Percent thread explained
Updated 2026-10-19
Charts give tap drills at about 75% thread. That number is a compromise, not a target: in most work 65–75% holds the screw just as well, and every point above that makes the tap work harder.
What percent thread means
The tap drill sets the hole’s inside diameter. The tap then cuts the thread form into the wall of that hole. Percent thread is how much of the full thread depth the tap actually cuts. A bigger drill leaves less material, so the tap cuts a shallower thread and the percentage drops.
The formula
With D the major diameter, d the drill and P the pitch, all in the same unit:
% thread = (D − d) ÷ (1.299 × P) × 100For inch taps P = 1 ÷ TPI, which gives the familiar shop form. For metric, 100 ÷ 1.299 = 76.98:
inch: % thread = TPI × (D − d) ÷ 0.01299
metric: % thread = 76.98 × (D − d) ÷ P
drill for a target: d = D − (% ÷ 100) × 1.299 × PWorked example: 1/4-20
A 1/4-20 screw has D = 0.2500" and 20 threads per inch. The same thread with five different drills:
| Drill | Diameter | Thread |
|---|---|---|
| #3 | 0.2130" | 57% |
| #5 | 0.2055" | 69% |
| 13/64" | 0.2031" | 72% |
| #7 | 0.2010" | 75% |
| #9 | 0.1960" | 83% |
The #7 is the chart drill. A 13/64" fractional drill, which most people own, lands at 72% and works just as well. Every drill for this size is on the 1/4-20 tap drill page.
Why not 100%?
Holding power climbs only a little once engagement passes about 60%, while the torque needed to cut a deeper thread rises steeply, so higher percentages mostly buy broken taps (Regal Cutting Tools). The US Cutting Tool Institute gives 75% as the rule of thumb for 60° threads, recommends no less than 65% when the hole is shallower than three diameters, and a slightly lower percentage in harder materials (USCTI).
Which percent to use
| Thread | Use it for |
|---|---|
| under 60% | deep holes only |
| 60–70% | hard or tough metals |
| 70–80% | general use |
| over 80% | more torque, more broken taps |
Deep holes have more threads engaged, so a lower percentage still holds. Shallow holes in soft material are where you want the full 75%.
Find a drill
The tap drill calculator takes a thread and a target percentage and gives the closest drill from every set. Charts: inch UNC / UNF and metric coarse.