List of drill and tap sizes

Rules of thumb

Regarding the proportion of tap drill to thread major diameter, for standard V threads (ISO V thread and UTS V thread), there are several rules of thumb with strong predictive power:

• A good tap drill is 85% (± 2 pp) of major diameter for coarse threads, and 90% (± 2 pp) of major diameter for fine threads.
• For metric V threads, the concept of major minus pitch (i.e., the major or widest diameter of the intended screw in millimeters minus the pitch of the threads of that screw in millimeters per thread) yields a good tap drill diameter. The major minus pitch technique also works for inch-based threads, but you must first calculate the pitch by converting the fraction of threads-per-inch (TPI) into a decimal. For example, a screw with a pitch of 1/20in (20 threads per inch) has a pitch of 0.050in and a in pitch (13 threads per inch) has a pitch of 0.077in. Your result will only land near a tap drill size (not directly on one).
• For both of these rules of thumb (85%/90% and major minus pitch), the tap drill size yielded is not necessarily the only possible one, but it is a good one for general use.
• The 85% and 90% rules works best in the range of 1/4 –, the sizes most important on many shop floors. Some sizes outside that range have different ratios.

Below, these guidelines are explored with examples.

Examples

• Example (inch, coarse): For size (this is the diameter of the intended screw in fraction form)-14 (this is the number of threads per inch; 14 is considered coarse), 0.437in × 0.85 = 0.371in. Therefore, a size screw ( ≈ 0.437) with 14 threads per inch (coarse) needs a tap drill with a diameter of about 0.371 inches.
  • The drill sizes that are near this are letter U (0.368in; 84.2%), 9.5 mm (0.374in; 85.6%), and in (0.375in; 85.8%); any of these will work well.
• Example (inch, fine): For -20 (same diameter as the previous example, but this time with 20 threads per inch, which is considered fine), 0.437in × 0.90 = 0.393in (i.e., if the threads are to be fine, then a slightly larger diameter drill bit should be used before tapping the hole for the screw).
  • The drill sizes that are near this are (0.391in; 89.4%), 10 mm (0.393in; 90%), and letter X (0.397in; 90.8%); any of these will work well.
• Example (metric, coarse): For M7.0×1.0 (an intended screw with a diameter of 7.0 mm and a pitch of 1 mm between each thread, which is considered coarse), 7.0mm × 0.85 = 5.95mm.
  • The drill sizes that are near this are in (85%), 6.0 mm (85.7%), and 6.1 mm (87.1%); any of these will work well.
• Example (metric, fine): For M7.0×0.5, 7.0mm × 0.90 = 6.30.
  • The drill sizes that are near this are 6.3 mm (90%), in (90.7%), 6.4 mm (91.4%), and 6.5 mm (92.9%); any of these will work well.
• For metric V threads, major minus pitch yields a good tap drill diameter.
  • Example (metric, coarse): For M7.0 × 1.0, 7.0 − 1.0 = 6.0
  • Example (metric, fine): For M7.0 × 0.5, 7.0 − 0.5 = 6.5
    • (The 85% coarse, 90% fine guideline, within its effective range, matches this in net effect)
  • The major minus pitch also works for inch-based threads, but you must first determine the pitch by looking at the number of treads per inch (TPI; for example, = 0.050 and ≈ 0.077), and your result will only land near a tap drill size (not directly on one).
  • Example (inch coarse): For -14, 1in ÷ 14 = 0.071in; 0.437in − 0.071in = 0.366in;
    • The drill sizes that are near this are 9.3 mm (0.366in) and letter U (0.368in). In addition, 9.5 mm (0.374in), and in (0.375in) will work well, although major minus pitch by itself does not tell you this; but the 85% ± 2 pp guideline supports it.
  • Example (inch fine): For -20, 1in ÷ 20 = 0.050in; 0.437in − 0.050in = 0.387in;
    • The drill sizes that are near this are letter W (0.386in) and in (0.391in). In addition, 10 mm (0.393in), and letter X (0.397in) will work well, although major minus pitch by itself does not tell you this; but the 90% ± 2 pp guideline supports it.
• For both of these rules of thumb (85%/90% and major minus pitch), the tap drill size yielded is not necessarily the only possible one, but it is a good one for general use.
  • Example (metric coarse): For M7.0×1.0, major minus pitch yields 6.0, but 6.1 also works well.
  • Example (metric fine): For M7.0×0.5, major minus pitch yields 6.5, which at 92.9% happens to be an example that pushes over the outer bound of the 90% ± 2 pp, but major minus pitch is still valid, although smaller drills (6.3 mm, , 6.4 mm) will work well.

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