Tap Drill Size Chart: Metric & Imperial Thread Sizes

Tap drill size is the diameter of the pilot hole drilled before threading. The rule: drill diameter = thread outer diameter − thread pitch. For M8 × 1.25 coarse, the tap drill is 6.8mm. For 1/4" BSP, it's 11.8mm. The compact reference below covers the most-used metric coarse sizes; full metric fine, BSP, UNC and UNF charts are further down.

(If you are looking for the Sutton Tap & Drill Chart PDF, click here.)

Tap Drill Size Chart — Metric Coarse Quick Reference

The most frequently used metric coarse thread sizes and their tap drill diameters:

How to Use This Chart

Tap drill size refers to the diameter of the hole you drill before running a tap through it. The hole must be smaller than the thread's outer diameter, leaving enough material for the tap to cut the thread profile. Too large and the thread is shallow and weak. Too small and you risk breaking the tap.

As a general rule, tap drill size = thread outer diameter − thread pitch. This gives approximately 75% thread engagement, which is standard for most applications. For softer materials or where tap breakage is a concern, go slightly larger. For maximum thread strength in hard materials, go slightly smaller.

By accurately matching the tap size to the drill size and choosing the right tap for the job, you can achieve optimal results in your thread cutting operations.

Metric Coarse Tap Drill Size Chart

Metric coarse is the standard thread series for most bolts, screws and tapped holes in general engineering. Pitch is expressed in millimetres — a lower number means finer threads. These are the sizes you'll use for the vast majority of metric tapping work.

Metric Fine Tap Drill Size Chart

Metric fine threads are used where vibration resistance, fine adjustment, or higher tensile strength is required — common in automotive, aerospace, and precision engineering applications. Multiple pitches exist per diameter; confirm your pitch before selecting the drill.

BSP Tap Drill Size Chart (British Standard Pipe)

BSP threads are used on pipe fittings, hydraulic connections, and pneumatic systems throughout Australia and the UK. Sizes refer to the nominal bore of the pipe — not the actual thread diameter, which is always larger. BSPP (parallel) and BSPT (taper) share the same thread form and the same tap drill size.

The 1/4" BSP tap drill size (11.8mm) is one of the most commonly referenced in Australian trade and industrial work. If you're unsure whether your fitting is BSPP or BSPT, the tap drill size is the same for both — the distinction only matters when selecting the tap itself.

UNC Tap Drill Size Chart (Unified National Coarse)

UNC is the standard US coarse thread series. Common in imported machinery, agricultural equipment, and items manufactured to American standards. Identified by thread count in threads per inch (TPI).

UNF Tap Drill Size Chart (Unified National Fine)

UNF has a finer pitch than UNC — more threads per inch, higher tensile strength, and better vibration resistance. Used in aerospace, precision equipment, and anywhere a finer thread is specified. When in doubt, check the thread count: more threads per inch means UNF.

Frequently Asked Questions

What drill size for M3 tap?

For M3 coarse thread (0.5mm pitch), use a 2.5mm tap drill. For M3 fine (0.35mm pitch), use a 2.65mm drill. Formula: 3 − 0.5 = 2.5mm.

What drill size for M4 tap?

For M4 coarse thread (0.7mm pitch), use a 3.3mm tap drill. For M4 fine (0.5mm pitch), use a 3.5mm drill. Formula: 4 − 0.7 = 3.3mm.

What drill size for M5 tap?

For M5 coarse thread (0.8mm pitch), use a 4.2mm tap drill. For M5 fine (0.5mm pitch), use a 4.5mm drill. Formula: 5 − 0.8 = 4.2mm.

What drill size for M6 tap?

For M6 coarse thread (1.0mm pitch), use a 5.0mm tap drill. For M6 fine (0.75mm pitch), use a 5.25mm drill. Formula: 6 − 1.0 = 5.0mm.

What drill size for M8 tap?

For M8 coarse thread (1.25mm pitch), use a 6.8mm tap drill. For M8 fine (1.0mm pitch), use a 7.0mm drill. Formula: 8 − 1.25 = 6.75mm, rounded to 6.8mm.

What drill size for M10 tap?

For M10 coarse thread (1.5mm pitch), use an 8.5mm tap drill. For M10 fine (1.25mm pitch), use 8.75mm; for M10 fine (1.0mm pitch), use 9.0mm. Formula: 10 − 1.5 = 8.5mm.

What drill size for M12 tap?

For M12 coarse thread (1.75mm pitch), use a 10.2mm tap drill. For M12 fine (1.5mm pitch), use 10.5mm; for M12 fine (1.25mm pitch), use 10.75mm. Formula: 12 − 1.75 = 10.25mm, rounded to 10.2mm.

What drill size for M14 tap?

For M14 coarse thread (2.0mm pitch), use a 12.0mm tap drill. For M14 fine (1.5mm pitch), use 12.5mm. Formula: 14 − 2.0 = 12.0mm.

What drill size for M16 tap?

For M16 coarse thread (2.0mm pitch), use a 14.0mm tap drill. For M16 fine (1.5mm pitch), use 14.5mm. Formula: 16 − 2.0 = 14.0mm.

What drill size for M20 tap?

For M20 coarse thread (2.5mm pitch), use a 17.5mm tap drill. For M20 fine (2.0mm pitch), use 18.0mm; for M20 fine (1.5mm pitch), use 18.5mm. Formula: 20 − 2.5 = 17.5mm.

What drill size for 1/4 inch BSP tap?

The recommended tap drill for 1/4 inch BSP (19 TPI) is 11.8mm, or 0.465 inches. This applies to both BSPP (parallel) and BSPT (taper) threads — the tap drill is the same.

What drill size for 1/8 inch NPT tap?

The recommended tap drill for 1/8 inch NPT (27 TPI) is 8.6mm, or 21/64 inch. NPT is a tapered thread used on American pipe fittings — distinct from BSP.

What drill size for 1/4 inch UNC tap?

For 1/4-20 UNC, use a 5.1mm tap drill (13/64 inch). UNC has 20 threads per inch and is the standard US coarse thread.

How do I calculate tap drill size for metric threads?

Tap drill size (mm) equals thread diameter minus thread pitch. Example: M10 × 1.5 = 10 − 1.5 = 8.5mm. This gives approximately 75% thread engagement, which is standard for most applications.

What is a tap drill size?

A tap drill size is the diameter of the hole you drill before cutting a thread with a tap. It must be smaller than the thread's outer diameter so the tap has material to cut the thread profile into.

What is the difference between BSPP and BSPT?

BSPP (British Standard Pipe Parallel) has straight threads and seals with an O-ring or washer. BSPT (British Standard Pipe Taper) has a tapered thread that seals as it tightens. Both share the same tap drill size for a given nominal size, but BSPT taps are designed to cut a taper.

If a tap breaks during the threading process, see our guide on how to remove a broken tap — covering all six removal methods from tap extractors through to EDM.