Tap Drill Size Calculator
Pilot drill diameter for any tap: Metric, Metric fine, BSP
किसी भी टैप के लिए सही ड्रिल साइज़ — M3 से M30, BSP threads
Before you tap a thread, you must drill a pilot hole of the correct diameter. Too small and the tap jams or breaks; too large and the threads are shallow and strip easily. The rule for standard metric coarse threads is: tap drill diameter = major diameter − pitch. So an M8 × 1.25 tap needs a 6.8 mm drill, M10 × 1.5 needs 8.5 mm, M12 × 1.75 needs 10.2 mm. This gives approximately 75% thread engagement — the industry standard that balances thread strength and tapping torque. For tougher materials or lower-torque taps, you can go slightly larger (60-70% engagement). BSP threads use a different rule based on Whitworth profile: a G1/4 tap needs an 11.8 mm drill, G1/2 needs 18.8 mm. This calculator covers metric coarse, metric fine, and BSP pipe threads, and also returns the recommended drill for 75% and 60% engagement options.
Calculator
M10 × 1.5: major Ø 10.00 mm, pitch 1.500 mm. 75% engagement formula: major − pitch = 10 − 1.5 = 8.50 mm. Always use cutting oil when tapping.
ƒFormula
Tap drill = Major Ø − Pitch (75% engagement, metric coarse threads)This empirical formula gives ~75% thread engagement — the industry default that balances strength and tapping torque. For 60% engagement (lower torque, slightly weaker thread): drill = major − (1.3 × pitch). For BSP threads, use the Whitworth-based lookup table (cannot be reduced to a simple formula).
Units: All dimensions in millimetres (mm)
Tap se pehle kaunsi drill chahiye?
Jab aap tap karna chahte hain, pehle sahi size ka pilot hole drill karna padta hai. Galat size drill use kiya toh ya tap tootega, ya thread weak ho jayega. Metric coarse threads ke liye formula simple hai: tap drill = major diameter − pitch. Yaani M8 × 1.25 tap ke liye 8 − 1.25 = 6.75 ≈ 6.8 mm drill. M10 × 1.5 ke liye 8.5 mm. M12 × 1.75 ke liye 10.2 mm. Yeh 75% thread engagement deta hai, strong thread + manageable tapping torque ka balance. Softer materials (aluminium, brass) mein 60-70% engagement kaafi hota hai, drill thoda bada kar sakte hain. BSP pipe threads ka formula alag hai, Whitworth profile hone ki wajah se.
Worked Examples
Tapping M10 hole in a steel bracket
Problem: You need to tap an M10 × 1.5 hole in 15 mm mild steel. What drill first?
Solution: M10 coarse = 10 − 1.5 = 8.5 mm pilot drill. Use a HSS 8.5 mm drill with cutting oil. Then use an M10 × 1.5 HSS tap, turning 1/2 rotation forward then 1/4 back to break chips.
BSP thread for a pneumatic fitting
Problem: Need to tap a G1/4 port for a SPAC push-in fitting.
Solution: G1/4 BSP tap drill = 11.8 mm. Use an 11.8 mm drill (or nearest, 11.9 mm is fine), then a G1/4 BSP parallel tap. Use cutting oil. After tapping, test with a plug gauge or by threading the fitting in.
Fine thread tapping for thin walls
Problem: M8 fine thread (M8 × 1.0) in a 4 mm aluminium bracket.
Solution: M8 × 1.0 fine = 8 − 1.0 = 7.0 mm pilot drill. Because material is thin aluminium, consider 60% engagement: 8 − (1.3 × 1.0) = 6.7 mm drill for easier tapping, still strong enough for this light application.
Tap Drill Chart, Metric Coarse, Metric Fine, BSP (75% engagement)
| Thread | Major Ø (mm) | Pitch (mm) | Tap Drill (mm) | Type |
|---|---|---|---|---|
| M3 × 0.5 | 3.0 | 0.50 | 2.5 | Metric coarse |
| M4 × 0.7 | 4.0 | 0.70 | 3.3 | Metric coarse |
| M5 × 0.8 | 5.0 | 0.80 | 4.2 | Metric coarse |
| M6 × 1.0 | 6.0 | 1.00 | 5.0 | Metric coarse |
| M8 × 1.25 | 8.0 | 1.25 | 6.8 | Metric coarse |
| M8 × 1.0 | 8.0 | 1.00 | 7.0 | Metric fine |
| M10 × 1.5 | 10.0 | 1.50 | 8.5 | Metric coarse |
| M10 × 1.25 | 10.0 | 1.25 | 8.8 | Metric fine |
| M12 × 1.75 | 12.0 | 1.75 | 10.2 | Metric coarse |
| M12 × 1.5 | 12.0 | 1.50 | 10.5 | Metric fine |
| M14 × 2.0 | 14.0 | 2.00 | 12.0 | Metric coarse |
| M16 × 2.0 | 16.0 | 2.00 | 14.0 | Metric coarse |
| M18 × 2.5 | 18.0 | 2.50 | 15.5 | Metric coarse |
| M20 × 2.5 | 20.0 | 2.50 | 17.5 | Metric coarse |
| M22 × 2.5 | 22.0 | 2.50 | 19.5 | Metric coarse |
| M24 × 3.0 | 24.0 | 3.00 | 21.0 | Metric coarse |
| M30 × 3.5 | 30.0 | 3.50 | 26.5 | Metric coarse |
| G1/8 | 9.73 | 0.907 | 8.8 | BSP parallel |
| G1/4 | 13.16 | 1.337 | 11.8 | BSP parallel |
| G3/8 | 16.66 | 1.337 | 15.2 | BSP parallel |
| G1/2 | 20.96 | 1.814 | 18.8 | BSP parallel |
| G3/4 | 26.44 | 1.814 | 24.3 | BSP parallel |
Values give ~75% thread engagement — the industry standard. For softer materials or hand-tapping, go slightly larger (up to 65% engagement). Always use cutting oil and back off every turn to break chips.
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Frequently Asked Questions: Tap Drill Size Calculator
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