Cutting clean, strong internal threads starts with drilling the correct hole. This guide explains tap drill sizes — what they are, how they’re worked out, and the exact drill bit to use for common UNC, UNF and metric taps.

What Is a Tap Drill Size?

Before you cut threads with a tap, you drill a hole slightly smaller than the thread’s outside diameter. That hole is the tap drill. Get it right and the tap cuts strong, clean threads with reasonable effort. Too small and the tap binds or snaps; too large and the threads are shallow and weak.

How Tap Drill Size Is Calculated

For a standard 75% thread, the rule of thumb is: tap drill = major diameter − thread pitch. For metric, subtract the pitch in millimetres (e.g. M6×1.0 → 6 − 1 = 5 mm). For inch threads, subtract 1/N of an inch, where N is threads per inch. Most shops just use a chart — here are the common ones:

75% vs. 50% Thread

The charts above give roughly 75% thread engagement, which is the usual choice for strength. In hard materials you can drill slightly larger for about 50% thread — it’s much easier to tap and only marginally weaker, which helps prevent broken taps in tough steel.

Tips for Clean Threads

• Use cutting fluid on metal — it protects the tap and improves the finish.
• Back off often to break and clear chips, especially in blind holes.
• Keep the tap square to the hole to avoid cross-threading.
• Deburr the hole first so the tap starts cleanly.

Get the Right Bits

• HSS & cobalt drills
• Tap drill chart tool
• Drill bit size chart
• Shop all drill bits →