Tap Drill Calculator
Tap drill size for a metric (ISO coarse or fine) or unified inch (UNC, UNF) thread at the percentage of thread you choose, drill = major − 1.299 × pitch × % ÷ 100, or the percentage of thread a drill you already have will give.
Result…
How it's calculated
Tap drill = major − 1.299 × pitch × % thread ÷ 100 Inch: pitch = 1 ÷ TPI, so drill = major − 0.01299 × % ÷ TPI % thread = (major − drill) ÷ (1.299 × pitch) × 100 Rule of thumb: major − pitch ≈ 75% thread
A tapped hole starts as a drilled hole a little larger than the thread's minor diameter, so the tap cuts only part of the full thread depth. The percentage of thread compares the depth actually cut with the full depth, which for the 60° ISO metric and unified inch forms is taken as 1.299 × pitch across both sides. The drill for a chosen percentage is the major diameter less 1.299 × pitch × the percentage, the formula given by the 6G Tools thread calculator; the shortcut major − pitch gives roughly a 75% thread. Inch pitch is 1 ÷ threads per inch, and 1 in = 25.4 mm exactly. The rounded sizes are plain arithmetic, not a list of drills on sale.
Your numbers, step by step
About the Tap Drill calculator
This tap drill calculator gives the drill size for a metric thread such as M8 × 1.25, coarse or fine, or a unified inch thread such as 1/4-20 UNC or 3/8-24 UNF, at the percentage of thread you choose. Charts usually list one drill for about 75% thread, but a softer or harder material, a deep hole or a tap that keeps breaking can call for a different figure. Switch to the other mode and it works backwards: enter a drill you already have and it tells you the percentage of thread that drill will leave for the tap. Results come in millimetres and inches, with the size rounded to 0.1 mm or to the nearest 1/64 inch and the thread that rounded size gives. The working is shown step by step with your numbers. The formula is for cutting taps, not forming taps.
How to use it
- Choose to “Find” the “Drill size” or the “Percentage of thread”, and pick the “Thread” type.
- Enter the “Major diameter” and the “Pitch” in mm, or the major diameter in inches and the “Threads per inch”.
- For a drill size, enter the “Percentage of thread” you want; 75 is the usual chart figure.
- To check a drill you have, enter its “Drill diameter” instead.
- Read the drill size, the rounded size and the thread it gives, and confirm the drill exists in your set.
Worked example
An M8 × 1.25 thread at 75%: drill = 8 − 1.299 × 1.25 × 75 ÷ 100 = 6.782 mm (0.2670 in). Rounded to 6.8 mm, it gives (8 − 6.8) ÷ (1.299 × 1.25) × 100 = 73.9% thread. Checking a 3/8-16 thread with a 5/16 in (0.3125 in) drill: (0.375 − 0.3125) ÷ (1.299 ÷ 16) × 100 = 77.0% thread.
Updated
Questions people ask
How do I calculate tap drill size?
Tap drill = major diameter − 1.299 × pitch × percentage of thread ÷ 100, with pitch = 1 ÷ threads per inch for inch threads. The 6G Tools thread engagement calculator uses the same 1.299 × pitch constant.
Is major diameter minus pitch a good tap drill?
It is a handy shortcut that gives roughly a 75% thread for both metric and inch threads, coarse or fine. An M6 × 1 tap then takes a 5 mm drill.
What percentage of thread should I use?
About 75% is the usual chart figure. Guhring suggests 60% to 70% for most work and notes a 100% thread is only about 5% stronger than 75% but needs about 3 times the torque to tap.
Why does my tap keep breaking?
A common cause is a hole drilled too small, which gives too high a percentage of thread and too much torque. Check the drill size here, use the right tap for the material, and clear the chips often.
Can I use this for forming or roll taps?
No. Forming taps push the metal instead of cutting it and need a larger hole; use the size the tap maker gives.
What does this tap drill result not cover?
It does not list which drills are sold where you are, and real drills can cut a little oversize. For tight specifications check the minor diameter limits in the thread standard or with a thread gauge. For milling speeds see the CNC feed rate calculator.