Workshop and Metalwork Calculators
Tap drill size.
Pick a common thread or enter the major diameter and the pitch. You get the tap drill from the major-minus-pitch rule and from the 85 or 90 percent rule, plus the minor diameter and thread depth. Runs in your browser; nothing is sent or stored.
Inputs
Result
Fill the form and press Calculate.
Answer first. For a metric thread, tap drill = major diameter minus pitch (M6 x 1.0: 6 - 1 = 5.0 mm). For an inch thread, subtract 1 divided by threads per inch (1/4-20: 0.250 - 0.050 = 0.200 in).
Worked example
M6 x 1.0 (coarse): major minus pitch = 6.0 - 1.0 = 5.0 mm. The 85% rule gives 6.0 x 0.85 = 5.1 mm. Both are accepted rules of thumb. A 1/4-20 thread works out as 0.250 - 1/20 = 0.200 in, the nearest fractional drill being 13/64 in (0.203 in).
How it works
Two rules, one answer. "Major minus pitch" works for metric threads and, once the pitch is converted from threads per inch, for inch threads. A second rule says a good tap drill is about 85 percent of the major diameter for coarse threads and 90 percent for fine threads, give or take 2 points. Both give a general-purpose drill, not the only possible one; if they differ, a drill between them is normal.
Geometry. For ISO metric threads the minor diameter is the major diameter minus 1.082532 times the pitch, and the thread depth is 0.54125 times the pitch. The tap drill sits slightly above the minor diameter, which is why the thread is not 100 percent deep.
Choosing the drill. The number is a target. Use the standard drill nearest to it that you own. Check the tap maker's table when strength matters.
Safety. Wear eye protection, clamp the work and back the tap out often to break chips; a broken tap in a hole is hard to remove. Use cutting fluid on steel. These figures are general rules of thumb, not professional machining advice.
Sources
- List of drill and tap sizes - Wikipedia (opened 2026-10-08)
- ISO metric screw thread - Wikipedia (opened 2026-10-08)
The two rules of thumb are from the first page; the minor diameter and thread depth formulas are from the second. Inch results are also shown to the nearest 1/64 in.
FAQ
What size drill for an M6 tap?
Major minus pitch gives 6.0 - 1.0 = 5.0 mm for the standard M6 x 1.0 coarse thread. The 85 percent rule gives 5.1 mm.
What is the tap drill for 1/4-20?
0.250 in minus 1/20 in is 0.200 in. The nearest fractional drill is 13/64 in (0.203 in).
How is the pitch found for inch threads?
Pitch is 1 divided by threads per inch. A 1/4-20 thread has a pitch of 0.05 in.
Coarse or fine: which percent do I use?
The rule of thumb is 85 percent for coarse threads and 90 percent for fine threads. Pick the thread series in the form.
Is this the same as a clearance drill?
No. A tap drill makes the hole that the tap cuts a thread in. A clearance hole is bigger so the bolt passes through; it is not covered here.