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Screw Size Chart
Short answer
A screw size is its major diameter plus its pitch: M5 x 0.8 is 5.0 mm across the thread crests with 0.8 mm between them, and 1/4-20 is 0.250 inch across the crests with twenty threads per inch. The tables below give both series with the coarse and fine pitch, the tap drill, the nut bore you would measure, and the spanner size.
Metric coarse and fine series (ISO 261/262)
| Size | Major ⌀ (mm) | Coarse pitch (mm) | Fine pitch (mm) | Tap drill (mm) | Nut bore (mm) | Spanner (mm) |
|---|---|---|---|---|---|---|
| M1.6 | 1.6 | 0.35 | — | 1.25 | 1.22 | 3.2 |
| M2 | 2.0 | 0.40 | — | 1.60 | 1.57 | 4.0 |
| M2.5 | 2.5 | 0.45 | — | 2.05 | 2.01 | 5.0 |
| M3 | 3.0 | 0.50 | 0.35 | 2.50 | 2.46 | 5.5 |
| M3.5 | 3.5 | 0.60 | 0.35 | 2.90 | 2.85 | 6.0 |
| M4 | 4.0 | 0.70 | 0.50 | 3.30 | 3.24 | 7.0 |
| M5 | 5.0 | 0.80 | 0.50 | 4.20 | 4.13 | 8.0 |
| M6 | 6.0 | 1.00 | 0.75 | 5.00 | 4.92 | 10.0 |
| M8 | 8.0 | 1.25 | 1.00 | 6.75 | 6.65 | 13.0 |
| M10 | 10.0 | 1.50 | 1.25 | 8.50 | 8.38 | 17.0 |
| M12 | 12.0 | 1.75 | 1.25 | 10.25 | 10.11 | 19.0 |
| M14 | 14.0 | 2.00 | 1.50 | 12.00 | 11.84 | 22.0 |
| M16 | 16.0 | 2.00 | 1.50 | 14.00 | 13.84 | 24.0 |
| M20 | 20.0 | 2.50 | 1.50 | 17.50 | 17.29 | 30.0 |
Unified inch series (ASME B1.1)
| Size | Major ⌀ (in) | Major ⌀ (mm) | UNC (TPI) | UNF (TPI) | Spanner (mm) |
|---|---|---|---|---|---|
| #0 | 0.06 | 1.52 | — | 80 | — |
| #1 | 0.073 | 1.85 | 64 | 72 | — |
| #2 | 0.086 | 2.18 | 56 | 64 | — |
| #3 | 0.099 | 2.52 | 48 | 56 | — |
| #4 | 0.112 | 2.85 | 40 | 48 | — |
| #5 | 0.125 | 3.17 | 40 | 44 | — |
| #6 | 0.138 | 3.50 | 32 | 40 | — |
| #8 | 0.164 | 4.17 | 32 | 36 | — |
| #10 | 0.19 | 4.83 | 24 | 32 | — |
| #12 | 0.216 | 5.49 | 24 | 28 | — |
| 1/4" | 0.25 | 6.35 | 20 | 28 | 11.11 |
| 5/16" | 0.3125 | 7.94 | 18 | 24 | 12.70 |
| 3/8" | 0.375 | 9.53 | 16 | 24 | 14.29 |
| 7/16" | 0.4375 | 11.11 | 14 | 20 | 17.46 |
| 1/2" | 0.5 | 12.70 | 13 | 20 | 19.05 |
| 9/16" | 0.5625 | 14.29 | 12 | 18 | 22.23 |
| 5/8" | 0.625 | 15.88 | 11 | 18 | 23.81 |
Metric and inch near neighbours
| Metric | Nearest inch | Gap (mm) | By diameter alone |
|---|---|---|---|
| M1.6 × 0.35 | #0 | 0.08 | Too close to call |
| M2 × 0.40 | #1-64 | 0.15 | Too close to call |
| M2.5 × 0.45 | #3-48 | 0.02 | Too close to call |
| M3 × 0.50 | #4-40 | 0.15 | Too close to call |
| M3.5 × 0.60 | #6-32 | 0.00 | Too close to call |
| M4 × 0.70 | #8-32 | 0.17 | Too close to call |
| M5 × 0.80 | #10-24 | 0.17 | Too close to call |
| M6 × 1.00 | 1/4"-20 | 0.35 | Distinguishable |
| M8 × 1.25 | 5/16"-18 | 0.06 | Too close to call |
| M10 × 1.50 | 3/8"-16 | 0.47 | Distinguishable |
| M12 × 1.75 | 1/2"-13 | 0.70 | Distinguishable |
| M14 × 2.00 | 9/16"-12 | 0.29 | Too close to call |
| M16 × 2.00 | 5/8"-11 | 0.13 | Too close to call |
| M20 × 2.50 | 5/8"-11 | 4.13 | Distinguishable |
How to read a designation
A metric designation runs diameter x pitch: M5 x 0.8 is 5 mm across the crests with 0.8 mm between them. When the pitch is left off, as in "an M5 screw", the coarse pitch is implied — that is the row marked coarse below.
An inch designation runs diameter-threads per inch: 1/4-20 is a quarter inch across the crests with twenty threads in an inch. Numbered sizes below a quarter inch use a gauge instead of a fraction, where the major diameter is 0.060 in + 0.013 in per gauge number. That is why a #8 is 0.164 in.
Tap drill sizes
The tap drill column is the standard rule for a roughly 75 percent thread: drill diameter equals the major diameter minus the pitch. An M6 x 1 thread wants a 5.0 mm hole, an M8 x 1.25 wants 6.75 mm.
A 75 percent thread is the usual compromise. Going closer to a full thread means a smaller hole, more torque to tap and a much higher risk of snapping the tap in the work.
Measuring a nut instead of a screw
When only the nut is left, measure across the flats to find the spanner size, then read the size off the across-flats column. A 13 mm spanner means an M8 thread; a 17 mm spanner means M10.
The nut bore column is the other route: measure the hole through the nut and match it against the minor diameter. It is less exact than across-flats, because the bore is the minor diameter of the internal thread rather than a flat surface, but it works when the nut is a plain round one.
Frequently asked questions
What does the M in M6 mean?
It marks the ISO metric thread series, and the number after it is the nominal major diameter in millimetres. M6 is 6 mm across the thread crests.
What size screw is 4.8 mm across the crests?
Nothing in the metric series — the nearest are M5 at 5.0 mm and M4.5, which is not a common size. 4.826 mm is exactly a #10 in the numbered inch series, so a reading of 4.8 mm on an unmarked screw is far more likely to be a #10 than an M5.
Which metric sizes have a fine pitch?
From M3 upward, most sizes have a fine variant in general use: M4 x 0.5, M6 x 0.75, M8 x 1, M10 x 1.25. Below M3 the coarse pitch is already fine enough that a separate series is rarely stocked.
Are coarse and fine threads interchangeable?
No. They share a diameter and nothing else. An M10 x 1.25 bolt will enter an M10 x 1.5 nut and lock solid within about two turns, damaging both.
Other tools
Would rather just take a photo?
These charts are the same tables the app matches against. Screw Identifier does the measuring for you: photograph the fastener next to a ruler, or measure it on the screen against a calibrated on-screen caliper.
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