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Calculate thread pitch, TPI, and major/minor diameters for metric and imperial screws and bolts. Convert between coarse and fine thread specs.
For metric threads, tap drill diameter equals nominal major diameter minus pitch. This approximates a 75% thread engagement, the standard balance of strength and tap life.
Tap drill (mm) = Major diameter − PitchFor UNC/UNF threads, tap drill size equals major diameter minus pitch (where pitch = 1/TPI). Alternatively use the standard tap drill chart for fractional sizes.
Tap drill (in) = Major diameter − (1 / TPI)The standard 75% engagement formula works for most through holes in steel and aluminum. Blind holes, soft materials, or high-strength requirements may use 65% (larger drill) or 100% (smaller drill) engagement.
Engagement % ≈ (Major − Drill) / (Major − Minor) × 100Updated: July 2026
M8 with 1.25 mm pitch: tap drill = 8 − 1.25 = 6.75 mm. Use a 6.8 mm drill (common) or 6.7 mm for closer to 100% engagement in blind holes.
1/4-inch-20 UNC: tap drill = 0.250 − 0.050 = 0.200 inch (#7 drill, 0.201 inch). This yields standard 75% thread in through holes in mild steel.
1/4-inch-28 UNF: tap drill = 0.250 − 0.036 = 0.214 inch (#3 drill, 0.213 inch). Fine threads need a slightly larger drill than coarse for the same nominal diameter.
Drilling to major diameter leaves almost no material for threads. Always subtract at least one pitch from the major diameter for proper thread formation.
Fine threads (higher TPI or smaller pitch) require a larger tap drill than coarse threads of the same nominal diameter. Always verify the pitch or TPI before selecting the drill.
Tapping requires a hole sized so the tap cuts threads to the correct depth without stripping or producing weak threads. This calculator determines the recommended tap drill diameter from nominal thread size and pitch for metric and imperial thread systems.