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1/4-20 UNC Tap Drill Size

1/4-20 UNC is the everyday inch coarse bolt in the US and on mixed-unit machines. 20 threads per inch on a 0.250 in major diameter. Do not mix it with 1/4-28 UNF — the pitches are not compatible.

Recommended tap drill

5.3mm

0.2094 in · nearest # / fraction on a letter-drill chart

75% thread: tap drill ≈ major − 0.974 × pitch (6.35 − engagement × 1.0825 × 1.27).

Designation
1/4-20 UNC
Major diameter
6.35 mm
Pitch
1.27 mm
TPI
20
Basic minor Ø
4.975 mm
Internal / external
Internal tap drill shown · external uses major Ø
Clearance · close
6.7 mm
Clearance · normal
7.1 mm
Clearance · loose
7.9 mm

75% tap drill ≈ major − (1/TPI) = 0.250 − 0.050 = 0.200 in. Shops use a #7 (0.201 in) or 13/64 in (0.203 in) drill, about 5.1 mm. Normal clearance is 6.6 mm (17/64 in). Fine 1/4-28 uses a #3 drill (0.213 in).

Formula

Major Ø 6.35 mm

Pitch 1.27 mm

Tap drill 5.3 mm

Minor Ø 4.975 mm

1/4-20 UNC at a glance

Tap drill 5.3 mm. Close / normal / loose clearance: 6.7 / 7.1 / 7.9 mm.

Full metric and imperial charts live on the thread & tap drill calculator. Other common sizes: M8 × 1.25, M10 × 1.5, M12 × 1.75, M6 × 1, M16 × 2, M4 × 0.7, M5 × 0.8.

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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