Thread percentage calculator
Enter the thread and the drill you have. See the thread percentage it leaves, and the drill you would need for a target percentage.
Thread with this drill
–%
–Drill for target
–Same, other unit
Formulas
% = (D − drill) / (0.01299 × P)
- D
- major diameter
- drill
- drill diameter, same unit
- P
- pitch; for inch threads P = 1 / TPI
Metric shortcut: 76.98 × (D − drill) / P
What to aim for
65–75 % suits most steel. Go higher in soft materials such as aluminium, brass and plastics; go lower in stainless and tough alloys to save the tap.
How much thread you need
Thread percentage to aim for by material.
Mild and alloy steel65–75 % thread
Stainless steel60–70 % thread
Cast iron70–80 % thread
Aluminium75–85 % thread
Brass and bronze75–85 % thread
Plastics75–85 % thread
Bars run from 50 % to 90 % thread. Typical shop practice for cut taps. Harder, tougher materials take less thread to save the tap; soft materials need more engagement to hold.
Common questions
What thread percentage should I use?
65–75 % for most steel. Up to 75–85 % for soft materials such as aluminium, brass and plastics. 60–70 % for stainless steel.
Does a smaller tap drill make a stronger thread?
Only slightly. Above about 75 % engagement the strength gain is small and the risk of breaking the tap rises fast.