Thread Pitch and TPI Converter

Metric pitch to threads per inch and back, with the standard ISO coarse and fine pitches and the UNC/UNF series so you can identify what you are holding — plus the traps: BSP is 55° not 60°, a BSP size is a bore, and M5×0.8 will start in a #10-32 hole.

The conversion is one line; the standards are the page

ISO metric: which pitch is standard at each diameter

Unified inch threads, with their pitches in millimetres

BSP: 55°, and the size is a bore rather than a diameter

BSP against NPT — the pair that will engage and must not

Near misses, computed from the two standards’ own values

One line of arithmetic, and three standards’ worth of everything else

The conversion itself is one line and there is no measurement in it. An inch is exactly 25.4 mm, so threads per inch is 25.4 divided by the pitch in millimetres, and back again. A 1 mm pitch is 25.4 tpi; 20 tpi is 1.27 mm. If that were all a reader needed, this page would not be worth writing. What they actually need is the two things around it: which pitch is standard at a given diameter, and which near-identical threads will engage without holding.

Standard pitch is a decision, not a formula. ISO 261 and ISO 262 record which coarse and fine pitches belong at each metric diameter, and that is why an M8 with nothing after it means M8 × 1.25 while a fine M8 × 1 must be written out. ASME B1.1 does the same job for UNC and UNF. All three are copyrighted; this page cites them by number, takes the pitch values from named engineering references, and computes everything else. Look down the TPI column of the metric table and you will see the structural reason the two systems can never share a tap: almost no standard metric pitch is a whole number of threads per inch, and none of the few that come close coincides with a standard UNC or UNF pitch.

The flank angle matters as much as the pitch, and this is where pipe threads bite. Metric, UNC, UNF and NPT threads are all 60° included, with flattened crests and roots. BSW, BSPP and BSPT are 55°, with rounded crests and roots. So two threads can share a pitch and still be different threads. The worked case is 1/2 inch: BSP and NPT both run 14 threads per inch, their major diameters are 20.955 and 21.34 mm — under two per cent apart — and NPT is tapered where BSPP is parallel. A fitting will start, feel right, and neither seal nor hold. The reference used here says so in terms: the two are “not compatible due to the differences in their thread forms, and not just the fact that most sizes have a different pitch”.

A BSP size is not a diameter either. It is a historic pipe bore, so a “1/2 inch BSP” measures 20.955 mm across the crests — 1.65 times the half-inch in its name. The overstatement is worst at the small end, where a 1/16 BSP is nearly five times its own label, and shrinks towards the large sizes. If you are identifying a fitting with calipers, measure it and look the diameter up; the name will mislead you every time.

The near-miss table on this page is computed rather than quoted. Every standard metric pitch was compared against every standard UNC and UNF pitch and the pairs within six per cent on both diameter and pitch kept. The one worth memorising is M5 × 0.8 against #10-32 UNF: pitches of 0.8 and 0.7937 mm, eight thousandths of a millimetre apart, so a bolt runs in several turns before it binds and by then the damage is done. It also corrects a piece of folklore. M6 × 1.0 and 1/4-20 UNC are frequently named as an interchangeable-feeling pair, and the arithmetic says otherwise: the diameters are 5.5% apart, which lets the M6 enter, but the pitches are 1.0 against 1.27, 21% apart. It cross-threads and strips; it does not run in. Same wrecked hole, different mechanism, and worth knowing which.

What is deliberately not here. Tap drill sizes and thread engagement depth belong to the drill bit size converter, which already owns that calculation, so this page gives the thread heights and links rather than rebuilding it. Acme and trapezoidal forms and pipe-thread taper dimensions are absent because no clean published table for them could be verified, and printing them from memory would be worse than the gap. For the fasteners themselves see the wood screw size converter and the nail size converter.

Frequently asked questions

Related calculators

References

1. ISO 261, ISO general purpose metric screw threads — General plan, ISO 262 (selected sizes) and ISO 724 (basic dimensions). All copyrighted; none fetched and none reproduced. They are cited by number for what they are: the documents that decide which pitch is standard at a given diameter, which is the only part of this page that is not arithmetic.
2. Wikipedia, ISO metric screw thread (read 27 September 2026), the secondary the pitch values here were read from — its table of the ISO 262 preferred sizes, M1 to M36, with the coarse pitch and the fine pitches for each. It also gives the 60° flank angle, the fundamental triangle height H = P × √3/2, and the rule that “the coarse pitch is the commonly used default pitch for a given diameter” and may be omitted from the designation. Its internal thread height is printed as “0.54125 × pitch”; the exact value from the profile is 5/8 × √3/2 = 0.5412659, and this page uses the exact one.
3. ASME B1.1, Unified Inch Screw Threads (UN, UNR and UNJ Thread Form). Copyrighted, not fetched, not reproduced. The UNC and UNF designations used here were read from The Engineering ToolBox, Unified Thread Standards: UNC & UNF Size Charts & Specifications (read 27 September 2026), both columns complete from #0 to 1½ in.
4. Hydraulic Insight, BSP Thread Size Chart: BSPP and BSPT Dimensions (read 27 September 2026), for the BSP table — nominal size, TPI, pitch and major diameter in millimetres from 1/16 to 2 in — and for the two facts the page exists for: “BSP threads use an included angle of 55 degrees” and “A BSP size is a nominal designation, not the actual outside diameter of the thread. The nominal size originated from historical pipe-bore dimensions”.
5. Valves Online, Explanation of common threads including BSP & NPT (read 27 September 2026), the independent check, agreeing at every shared BSP row and adding the NPT column. BSP: “the same pitch, angle (55 degrees) and shape (rounded peaks and valleys)”. NPT: “the same pitch, angle (60 degrees) and shape (flat peaks and valleys)”. And the conclusion: “NPT/NPS and BSP threads are not compatible due to the differences in their thread forms, and not just the fact that most sizes have a different pitch.”
6. ISO 228 (parallel pipe threads, the BSPP family) and ISO 7 / BS 21 (taper, the BSPT family), and ASME B1.20.1 for NPT. Cited by number only: all four are copyrighted, none was fetched, and this page deliberately carries no taper dimensions at all because no clean published table for them could be verified.