Inch or metric? Reading bolt sizes like 1/2" x 1-1/2" and M12 x 40
Write the diameter first, the length second, then the thread. M12 x 40 is a 12 mm metric bolt, 40 mm long. 1/2" x 1-1/2" UNC is a half-inch bolt, one and a half inches long, with a coarse unified thread. Most mix-ups come from leaving one of those three out.
Metric sizes
Metric bolts start with M and the diameter in millimetres. The pitch is only written when it is fine: M10 is the coarse 1.5 mm pitch, M10 x 1.25 the fine one used on many vehicle and machine parts. Length is measured under the head for hex bolts, and overall for countersunk screws.
Inch sizes and threads
Inch bolts give the diameter as a fraction and the length in inches. The thread system matters as much as the size:
- UNC (unified coarse): the common inch thread on structural and general bolts, 13 threads per inch on a 1/2" bolt.
- UNF (unified fine): machinery and automotive parts, 20 threads per inch at 1/2".
- BSW (British Standard Whitworth): older equipment, 12 threads per inch at 1/2".
A 1/2" bolt is 12.7 mm across, so it looks almost like an M12 (12 mm), but the threads differ: an M12 nut will not run on a 1/2" bolt, and forcing it ruins both.
Nearest sizes at a glance
| Inch | Millimetres | Nearest metric |
|---|---|---|
| 1/4" | 6.4 | M6 |
| 5/16" | 7.9 | M8 |
| 3/8" | 9.5 | M10 |
| 1/2" | 12.7 | M12 |
| 5/8" | 15.9 | M16 |
| 3/4" | 19.1 | M20 |
| 1" | 25.4 | M24 |
These are the nearest sizes, not swaps: an inch nut and a metric bolt do not interchange. For lengths, 1" is 25.4 mm, 1-1/2" about 38 mm and 2" about 51 mm.
When the list says it the other way round
Lists often arrive as "1 1/2 x 1/2". A bolt one and a half inches thick and half an inch long is rare, so it almost always means 1/2" diameter by 1-1/2" long. Say which you mean, or send a photo beside a tape measure, and we will confirm before quoting.
Not sure which fits your job? Send a photo and the load to us on WhatsApp +254 723 525243, or request a quote with your list.
Inch and metric HT bolts
This hex head bolt is manufactured from steel and conforms to the BS 18.2 (1768) standard. It features a nominal diameter of 1 1/2 inches and a total length of 10 inches with a half-threaded configuration. The fastener is supplied with a black surface finish and an UNC thread series.
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Questions buyers ask
Will an M12 nut fit a 1/2 inch bolt?
No. They are close in size, but the thread pitch and form differ, so the nut binds or strips.
How do I measure a bolt I already have?
Measure the diameter across the threads and the length under the head, then count the threads over 25 mm or one inch, or use a thread gauge. A photo beside a tape measure helps us confirm it.
Do you stock inch bolts?
Yes. We stock HT hex bolts in UNC and UNF threads as well as metric sizes. Send the size and thread and we will confirm what is available.