Thread Pitch Calculator
Convert threads per inch to pitch and back, and look up the major, pitch and minor diameters and tap drill for UNC, UNF and ISO metric threads. Every dimension is worked out from the 60° basic profile, with the formulas shown.
Your numbers
Threads per inch from a thread gauge or a drawing, or a pitch in mm or inches.
Pitch
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- Pitch in inches
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- Closest UNC/UNF thread
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- Closest ISO metric pitch
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Basic dimensions from the 60° profile, the same for Unified and ISO metric threads. Real parts sit inside tolerances (class 2A/2B, 6g/6H): check limits in the standard or the maker's chart before you gauge or machine.
The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.
How the thread pitch calculator works
Pitch is the distance from one thread crest to the next. Inch threads are named by threads per inch (TPI), metric threads by the pitch in millimeters, and one is 25.4 divided by the other. Pick TPI ↔ pitch to convert, or pick a UNC/UNF or metric size to see its dimensions.
Unified and ISO metric threads share the same 60° basic profile. From the major diameter D and the pitch P, the height of the sharp V is H = 0.866025 × P, the pitch diameter is D − 0.649519 × P, the basic minor diameter of the nut is D − 1.082532 × P and the minor diameter of the bolt is D − 1.226869 × P. The page computes these from the formulas; for the inch sizes they match the basic dimensions printed in NBS Handbook H28, Tables 2.8 and 2.9, to the fourth decimal.
Common thread sizes and tap drills
Worked out with the same formulas as the calculator. Inch tap drills are for 75% thread; metric tap drills are major minus pitch.
| Size | TPI | Pitch | Minor (nut) | Tap drill |
|---|---|---|---|---|
| 1/4-20 UNC | 20 | 1.27 mm | 0.1959 in | 0.2013 in (13/64) |
| 1/4-28 UNF | 28 | 0.907 mm | 0.2113 in | 0.2152 in (7/32) |
| 5/16-18 UNC | 18 | 1.411 mm | 0.2524 in | 0.2584 in (17/64) |
| 5/16-24 UNF | 24 | 1.058 mm | 0.2674 in | 0.2719 in (17/64) |
| 3/8-16 UNC | 16 | 1.588 mm | 0.3073 in | 0.3141 in (5/16) |
| 3/8-24 UNF | 24 | 1.058 mm | 0.3299 in | 0.3344 in (21/64) |
| 7/16-14 UNC | 14 | 1.814 mm | 0.3602 in | 0.3679 in (3/8) |
| 7/16-20 UNF | 20 | 1.27 mm | 0.3834 in | 0.3888 in (25/64) |
| 1/2-13 UNC | 13 | 1.954 mm | 0.4167 in | 0.4251 in (27/64) |
| 1/2-20 UNF | 20 | 1.27 mm | 0.4459 in | 0.4513 in (29/64) |
| 5/8-11 UNC | 11 | 2.309 mm | 0.5266 in | 0.5364 in (17/32) |
| 5/8-18 UNF | 18 | 1.411 mm | 0.5649 in | 0.5709 in (37/64) |
| 3/4-10 UNC | 10 | 2.54 mm | 0.6417 in | 0.6526 in (21/32) |
| 3/4-16 UNF | 16 | 1.588 mm | 0.6823 in | 0.6891 in (11/16) |
| 7/8-9 UNC | 9 | 2.822 mm | 0.7547 in | 0.7667 in (49/64) |
| 7/8-14 UNF | 14 | 1.814 mm | 0.7977 in | 0.8054 in (13/16) |
| 1-8 UNC | 8 | 3.175 mm | 0.8647 in | 0.8782 in (7/8) |
| 1-12 UNF | 12 | 2.117 mm | 0.9098 in | 0.9188 in (59/64) |
| Size | TPI | Pitch | Minor (nut) | Tap drill |
|---|---|---|---|---|
| M3 | 50.8 | 0.5 mm | 2.459 mm | 2.5 mm (0.098 in) |
| M4 | 36.29 | 0.7 mm | 3.242 mm | 3.3 mm (0.13 in) |
| M5 | 31.75 | 0.8 mm | 4.134 mm | 4.2 mm (0.165 in) |
| M6 | 25.4 | 1 mm | 4.917 mm | 5 mm (0.197 in) |
| M8 | 20.32 | 1.25 mm | 6.647 mm | 6.75 mm (0.266 in) |
| M10 | 16.93 | 1.5 mm | 8.376 mm | 8.5 mm (0.335 in) |
| M12 | 14.51 | 1.75 mm | 10.106 mm | 10.25 mm (0.404 in) |
| M14 | 12.7 | 2 mm | 11.835 mm | 12 mm (0.472 in) |
| M16 | 12.7 | 2 mm | 13.835 mm | 14 mm (0.551 in) |
| M20 | 10.16 | 2.5 mm | 17.294 mm | 17.5 mm (0.689 in) |
| M24 | 8.47 | 3 mm | 20.752 mm | 21 mm (0.827 in) |
Tap drill and percent of thread
For inch threads the page gives the hole for a percent of thread you choose, 75% by default, the common shop rule. NBS Handbook H28's tap drill tables count percent of thread against 0.75H, so a 100% thread would be a hole of D − 1.299 × P. It also shows the nearest fractional drill in 64ths and the percent that drill gives.
For metric threads the page uses the usual rule, major diameter minus pitch, which works out to about 77% of thread. Drills tend to cut slightly oversize, and harder materials or deep holes may need a slightly bigger drill; the tap maker's chart and the internal thread's minor diameter limits decide for critical parts.
Threads that look alike
Some inch and metric pitches are close: 20 TPI is 1.27 mm against the metric 1.25 mm, and 16 TPI is 1.5875 mm against 1.5 mm. A nut may start on the wrong bolt and then bind. When a converted pitch is within 3% of the nearest standard thread in the other system, the page says so; check with a thread pitch gauge and a caliper on the major diameter before you order.
For fastener, tooling and industrial sales
A customer on the plant floor reads you a thread off a worn part, a drawing or a gauge: "three-eighths sixteen", "M12 fine", "it's about 1.25 pitch". Write down the size, the pitch or TPI, the series and the class, and confirm whether it is inch or metric before you quote fasteners, taps or inserts.
Sources, each read on 2026-11-01. H28 is the federal edition of the Unified standard that ASME B1.1 carries on; the ISO standards themselves are sold, so the metric dimensions are computed from the basic profile:
- NBS Handbook H28 (1969), Part I: Unified screw threads, Tables 2.8 and 2.9 and Appendix A3 (tap drills)
- ISO metric screw thread: ISO 68-1 basic profile and ISO 262 sizes and pitches (Wikipedia summary)
- Engineering ToolBox, ISO 724 metric threads (coarse pitches and tap drills, cross-check)
Related: the bolt torque calculator for the tightening torque and clamp load of a bolt (this page gives the thread's geometry, not torque), and the feeds and speeds calculator for drilling the tap hole.
More for manufacturing and industrial sales: Manufacturers' rep, Industrial distributor, Machine tool sales.
Thread specs get read out on the plant floor
On a plant visit the customer reads a thread off a worn part or a drawing, along with a dozen other part numbers. Record the visit in ParrotNotes and you get a transcript and a summary, so the size, pitch and series are in writing when you quote.
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