Tap Drill Chart – Metric, UNC, UNF & BSW

Look up tap drill size, clearance drill, pitch and minor diameter. Metric M2–M64 (M8 × 1.25 → 6.8 mm) plus UNC, UNF, UNEF, BSW and BSF.

Thread

Recommended tap drill

7mm

0.2756 in

75% thread: tap drill ≈ major − pitch (8 − engagement × 1.0825 × 1.25).

Designation: M8
Major diameter: 8 mm
Pitch: 1.25 mm
TPI: 20.32
Basic minor Ø: 6.647 mm
Internal / external: Internal tap drill shown · external uses major Ø
Clearance · close: 8.4 mm
Clearance · normal: 8.8 mm
Clearance · loose: 9.6 mm

How tap drill size is determined

A 60° metric or Unified thread has a theoretical height H = 0.866025 × pitch. The internal thread is truncated; a 75% engagement tap drill removes about ¾ of the remaining height, which simplifies to a workshop rule of tap drill ≈ major diameter − pitch for metric coarse and fine series.

Formula

H = (√3 / 2) × P

Tap drill ≈ D − 0.75 × 1.0825 × P

Metric shortcut: D − P

Basic minor Ø = D − 1.0825 × P

M8 × 1.25

8.00 − 1.25 = 6.75 mm, issued as a 6.8 mm tap drill. Normal clearance for an M8 bolt is 9.0 mm.

Popular sizes

After tapping, check the hole against the tolerance calculator if the print calls up an H7 or similar locational fit on a plain diameter.

Frequently asked questions

How is a metric tap drill size determined?+

For a typical 75% thread engagement, tap drill ≈ major diameter − pitch. For M8 × 1.25 that is 8.00 − 1.25 = 6.75 mm, rounded to the standard 6.8 mm drill. Fine pitches subtract a smaller pitch and therefore use a larger drill.

What is the difference between tap drill and clearance drill?+

A tap drill is the hole you tap (internal thread). A clearance drill is a through-hole for a bolt to pass freely. Clearance holes are given as close, normal and loose fits per common workshop practice (aligned with ISO 273 medium series).

Do you support imperial threads?+

Yes. UNC, UNF, UNEF, BSW and BSF are included with TPI, major diameter, tap drill (inch and nearest fractional), and clearance drill recommendations.

Is 75% thread always correct?+

75% is the usual workshop target for strength versus tapping torque. Softer materials or difficult holes sometimes use 60–70%; high-strength joints in steel may go higher. The calculator shows the 75% recommendation and the formula so you can adjust.

What does M8 × 1.25 mean?+

M8 is a metric thread with 8 mm major diameter. 1.25 mm is the pitch (distance between crests) — the ISO coarse pitch for M8. Fine M8 variants include 1.0 mm and 0.75 mm pitch.

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