How to read thread sizes (inch and metric)
Last updated 2026-09-30
A fastener size tells you three things: the diameter, the thread and the length.
Inch sizes: 3/8"-16 x 1-1/2"
- 3/8": the nominal diameter of the threaded shank.
- 16: threads per inch (TPI).
- 1-1/2": the length.
Small screws use numbers instead of fractions: #4, #6, #8, #10 and #12 get larger as the number goes up (a #10 is about 3/16"). A #10-24 is a #10 diameter with 24 threads per inch.
Coarse (UNC) or fine (UNF)?
Most diameters come in a coarse and a fine thread. Coarse (UNC) is the everyday standard: faster to assemble and more forgiving of dirt and damage. Fine (UNF) has more threads per inch, gives finer adjustment and is common in automotive and precision work. A nut must match both the diameter and the thread of its bolt.
| Diameter | Coarse (UNC) | Fine (UNF) |
|---|---|---|
| 1/4" | 1/4"-20 | 1/4"-28 |
| 5/16" | 5/16"-18 | 5/16"-24 |
| 3/8" | 3/8"-16 | 3/8"-24 |
| 7/16" | 7/16"-14 | 7/16"-20 |
| 1/2" | 1/2"-13 | 1/2"-20 |
| 5/8" | 5/8"-11 | 5/8"-18 |
| 3/4" | 3/4"-10 | 3/4"-16 |
Metric sizes: M8 x 1.25 x 30
- M8: 8 mm diameter.
- 1.25: the thread pitch, the distance between threads in millimeters. Larger means coarser.
- 30: length in millimeters.
When the pitch isn't written (for example "M8 x 30"), it's usually the standard coarse pitch: M6 x 1.0, M8 x 1.25, M10 x 1.5, M12 x 1.75, M16 x 2.0.
How length is measured
For most bolts and screws, length is measured from under the head to the tip. For flat-head (countersunk) screws, which sit flush, it's the overall length including the head.
Find it on our site
On any fasteners page, use the Diameter, Thread size (for example 3/8"-16 or M8 x 1.25), Thread type (coarse, fine, metric) and Length filters. Unsure what you have? Bring or describe a sample: call (908) 824-0916. See also hex bolt grades explained.