How to use it
- Pick the thread system: UNC/UNF, metric coarse, metric fine or BSP.
- Type a size (1/4, #10, M8, 10x1.25, G1/2) to filter the table.
- Or type a measured diameter, like 9.5 mm or 0.375 in.
- Read the pitch, minor diameter and tap drill.
How to identify a thread
- Measure the outside diameter of the bolt (the major diameter) with calipers. Type it in, for example
0.31 inor7.9 mm; the chart shows threads within a few percent of that size. - Count threads or measure pitch. Inch threads are specified in TPI (threads per inch): count the crests in one inch, or use a thread gauge. Metric threads use pitch in mm, the distance between two crests.
- Decide between coarse and fine. UNC (coarse) is the default for most hardware; UNF (fine) is used where vibration resistance or fine adjustment matters, such as automotive and aerospace parts.
A bolt that measures about 0.24–0.25 in with 20 TPI is a 1/4-20 UNC; one that measures 7.9 mm with a 1.25 mm pitch is an M8 coarse.
Common UNC and UNF threads with tap drills
| Thread | Major diameter | Tap drill (about 75% thread) |
|---|---|---|
| #6-32 UNC | 0.138 in | #36 (0.1065 in) |
| #8-32 UNC | 0.164 in | #29 (0.136 in) |
| #10-24 UNC | 0.190 in | #25 (0.1495 in) |
| #10-32 UNF | 0.190 in | #21 (0.159 in) |
| 1/4-20 UNC | 0.250 in | #7 (0.201 in) |
| 1/4-28 UNF | 0.250 in | #3 (0.213 in) |
| 5/16-18 UNC | 0.3125 in | F (0.257 in) |
| 3/8-16 UNC | 0.375 in | 5/16 in (0.3125 in) |
| 3/8-24 UNF | 0.375 in | Q (0.332 in) |
| 1/2-13 UNC | 0.500 in | 27/64 in (0.4219 in) |
The chart in the tool lists every size from #0 to 2 in, with the d − P rule-of-thumb tap drill in inches and mm, so you can pick the nearest number, letter or fractional drill.
Metric threads
Metric threads are written as M + diameter in mm, with the pitch added for fine threads: M8 is 8 mm coarse (1.25 mm pitch), M8 × 1 is the fine version. Common coarse threads and tap drills: M3 × 0.5 → 2.5 mm, M4 × 0.7 → 3.3 mm, M5 × 0.8 → 4.2 mm, M6 × 1 → 5 mm, M8 × 1.25 → 6.8 mm, M10 × 1.5 → 8.5 mm, M12 × 1.75 → 10.2 mm.
The tool shows the pitch diameter (d2), the nut's minor diameter (D1) and the bolt's minor diameter (d3) for every size from M1 to M64, calculated from the ISO basic profile.
Pipe threads: BSP vs. NPT
BSP (G) threads are British parallel pipe threads (ISO 228-1, 55° profile), common on European fittings, garden hose adapters outside the US, and imported equipment. They seal with a washer or O-ring, not on the thread. The size name is nominal: a G 1/2 has an outside diameter of 20.955 mm (0.825 in), not half an inch.
US plumbing uses NPT, a tapered thread with a 60° profile that seals on the thread. BSP and NPT share size names and, for 1/2 and 3/4 in, the same 14 TPI, but they don't seal reliably together. Use an adapter rather than forcing one into the other.
About the numbers
Inch thread data follows the Unified (UN) standard; metric pitches follow ISO 261, with basic diameters from the ISO 68-1 profile. Values are basic sizes without tolerances. Tap drills give roughly 75% thread engagement; harder materials may call for a slightly larger drill, and a tap maker's chart can differ by a few thousandths of an inch. To convert individual measurements, use the inches to mm converter.
Frequently asked questions
What does 1/4-20 mean?
A 1/4 in diameter thread with 20 threads per inch. It's the coarse (UNC) 1/4 in thread; the fine version is 1/4-28 UNF.
What size drill for a 1/4-20 tap?
A #7 drill (0.201 in) for about 75% thread. 13/64 in (0.203 in) also works.
What is the difference between UNC and UNF?
UNC is coarse (fewer threads per inch), the general-purpose standard. UNF is fine, used where vibration or precise adjustment matters.
Is M6 the same as 1/4 inch?
No. M6 is 6 mm (0.236 in) with a 1 mm pitch; 1/4-20 is 6.35 mm with a 1.27 mm pitch. They look similar but don't interchange.
How do I measure thread pitch without a gauge?
Count crests over 1 in for TPI, or measure across 10 crests in mm and divide by 10 for metric pitch.
Last updated: November 12, 2026