About the Thread Calculator
This thread calculator gives the tap drill size and the key basic dimensions of a 60° screw thread — pitch, threads per inch, thread height, pitch diameter and minor diameter — for ISO metric and Unified (UNC/UNF) threads. Pick a common size such as M10 × 1.5 or 1/4-20, or enter any diameter and pitch, then choose the percentage of thread you want the tapped hole to have.
It is built for machinists, mechanics and makers who need a tap drill when the chart is not to hand, and for designers checking thread geometry. ISO metric and Unified threads share the same basic profile, so one set of formulas covers both; inch sizes are converted to millimetres internally and every key size is shown in both units.
Percentage of thread follows the convention used in NIST Handbook H28, where 100% is a full basic thread height of 3H/4 per side. Around 75% is the usual target: it keeps nearly all of the strength while making tapping far easier than 100%. The calculator gives the exact drill for your target and the nearest 0.1 mm and 1/64 in drills with the thread percentage they produce. Drills often cut slightly oversize, so for tight minor-diameter tolerances check the thread standard or ream the hole.
With the default inputs, the tap drill for target thread % is 8.539 mm. Change any value above to recalculate instantly.
How to use the thread calculator
- 1Choose a standard thread size, or a custom metric or inch thread.
- 2For a custom thread, enter the major diameter and the pitch or TPI.
- 3Set the target percentage of thread — 75% suits most steel and aluminium work.
- 4Read the exact tap drill and the nearest 0.1 mm and 1/64 in drills.
- 5Check the thread percentage of the drill you actually have before drilling.
Formula and method
ISO metric and Unified inch threads share the same 60° basic profile. H is the height of the sharp V triangle, the basic thread height is 5H/8, the pitch diameter sits 3H/8 below the major diameter on each side and the basic internal minor diameter is 5H/8 below it on each side. For inch threads the pitch is 25.4 ÷ TPI millimetres.
Percentage of thread compares how much thread is cut with a full thread height of 3H/4 per side, the convention NIST Handbook H28 uses for its tap drill tables. Because 2 × 3H/4 = 1.299038·P, the drill for a target percentage is D minus 1.299038·P times that percentage. The quick “D − P” rule for metric taps gives about 77% thread. Real holes are usually a few hundredths of a millimetre oversize, which slightly lowers the actual percentage.
- D
- Major (nominal) diameter
- P
- Pitch, mm (25.4 ÷ TPI for inch threads)
- H
- Height of the fundamental sharp-V triangle
- h
- Basic thread height (5H/8)
Worked examples
M10 × 1.5 at 75% thread
For 75% thread the drill is 10 − 1.299038 × 1.5 × 0.75 ≈ 8.54 mm. The standard 8.5 mm drill (10 − 1.5) gives 1.5 ÷ 1.9486 ≈ 77% thread. The basic pitch diameter is 10 − 0.649519 × 1.5 ≈ 9.026 mm and the internal minor diameter is about 8.376 mm.
1/4-20 UNC at 75% thread
A 1/4-20 thread is 6.35 mm in diameter with a 25.4 ÷ 20 = 1.27 mm pitch. The 75% drill is 6.35 − 1.299038 × 1.27 × 0.75 ≈ 5.113 mm, or 0.2013 in — the familiar #7 (0.201 in) tap drill.
Custom #10-32 at 65% thread
With a 0.19 in diameter and 32 TPI, P = 0.79375 mm. Aiming for 65% thread — common in tough stainless — the drill is 4.826 − 1.299038 × 0.79375 × 0.65 ≈ 4.156 mm, or about 0.1636 in.
Frequently asked questions
How do I calculate a metric tap drill size?+
Subtract the pitch from the major diameter: an M8 × 1.25 needs an 8 − 1.25 = 6.75 mm (in practice 6.8 mm) drill, and an M10 × 1.5 needs 8.5 mm. This gives about 77% thread, which is the usual target.
What percentage of thread should I use?+
About 75% is standard for general work. Testing shows strength barely rises beyond that while tapping torque climbs quickly. Use 50–65% for hard, tough or gummy materials and deep holes, and up to around 80% for soft or thin materials.
How do I convert TPI to pitch?+
Pitch in millimetres equals 25.4 divided by threads per inch. For example, 20 TPI is 1.27 mm and 13 TPI is about 1.954 mm. Going the other way, TPI = 25.4 ÷ pitch, so an M10 × 1.5 thread has about 16.9 threads per inch.
What is the tap drill for 1/4-20?+
The standard tap drill for 1/4-20 UNC is a #7 drill (0.201 in, about 5.1 mm), which gives roughly 75% thread. A 13/64 in (0.2031 in) drill is a common substitute and gives about 72%.
What is pitch diameter?+
Pitch diameter is the diameter of an imaginary cylinder that cuts the thread where the thread ridge and the groove are equal in width. For 60° threads it equals the major diameter minus 0.649519 × pitch, and it is the dimension thread gauges and wires check.
Are metric and UNC thread profiles the same?+
Yes, the basic profile is the same 60° form in both systems, which is why one set of formulas works for both. They differ in diameter and pitch series and in tolerance classes, so the sizes are not interchangeable.