Tap Drill Size Chart — Metric Coarse Quick Reference
For M8 × 1.25 coarse thread, the tap drill is 6.8mm. For M10 × 1.5, use 8.5mm. For M12 × 1.75, use 10.2mm.
| Thread | Pitch (mm) | Tap Drill |
|---|---|---|
| M3 | 0.5 | 2.5mm |
| M4 | 0.7 | 3.3mm |
| M5 | 0.8 | 4.2mm |
| M6 | 1.0 | 5.0mm |
| M8 | 1.25 | 6.8mm |
| M10 | 1.5 | 8.5mm |
| M12 | 1.75 | 10.2mm |
| M16 | 2.0 | 14.0mm |
| M20 | 2.5 | 17.5mm |
| M24 | 3.0 | 21.0mm |
BSP (British Standard Pipe) tap drills: 1/8" BSP = 8.8mm, 1/4" BSP = 11.8mm, 3/8" BSP = 15.0mm, 1/2" BSP = 19.0mm, 3/4" BSP = 24.3mm, 1" BSP = 30.5mm.
UNC tap drills: 1/4"-20 = 5.1mm (#13), 5/16"-18 = 6.9mm (F), 3/8"-16 = 8.0mm (5/16"), 1/2"-13 = 10.7mm (27/64"). UNF tap drills: 1/4"-28 = 5.5mm (3), 5/16"-24 = 7.1mm (I).
Tap drill size determines the percentage of thread engagement. Standard is 75% engagement — provides adequate strength in most materials while reducing tap breakage risk. Reduce to 65% for hard or brittle materials; increase to 85% for soft materials where maximum strength is needed.
People Also Ask — Tap Drill Size Chart: Metric & Imperial Thread Sizes
Q: What drill size do I use for an M10 tap?
For M10 × 1.5 coarse thread, use an 8.5 mm pilot drill. For M10 × 1.25 fine thread, use a 8.75 mm drill (often rounded to 9.0 mm in practice). Using the correct pilot hole is critical — too small risks tap breakage; too large produces insufficient thread engagement and weak joints.
Q: What is the difference between a taper tap and a plug tap?
A taper tap has a long chamfer (8–10 threads) that guides it into the hole gradually — best for starting threads in blind or through holes. A plug tap (4–5 thread chamfer) picks up where the taper left off and is the most common general-purpose tap. A bottoming tap has just 1–2 threads of chamfer for cutting threads to the very bottom of a blind hole.
Q: What is the difference between BSP and NPT threads?
BSP (British Standard Pipe) uses a 55° thread angle and is the standard for most Australian, British, and European hydraulic and pneumatic fittings. NPT (National Pipe Taper) uses a 60° thread angle and is common on American equipment. The two are not interchangeable — mismatching causes leaks and can damage fittings even if they appear to thread together.
Q: How do I know which tap size to buy?
Match the tap to the bolt thread you need — an M6 × 1.0 tap cuts the thread for an M6 coarse bolt. Always pair taps with the correct pilot drill from a tap drill chart. For blind holes, buy a taper, plug, and bottoming tap set. For through holes, a plug tap alone is usually sufficient for most trade applications.
