Metric
M8 × 30
- M metric thread
- 8 thread diameter, 8 mm
- 30 length, 30 mm
Size guide
Fastener sizes made simple
How to read a size, how to measure the one you have, and the spanner, hex key and drill sizes for every metric thread we stock.
Read a size in five seconds
Every size is the thread first, then the length. Our product titles write the × as an X, so M8 × 30 appears as M8 X 30.
Metric fine thread
M10 × 1.25
- 10 thread diameter, 10 mm
- 1.25 pitch in mm, not a length
- Standard M10 is 1.5, so this needs a fine thread bolt
Imperial
1/4" UNC × 1"
- 1/4" thread diameter, a quarter inch
- UNC thread type (Unified coarse)
- 1" length, one inch. 1.1/2" means one and a half inches
Wood & self-tapping
4.0 × 40
- 4.0 screw diameter, 4 mm. No M, because it is not a machine thread
- 40 length, 40 mm
Measure the one you have
1
Diameter
Measure across the outside of the thread with a caliper. A metric thread measures a little under its size, so an M8 bolt reads about 7.8 to 7.9 mm.
2
Pitch
Use a thread gauge, or measure across 10 thread peaks and divide by 10. 12.5 mm across 10 peaks is a 1.25 mm pitch, standard for M8. If the numbers come out uneven, count threads per inch instead: it may be imperial.
3
Length
Where you measure from depends on the head. Flush heads include the head, everything else is measured from under the head.
Metric size picker
Pick a thread size to see its pitch, the spanner and hex key it takes, and the holes to drill. These are standard ISO values for the sizes we stock.
See the full metric chart
| Size | Coarse pitch | Fine pitches we stock | Spanner | Hex key (socket cap) | Clearance hole | Tap drill |
|---|
All figures in mm. Spanner sizes are for ISO hex heads and nuts; older DIN parts can take the size shown in brackets. The clearance hole is the normal (medium) fit, and the tap drill is for the coarse thread.
Wood & self-tapping screw gauges
Older plans and American parts give screw sizes as a gauge number (No. 8, often written #8). Our screws are sized in millimetres, so here is how they line up. A bigger gauge number means a thicker screw.
| Gauge | Diameter | Self-tapping screw | Closest wood screw |
|---|---|---|---|
| No. 4 | 2.8 mm | 2.9 mm | 3.0 mm |
| No. 6 | 3.5 mm | 3.5 mm | 3.5 mm |
| No. 7 | 3.8 mm | 3.9 mm | 4.0 mm |
| No. 8 | 4.2 mm | 4.2 mm | 4.0 or 4.5 mm |
| No. 10 | 4.8 mm | 4.8 mm | 5.0 mm |
| No. 12 | 5.5 mm | 5.5 mm | 6.0 mm |
| No. 14 | 6.2 mm | 6.3 mm | 6.0 mm |
Wood and self-tapping screws cut their own thread, so a close match works where a machine thread would not. In our size filter they sit under the nearest M size, so 4.0 mm wood screws appear under M4.
Nuts & washers sized by the bolt
Nuts
A nut has a thread size but no length. An M10 nut fits an M10 bolt with the same pitch. A plain M10 nut is coarse (1.5 mm); if the title says M10 × 1.25 or M10 × 1, it is a fine thread nut.
Washers
A washer is named after the bolt it fits, so the hole is a little bigger than the size. An M10 flat washer has a 10.5 mm hole. Penny and repair washers have the same hole with a much larger outside diameter, to spread the load.
Imperial at a glance
Imperial sizes are in inches. They look close to some metric sizes but they never interchange: the diameter and the thread spacing are both different, so a 5/16" bolt will jam or strip an M8 nut.
| Thread | Written as | Where you find it |
|---|---|---|
| UNC (coarse) | 1/4" UNC, 10-24 UNC | American and Canadian equipment, machinery, vehicles |
| UNF (fine) | 1/4" UNF, 10-32 UNF | American vehicles, aircraft, instruments |
| BSW (Whitworth) | 1/4" BSW | Older British machinery, buildings, gates and railings |
| BSF (British fine) | 1/4" BSF | Classic British cars, motorbikes and machine tools |
| BA | 4BA, 6BA | Electronics, clocks, model engineering, instruments |
Inch and mm converter
Size questions
What does M8 mean on a bolt?
M8 is a metric thread with an 8 mm diameter. It says nothing about length: an M8 × 30 bolt is 8 mm thick and 30 mm long. Standard M8 has a 1.25 mm pitch and takes a 13 mm spanner.
How is the length of a screw measured?
From under the head to the end for hex, pan, socket cap and button heads. Countersunk screws are measured overall because the head sits flush. Raised countersunk screws are measured from the rim of the head, leaving out the dome.
Will an M8 nut fit a 5/16" bolt?
No. They are within a fraction of a millimetre in diameter, but the threads are spaced differently. The nut may start, then bind and strip after a turn or two. Match metric with metric and imperial with the same imperial thread type.
What size spanner does an M10 bolt need?
16 mm for ISO hex heads and nuts. Older DIN 933 and DIN 934 parts use 17 mm. M12 is 18 mm (DIN 19 mm) and M6 is 10 mm.
What is the difference between coarse and fine thread?
Fine threads have more turns per millimetre, so they adjust more precisely and resist shaking loose. Coarse threads are standard, quicker to fit and less likely to cross-thread. A nut must match the bolt's pitch.
What size hole should I drill for an M6 bolt?
6.6 mm for a normal clearance hole. To tap an M6 thread, drill 5.0 mm. The picker above gives both figures for every metric size.