Tap Drill Size Calculator
Estimate a tap hole diameter from thread size and pitch.
Results
Approximate tap drill diameter
The calculated tap drill size is a general approximation. Actual drill size depends on thread engagement, material, tap type and the tool manufacturer's recommendations. This formula does not size holes for forming or roll taps. Check the tap manufacturer's specified drill diameter before machining.
Formula · How it works
Formula
Ddrill ≈ Dmajor − pitch; pitch (in) = 1 / TPIApproximate drill diameter = major diameter − pitch. For inch threads, pitch = 1 / TPI inches, so drill diameter ≈ major diameter − 1 / TPI. Both dimensions are converted to metres before subtraction, then displayed in mm and inches.
Worked example
M10 × 1.5: 10 − 1.5 = 8.5 mm. M6 × 1.0: 6 − 1 = 5.0 mm. For 1/4-20 UNC, the approximation is 0.25 − 1/20 = 0.2000 in (5.08 mm).
How it works
The presets supply nominal major diameter and pitch. They are thread dimensions, not a drill recommendation table. The simple subtraction gives an approximate starting diameter for a cutting tap. Fine-pitch metric and other straight thread dimensions can be entered manually.
Common questions
Can I use the result for a forming or roll tap?
No. Forming taps displace material rather than remove it, so they require different hole sizing. Use the tool manufacturer's recommendations for your material and thread engagement.
Why can the result differ from a standard tap drill chart?
The subtraction is a general approximation. Charts may target a particular thread engagement and choose an available drill size. This calculator displays the calculated diameter without rounding it to a number, letter or fractional drill.
How do I enter a fine pitch or a thread missing from the list?
Edit the major diameter and pitch directly in mm or inches, or enter TPI for an inch thread. The selector shows custom when your values do not match a preset. Both values must be positive and the pitch must be smaller than the major diameter.