Tap Drill Chart — Inch & Metric

What it is

A tap drill is the drill used to make the hole before tapping. Its size sets the minor diameter of the internal thread and, with it, the percentage of thread. The sizes below are the ones conventionally used with cut (fluted) taps for general work. Inch sizes target about 75% thread; metric sizes follow the shop rule tap drill ≈ major diameter − pitch, which gives a similar result.

How to identify

Read the thread callout: inch threads are written as size-TPI (1/4-20 UNC), metric as M diameter × pitch (M8 × 1.25). A metric callout without a pitch (M8) means the coarse pitch. Match both the diameter and the pitch before choosing a drill — 1/4-20 and 1/4-28 need different drills.

Chart

Key points

• These sizes are for cut taps. Thread-forming (roll) taps need a larger drill — use the forming-tap chart or the tap maker's data.
• Pipe taps (NPT, NPTF, BSPT) use their own drill sizes; see the pipe tap drill chart.
• Higher thread percentage adds little strength but much more tapping torque. In hard or tough materials, a slightly larger drill (lower thread percentage) can prevent tap breakage if the design allows it.
• Where a drawing specifies a minor-diameter limit, the drawing governs, not this chart.

Common mistakes

• Using the coarse-thread drill for a fine-thread tap of the same diameter.
• Using a cut-tap drill size with a forming tap, which overloads and breaks the tap.
• Substituting the "close" fractional size for a number or letter drill — 13/64 for #7, for example — without checking the resulting thread.

Governing standards: ASME B1.1 (Unified inch screw threads); ASME B1.13M (metric screw threads); ASME B94.9 (taps). Tap manufacturer's recommendations for specific materials.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.