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Find the common tap drill for ISO metric coarse, UNC and UNF threads. Select the thread and read the recommended drill, major diameter and pitch.
This is a starting-size reference for general-purpose tapping. The required drill can change with thread percentage, material, tap geometry, coating and whether the tap cuts or forms the thread.
A tap drill size is the hole diameter prepared before an internal thread is cut or formed, usually called the tapping drill or pre-tap drill. For ISO metric coarse threads at approximately 75% engagement, a practical starting rule is drill diameter = nominal diameter − pitch; an M8 × 1.25 thread therefore uses about 6.75 mm and the common 6.8 mm drill. For inch threads, a common approximation is major diameter − 1 / threads per inch at roughly 75% engagement; 1/4-20 gives about 0.200 inch, so the #7 drill at 0.201 inch is normally used. Material conditions matter: stainless steel can work-harden, aluminium can seize, and forming taps require a different hole. Blind holes also need drill depth beyond the required full thread to clear the tap chamfer and chips. Zhejiang Balford Mechatronics Co., Ltd. reviews tapped features within its 12,000 m² production facility against the drawing, material and inspection requirement.
These are common starting drill sizes for general-purpose tapping. Material, coating, tap class, blind-hole depth, cutting versus forming taps and the required thread engagement can require a different drill. Confirm the drawing, tap manufacturer and internal thread inspection requirement.
The familiar rules estimate the drill at roughly 75% thread engagement. Exact values depend on the thread standard, class and tap geometry.
| Thread | Pitch | Common tap drill | Starting rule |
|---|---|---|---|
| M3 | 0.5 mm | 2.5 mm | 3.0 − 0.5 |
| M4 | 0.7 mm | 3.3 mm | 4.0 − 0.7 |
| M5 | 0.8 mm | 4.2 mm | 5.0 − 0.8 |
| M6 | 1.0 mm | 5.0 mm | 6.0 − 1.0 |
| M8 | 1.25 mm | 6.8 mm | 8.0 − 1.25 |
| M10 | 1.5 mm | 8.5 mm | 10.0 − 1.5 |
| M12 | 1.75 mm | 10.2 mm | 12.0 − 1.75 |
| M16 | 2.0 mm | 14.0 mm | 16.0 − 2.0 |
| M20 | 2.5 mm | 17.5 mm | 20.0 − 2.5 |
Map to the nearest commonly stocked drill. Use the exact tap manufacturer's recommendation where thread class, material or forming taps control the process.
An M8 × 1.25 metric coarse thread at approximately 75% engagement gives 8.0 − 1.25 = 6.75 mm. The common 6.8 mm tap drill is the nearest standard size. For a 1/4-20 UNC thread, 0.250 − 1/20 = 0.200 in, and the #7 drill at 0.201 in is the normal starting size.
A tap drill is the hole drilled before cutting an internal thread with a tap. Its diameter removes enough material for the tap to form the thread while leaving sufficient wall engagement. The correct size depends on the thread system, nominal diameter, pitch and required thread percentage.
For common ISO metric coarse threads at approximately 75% engagement, a practical starting rule is tap drill diameter = nominal diameter − pitch. For example, M8 × 1.25 uses about 6.75 mm; the common 6.8 mm drill is the nearest standard size.
A common approximation is tap drill diameter = major diameter − 1 / threads per inch for approximately 75% thread. For 1/4-20, this is 0.250 − 0.050 = 0.200 in, so the #7 drill at 0.201 in is normally specified.
Stainless steel may need a slightly larger starting hole or a specific tapping strategy because it work-hardens and can seize. Forming taps, roll taps and high-engagement threads also use different drill sizes. Follow the tap manufacturer's table for the exact material and tap class.
The drill depth must exceed the required full thread depth by enough room for the tap chamfer and chips. A common starting allowance is several thread pitches beyond the usable thread, with more clearance for ductile materials and bottoming taps. Confirm the drawing and inspection method.
Send the drawing, material, thread specification, class, hole depth and inspection requirement. Send the drawing to shawn@balford.net. We normally reply within 24 hours, China Standard Time.