill
- M5×0.8 4.2 mm tap drill
- M12×1.75 10.2 mm tap drill
- 1/2"-13 UNC 27/64" tap drill
- M4×0.7 3.3 mm tap drill
- M3×0.5 2.5 mm tap drill
- #6-32 UNC #36 tap drill
- #10-32 UNF #21 tap drill
- M12×1.5 10.5 mm tap drill
- 5/16"-18 UNC F tap drill
- M12×1.25 10.8 mm tap drill
- #4-40 UNC #43 tap drill
Common questions
- How do I know if a bolt is metric or imperial?
- Measure the diameter. A round number of millimetres means metric; a round fraction of an inch means Unified. Where it is ambiguous, the pitch settles it — a thread pitch gauge is the reliable way and costs very little.
- What drill size do I need to tap a thread?
- Roughly the major diameter minus the pitch, which gives about 75% thread engagement. In practice use the chart figure rather than the arithmetic, because the charts round to drills that actually exist — M8 wants 6.75 mm and the world sells 6.8 mm.
- Why is my bolt measuring smaller than its stated size?
- Because the major diameter is a maximum, not a target. The standard truncates the crests, manufacturing tolerance takes a little more, and wear and plating take the rest. A used M8 measuring 7.8 mm is entirely normal.
- What is the difference between coarse and fine threads?
- Pitch, at the same diameter. Coarse threads assemble faster and tolerate damage and corrosion better; fine threads have a larger stress area, hold adjustment more precisely and resist vibration loosening slightly better. They are not interchangeable.