1/2-20 tap drill size

Drill a 29/64" hole (0.4531" / 11.51 mm) for a 1/2-20 UNF tap. That leaves about 72% thread, the standard for general-purpose tapping.

Tap drill
29/64"
0.4531" · 11.51 mm · 72% thread
Metric drill
11.5 mm 73% thread
Clearance (close)
33/64" 0.5156"
Helicoil (STI) drill
33/64"
1/2-20 tap drill: 29/64" (0.4531 in / 11.51 mm), 72% thread

1/2-20 thread specs

SeriesUnified fine (UNF)
Major diameter0.5000" (12.70 mm)
Pitch20 TPI (1.27 mm)
Basic minor diameter0.4459" (11.33 mm)
Common internal class2B

Every drill that works for 1/2-20

Drills from all four sets that give 50–85% thread, largest first. Lower percentages tap easier; see how much thread you need.

DrillThreadinmmSetGood for
11.8 mm55%0.464611.80Metricdeep holes only
11.7 mm61%0.460611.70Metrichard or tough metals
11.6 mm67%0.456711.60Metrichard or tough metals
29/64"standard72%0.453111.51Fractionalgeneral use
11.5 mm73%0.452811.50Metricgeneral use
11.4 mm79%0.448811.40Metricgeneral use
11.3 mm85%0.444911.30Metricmore torque, more broken taps

1/2-20 clearance hole

The hole a 1/2-20 screw passes through. Close fit locates parts; free fit is forgiving.

FitDrillinmm
Close fit33/64"0.515613.10
Free fit17/32"0.531313.49

1/2-20 counterbore and countersink

For a socket head cap screw to sit flush, and for a flat head socket screw.

Socket head counterboreDiameterDepth
AMES13/16" 0.8125"0.5000"

Flat head socket screw: countersink at 82° (inch screws, ASME B18.3) to 0.9380" (23.83 mm) across for a flush head. Metric flat heads use 90°, so don't mix countersinks.

1/2-20 holes in a 3D print

Printed holes usually come out undersize. Model the hole at the drill size plus what your printer loses, then tap or drill to size. Why printed holes shrink.

Print a test hole, measure it with calipers, enter the difference. 0 models the nominal size.

Hole to tap
11.51 mm
Clearance hole
13.10 mm

1/2-20 thread repair (Helicoil)

Stripped 1/2-20 thread? Drill out with the STI drill, tap with a 1/2-20 UNF STI tap, and fit a wire thread insert. The drill in your repair kit’s instructions wins over any chart.

ChartSTI drillinmm
AMES33/64"0.515613.10
MISUMI33/64"0.515613.10

Related sizes

3/8-24next size down1/2-13coarse threadM12closest metricFull chartUNC / UNF

Frequently asked questions

What size drill do I use for a 1/2-20 tap?
Use a 29/64" (0.4531 in / 11.51 mm) drill. It leaves about 72% thread, the standard for general-purpose tapping.
What metric drill can I use for a 1/2-20 tap?
11.5 mm (0.4528 in / 11.50 mm) is the closest, giving about 73% thread. Anything from about 65% to 80% thread taps well in most materials.
What size clearance hole for a 1/2-20 screw?
33/64" (0.5156") for a close fit, 17/32" (0.5313") for a free fit.
What is the metric equivalent of 1/2-20?
M12 is the closest size (12.00 mm vs 12.70 mm major diameter). The pitches differ, so the screws and tapped holes are not interchangeable.

Sources

Spotted a wrong number? Email contact@clove3d.com and it gets fixed.

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% thread = (major − drill) ÷ (1.299 × pitch) × 100. The standard drill comes from published charts and is checked against this formula on every build.