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Tap drill sizes and thread reference
Pick a thread family, then a size, and it gives you everything about that thread at once: its standard designation and the standard that defines it, the thread form and angle, pitch, major, pitch and minor diameters, the tap drill, and the starting blank for rolling an external thread. A cutting tap and a forming tap need different holes for the same thread, and getting that wrong is how taps break — so the tool asks which you are using rather than assuming.
The reference below is free to read — no account, no signup. The tool itself is inside MechCircle, which is free to start.
Coarse-pitch metric tap drill sizes
| Thread | Pitch | Cutting tap at 75% | Forming tap at 70% | Nearest drill (cutting) |
|---|---|---|---|---|
| M3 × 0.6 | 0.60 mm | 2.42 mm | 2.76 mm | 2.40 mm |
| M4 × 0.75 | 0.75 mm | 3.27 mm | 3.70 mm | 3.30 mm |
| M5 × 0.9 | 0.90 mm | 4.12 mm | 4.64 mm | 4.10 mm |
| M6 × 1 | 1.00 mm | 5.03 mm | 5.60 mm | 5.00 mm |
| M8 × 1.25 | 1.25 mm | 6.78 mm | 7.50 mm | 6.80 mm |
| M10 × 1.5 | 1.50 mm | 8.54 mm | 9.40 mm | 8.50 mm |
| M12 × 1.75 | 1.75 mm | 10.30 mm | 11.30 mm | 10.30 mm |
| M14 × 2 | 2.00 mm | 12.05 mm | 13.21 mm | 12.10 mm |
| M16 × 2 | 2.00 mm | 14.05 mm | 15.21 mm | 14.10 mm |
| M20 × 2.5 | 2.50 mm | 17.56 mm | 19.01 mm | 17.60 mm |
A forming (roll) tap always needs a LARGER hole than a cutting tap for the same thread and engagement, because it displaces material instead of removing it. The tool covers 91 metric size-and-pitch combinations, not only the coarse pitches shown here.
BSP parallel threads (G) — ISO 228-1
| Designation | TPI | Pitch (mm) | Major Ø (mm) | Pitch Ø (mm) | Minor Ø (mm) | Tap drill (mm) |
|---|---|---|---|---|---|---|
| G 1/16 | 28 | 0.907 | 7.723 | 7.142 | 6.561 | — |
| G 1/8 | 28 | 0.907 | 9.728 | 9.147 | 8.566 | 8.70 |
| G 1/4 | 19 | 1.337 | 13.157 | 12.301 | 11.445 | 11.60 |
| G 3/8 | 19 | 1.337 | 16.662 | 15.806 | 14.950 | 15.00 |
| G 1/2 | 14 | 1.814 | 20.955 | 19.793 | 18.632 | 19.00 |
| G 5/8 | 14 | 1.814 | 22.911 | 21.749 | 20.588 | 20.75 |
| G 3/4 | 14 | 1.814 | 26.441 | 25.279 | 24.118 | 24.50 |
| G 7/8 | 14 | 1.814 | 30.201 | 29.039 | 27.878 | 28.00 |
| G 1 | 11 | 2.309 | 33.249 | 31.770 | 30.292 | 30.50 |
| G 1-1/8 | 11 | 2.309 | 37.897 | 36.418 | 34.940 | 35.00 |
| G 1-1/4 | 11 | 2.309 | 41.910 | 40.431 | 38.953 | 39.50 |
| G 1-3/8 | 11 | 2.309 | 44.323 | 42.844 | 41.366 | 41.50 |
| G 1-1/2 | 11 | 2.309 | 47.803 | 46.324 | 44.846 | 45.00 |
| G 1-3/4 | 11 | 2.309 | 53.746 | 52.267 | 50.789 | 51.00 |
| G 2 | 11 | 2.309 | 59.614 | 58.135 | 56.657 | 57.00 |
| G 2-1/4 | 11 | 2.309 | 65.710 | 64.231 | 62.753 | 63.00 |
| G 2-1/2 | 11 | 2.309 | 75.184 | 73.705 | 72.227 | 72.50 |
| G 2-3/4 | 11 | 2.309 | 81.534 | 80.055 | 78.577 | 79.00 |
| G 3 | 11 | 2.309 | 87.884 | 86.405 | 84.927 | 85.50 |
| G 3-1/4 | 11 | 2.309 | 93.980 | 92.501 | 91.023 | 91.00 |
| G 3-1/2 | 11 | 2.309 | 100.330 | 98.851 | 97.373 | 97.75 |
| G 3-3/4 | 11 | 2.309 | 106.680 | 105.201 | 103.723 | 104.00 |
| G 4 | 11 | 2.309 | 113.030 | 111.551 | 110.073 | 110.50 |
| G 4-1/2 | 11 | 2.309 | 125.730 | 124.251 | 122.773 | 123.00 |
| G 5 | 11 | 2.309 | 138.430 | 136.951 | 135.473 | 136.00 |
| G 5-1/2 | 11 | 2.309 | 151.130 | 149.651 | 148.173 | 148.50 |
| G 6 | 11 | 2.309 | 163.830 | 162.351 | 160.873 | 161.50 |
Pitch and minor diameters are derived from the major diameter and TPI (p = 25.4 ÷ TPI, H1 = 0.6403 × p, pitch Ø = major − H1, minor Ø = major − 2 × H1), never copied from a table. The tap drills are the published figures; none is published for G 1/16, so none is shown. BSP taper (R, Rc, Rp — ISO 7-1) shares these majors and TPI at the gauge plane, but a taper thread is gauged rather than measured to one diameter, so it carries no tap drill.
How to use it
Choose the family — metric, inch, BSP parallel or BSP taper — then search for the size. Every pitch is listed as its own entry, so M10 × 1.25 can never be mistaken for M10 × 1.5. For metric and inch threads, pick cutting or forming and adjust the engagement if you want something other than the default; the arithmetic is shown so you can check it against the chart on your wall. Inch results carry their millimetre equivalent, and the nearest metric drill, because an inch drawing still gets drilled from the metric cabinet.
What it does and does not cover
- Metric and inch sizes show the nearest drill using a step rule: 0.05 mm below 3 mm, 0.1 mm above. That is a rule, not a stock-drill index. Number (#1 to #80) and letter (A to Z) inch drill designations are not included.
- BSP parallel (G) tap drills are the published figures, used as given — the engagement formulas describe the 60° form and are not applied to a 55° Whitworth thread. No drill is published for G 1/16, so none is shown.
- BSP taper (R, Rc, Rp) is shown with its diameters at the gauge plane and its taper, but carries no tap drill and no minimum or maximum length: a taper thread is gauged, not measured to a single diameter, and the gauge lengths live in ISO 7-1.
- Metric and inch threads show a pitch diameter but no minor diameter — the minor depends on the root truncation of the class, which is not held here, and the tap drill is the number you actually need.
- Inch threads are listed by size and TPI without a UNC or UNF designation until that mapping is supplied — it is not inferred from the TPI.
- The external rolling blank is a starting figure, never a finished answer: begin trials above it and reduce until the ring gauge of the required class passes.
- Confirm the final hole with a tapping test in the actual material.
Ready to get started?
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