Tap Drill Size Chart: Metric & Imperial Thread Sizes

Use this tap drill size chart to find the correct pilot hole diameter before threading — whether you're cutting metric, BSP pipe, UNC or UNF threads. Select your thread type, find your nominal size, and read across for the recommended drill diameter in both metric and imperial. Getting the drill size right is the single most important step in producing clean, full-strength threads.

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

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 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 into.

How do I calculate tap drill size for metric threads?
Tap drill size (mm) = thread diameter − thread pitch. For example, M10 × 1.5 = 10 − 1.5 = 8.5mm. This gives approximately 75% thread engagement, which is standard for most applications.

What drill size for 1/4" BSP?
The recommended tap drill for 1/4" BSP (19 TPI) is 11.8mm, or 0.465 inches.

What's the difference between BSPP and BSPT?
BSPP (British Standard Pipe Parallel) has straight threads and relies on a sealing washer or O-ring to make the joint. BSPT (British Standard Pipe Taper) has a tapered thread that forms a seal as it tightens. Both use the same tap drill size for a given nominal size.

What drill size for M8?
For M8 coarse thread (standard, 1.25mm pitch), use a 6.8mm drill. For M8 fine thread (1.0mm pitch), use a 7.0mm drill.

What percentage thread engagement should I use?
75% is standard for most materials and applications. For soft materials like aluminium or brass, 75% is ideal. For particularly hard materials, you may reduce engagement slightly to protect the tap. Structural applications in high-strength steel may call for higher engagement, but 75% is the default starting point.

What's the difference between UNC and UNF?
UNC (Unified National Coarse) has fewer threads per inch and is used for general fastening where ease of assembly matters. UNF (Unified National Fine) has more threads per inch, giving higher tensile strength and better resistance to loosening under vibration. If you're unsure which you have, count the threads over one inch.

Can I use the same tap for BSPP and BSPT?
No. They share the same thread form and pitch but BSPT taps are designed to cut a tapered thread. Using the wrong tap will produce an incorrect joint. Always use the correct tap for the application.

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.