Screw and bolt sizes explained: M sizes, # sizes and how to measure
Screw sizes look cryptic until you know they’re just measurements. A metric M6 × 20 is 6 mm across the thread and 20 mm long. US sizes like #8 × 3/4” work the same way, just with a numbered diameter.
Metric screws (M sizes)
The number after the M is the outside diameter of the thread in millimeters. A second number, if there is one, is the length: M5 × 16 is 5 mm across and 16 mm long.
Each size also has a standard thread pitch, the distance from one thread to the next. Most hardware uses the coarse pitch:
| Size | Diameter | Coarse pitch |
|---|---|---|
| M3 | 3 mm | 0.5 mm |
| M4 | 4 mm | 0.7 mm |
| M5 | 5 mm | 0.8 mm |
| M6 | 6 mm | 1.0 mm |
| M8 | 8 mm | 1.25 mm |
| M10 | 10 mm | 1.5 mm |
| M12 | 12 mm | 1.75 mm |
US screws (# sizes)
Smaller US screws use a gauge number. The higher the number, the thicker the screw. Above #12 or so, sizes are given in fractions of an inch: 1/4”, 5/16” and so on.
| Size | Diameter |
|---|---|
| #4 | 0.112 in (2.84 mm) |
| #6 | 0.138 in (3.51 mm) |
| #8 | 0.164 in (4.17 mm) |
| #10 | 0.190 in (4.83 mm) |
| 1/4” | 0.250 in (6.35 mm) |
For the numbered sizes the diameter follows a simple rule: 0.060 inch plus 0.013 inch for each step, so #8 is 0.060 + 8 × 0.013 = 0.164 inch.
How to measure a screw
Diameter. Measure across the outside of the thread at its widest point. A caliper is ideal for this. The result will be a touch under the nominal size: an M6 usually measures about 5.8–5.9 mm, because thread tips are slightly flattened.
Length. It depends on the head:
- For flat-topped (countersunk) heads, which sit flush with the surface, measure the whole screw, head included.
- For domed, pan and hex heads, which sit on top of the surface, measure from under the head to the tip.
Thread pitch. Measure across ten threads, from one peak to the peak ten threads further along, and divide by ten. If that’s 8 mm, the pitch is 0.8 mm, and with a 5 mm diameter the screw is an M5. For US screws, the pitch is given as threads per inch (TPI).
Tips
- Metric and US screws of similar size don’t fit each other’s holes. An M5 and a #10 look alike but have different threads.
- If you’re replacing a screw, take the old one to the store, or measure it before you go.
- A screen measurement gets you within about a millimeter. That’s enough to tell an M4 from an M5, but not two pitches apart. For that, use a thread gauge or a real caliper.