What is the tap drill for 5/16-18 UNC?

6.8 mm (0.2673 in) for 75% thread engagement.

Engineering

Look up tap drill

5/16-18 UNC Tap Drill Size

5/16-18 UNC is the next step up from 1/4-20 on US machinery: clamps, motor feet, mixed-unit fixtures. 18 threads per inch on a 0.3125 in major diameter.

Thread

Recommended tap drill

6.8mm

0.2673 in · nearest # / fraction on a letter-drill chart

75% thread: tap drill ≈ major − 0.974 × pitch (7.94 − engagement × 1.0825 × 1.411).

Designation: 5/16-18 UNC
Major diameter: 7.938 mm
Pitch: 1.411 mm
TPI: 18
Basic minor Ø: 6.41 mm
Internal / external: Internal tap drill shown · external uses major Ø
Clearance · close: 8.3 mm
Clearance · normal: 8.7 mm
Clearance · loose: 9.6 mm

75% tap drill ≈ 0.3125 − 1/18 ≈ 0.257 in. Shops use an F drill (0.257 in) or 6.5 mm. Normal clearance is about 8.4 mm (21/64 in). Fine 5/16-24 UNF uses a 6.9 mm / I drill.

Common metric tap drills: M6 tap drill 5.0 mm · M8 tapping drill 6.8 mm · M10 tap drill 8.5 mm · M12 tap drill 10.2 mm.

Formula

Major Ø 7.94 mm

Pitch 1.411 mm

Tap drill 6.8 mm

Minor Ø 6.41 mm

5/16-18 UNC at a glance

Tap drill 6.8 mm. Close / normal / loose clearance: 8.3 / 8.7 / 9.6 mm.

How we use this in the toolroom

Same trap as 1/4-20: a metric-trained operator will reach for 6.8 mm (M8 tap drill). Mark the F drill.

Accurate Industries in Pune still looks these sizes up the same way: tap drill ≈ major − pitch, then a clearance hole for the bolt. AlwaysReach is that chart in the browser — our story.

Full metric and imperial lookup lives on the thread & tap drill calculator. After tapping, check a locational bore on the ISO tolerance calculator. Other pages: M8 × 1.25, M10 × 1.5, M12 × 1.75, M6 × 1, M16 × 2, M4 × 0.7, M5 × 0.8, 1/4-20 UNC, M3 × 0.5, M20 × 2.5, M24 × 3, 3/8-16 UNC, 1/2-13 UNC, UNC chart, UNF chart, BSW chart, UNEF chart, BSF chart.

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. Dedicated UNEF and BSF tap drill charts sit next to the UNC, UNF and BSW charts.

Is this a metric tap drill calculator and tap size chart?+

Yes. It is a metric tap drill calculator for M2–M64 plus a tap size chart for Unified and Whitworth. Use it for thread drill size, tap drill size chart lookups, and the drill size for tapping at 75% engagement.

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.

What is the M8 tapping drill size?+

The M8 tapping drill (M8 × 1.25 coarse) is 6.8 mm. That is major minus pitch, 8.00 − 1.25 = 6.75 mm, issued as a 6.8 mm drill. M8 tap pitch is 1.25 mm coarse. A normal clearance hole for an M8 bolt is 9.0 mm.

What drill size for M6 and M10 taps?+

M6 drill size (M6 × 1 tap) is 5.0 mm; M6 tap size is the M6 × 1 thread itself. Tap drill for M10 coarse (M10 × 1.5) is 8.5 mm. Open the M6 or M10 page for clearance drills and fine pitches.

What size drill bit for an M8 × 1.25 tap?+

The drill bit for an M8 × 1.25 tap is 6.8 mm (M8 tap drill bit size). That is the coarse M8 tapping drill. M8 tap pitch is 1.25 mm coarse; fine M8 × 1 uses 7.0 mm and M8 × 0.75 uses 7.2 mm.

Is this a tap drill chart for metric and imperial?+

Yes. It is a tap drill chart metric and imperial: M2–M64 coarse and fine, plus UNC, UNF, UNEF, BSW and BSF. Use it as a tap drill size calculator, then open a size page (M6, M8, M10, 1/4-20) when you want one row without the full chart.

What is the tap drill for M6 × 1.0 (6 mm × 1)?+

M6 × 1.0 tap drill is 5.0 mm. M6 tap pitch is 1.0 mm coarse. Searches like 6mmx1 tap drill size and drill size for an M6 1.0 tap are that same hole. Open the M6 page for clearance drills.