The tap drill for 1/2-20 UNF is 29/64" (0.4531 in, 11.51 mm), which gives about 72% of full thread depth. The exact 75% hole is 0.4513 in (11.46 mm). The table lists every number, letter, fractional and metric drill that gives between 55% and 90% thread, so you can work from the bits you own.
| Drill | Type | in | mm | Thread | Note |
|---|---|---|---|---|---|
| 11.3 mm | metric | 0.4449 | 11.30 | 85% | deep, hard to tap |
| 11.4 mm | metric | 0.4488 | 11.40 | 79% | |
| 11.5 mm | metric | 0.4528 | 11.50 | 73% | |
| 29/64" | fraction | 0.4531 | 11.51 | 72% | chart drill |
| 11.6 mm | metric | 0.4567 | 11.60 | 67% | easier to tap |
| 11.7 mm | metric | 0.4606 | 11.70 | 61% | easier to tap |
About the 1/2-20 thread
1/2-20 is the fine thread at half an inch: 20 threads per inch against 13 for 1/2-13 UNC. Two places most people meet it are the mounting thread of the larger chucks on portable drills (smaller chucks often use 3/8-24) and the wheel studs and lug nuts of many US cars and light trucks. In machinery it is used where a half-inch bolt needs finer adjustment or a thinner tapped section than the coarse thread allows.
29/64 in (0.4531 in) is the chart drill, and the only number, letter or fractional drill in the 55 to 90% band near the target; it gives about 72% thread. 11.5 mm (0.4528 in) is practically the same size if you work with metric drills.
1/2-20 thread dimensions
| Dimension | Value | Also |
|---|---|---|
| Nominal diameter | 0.5000 in | 12.70 mm |
| Threads per inch | 20 | pitch 0.0500 in (1.270 mm) |
| Basic pitch diameter | 0.4675 in | 11.875 mm |
| Basic minor diameter (nut) | 0.4459 in | 11.325 mm |
| Tensile stress area | 0.1600 in² | 103.20 mm² |
| 1/2-13 UNC (same diameter): tap drill | 27/64" (0.4219 in) | 78% thread |
Clearance hole for 1/2
The tapped hole is in the part that holds the thread; the part being clamped gets a clearance hole the bolt passes through. For 1/2 a normal fit is 17/32" (0.5312 in). Never drill the clearance part with the tap drill: the screw will not pass.
| Fit | Drill | Diameter |
|---|---|---|
| Close fit | 33/64" | 0.5156 in |
| Normal fit | 17/32" | 0.5312 in |
| Loose fit | 9/16" | 0.5625 in |
How deep to drill a blind 1/2-20 hole
| Part material | Full thread wanted | Drill depth (full diameter) | Depth to drill tip (118°) |
|---|---|---|---|
| Steel or cast iron (1 × diameter) | 0.500 in | 0.850 in (about 27/32″) | 0.986 in (about 1″) |
| Aluminium or brass (2 × diameter) | 1.000 in | 1.350 in (about 1-11/32″) | 1.486 in (about 1-1/2″) |
Wrench size and torque for 1/2
- Hex cap screw: 3/4 in wrench; heavy hex: 7/8 in
- Socket head cap screw: 3/8 in hex key
| Bolt grade | Dry | Lubricated |
|---|---|---|
| Grade 2 | 55 ft-lb | 41 ft-lb |
| Grade 5 | 85 ft-lb | 64 ft-lb |
| Grade 8 | 120 ft-lb | 90 ft-lb |
Threads easily mistaken for 1/2-20
| Thread | Diameter (in) | Diameter (mm) | Pitch |
|---|---|---|---|
| 1/2-20 UNF (this page) | 0.5000 | 12.70 | 20 TPI (1.27 mm) |
| 1/2-13 UNC | 0.5000 | 12.70 | 13 TPI (1.95 mm) |
| M12 x 1.75 | 0.4724 | 12.00 | 1.75 mm (14.5 TPI) |
| M12 x 1.5 | 0.4724 | 12.00 | 1.5 mm (16.9 TPI) |
| M12 x 1.25 | 0.4724 | 12.00 | 1.25 mm (20.3 TPI) |
Tips for tapping 1/2-20
- Wheel studs and lug nuts are safety parts: replace a damaged stud rather than chase or re-tap it, and torque lug nuts to the vehicle maker’s figure.
- A 1/2-20 nut will not run on a 1/2-13 bolt; the coarse thread is the one on most hardware-store bolts and threaded rod.
- In aluminium, aim for at least 1 in of engagement (2 x diameter).
- Hex cap screws of this size take a 3/4 in wrench; heavy hex 7/8 in.
Frequently asked questions
What size drill bit for a 1/2-20 tap?
29/64" (0.4531 in), which gives about 72% thread. The exact 75% size is 0.4513 in (11.46 mm).
How deep should a blind 1/2-20 hole be?
For a full thread of 1 × diameter in steel, drill about 0.850 in (about 27/32″) at full diameter with a plug tap, or 0.986 in (about 1″) to the drill tip; in aluminium, about 1.350 in (about 1-11/32″).
What is the 1/2-20 tap drill in mm?
11.5 mm is the nearest 0.1 mm drill to the 75% size and gives 73% thread.
Is 29/64 the right drill for 1/2-20?
Yes. 29/64 in (0.453 in) gives about 72% thread, close to the 75% target; 11.5 mm is a near-identical metric alternative.
Are lug nuts 1/2-20?
Many US cars and light trucks have used 1/2-20 wheel studs; many other vehicles use metric threads such as M12 x 1.5 or M14 x 1.5. Check the vehicle maker’s specification rather than guessing.
Sources and method
Every dimension and percentage on this page is computed from the formulas listed, not copied from another chart. Values are basic (nominal); real parts are made to tolerance classes (2A/2B for inch, 6g/6H for metric).
- ASME B1.1 and FED-STD-H28/2B (Unified inch), ISO 68-1 and ISO 261 (metric): basic thread profile and stress area formulas.
- Machinery’s Handbook (Industrial Press): percentage-of-thread relation used by our tap drill size calculator.
- ASME B94.11M twist drill series (number, letter, fractional); 0.1 mm metric drill steps.
- ISO 273 metric clearance holes; inch close, normal and loose fits as published in the ASME B18.2.8 clearance classes.
- SAE J429 and ISO 898-1 proof strengths; T = K × D × F torque relation (see our bolt torque chart for the full method).
- Wrench and hex key sizes: ASME B18.2.1, ASME B18.3, ASME B18.6.3, ISO 4014/4017, DIN 933 and ISO 4762 values as widely published.
More thread and fastener tools
- All thread and fastener charts
- UNC and UNF thread chart
- Metric thread pitch chart
- Tap drill chart (inch and metric)
- Clearance hole chart
- Nail size chart
- Wood screw size chart
- Lag screw pilot hole chart
- Bolt torque chart
- Wrench size chart
- Drill bit size chart
- Thread engagement chart
- Nail gun nail sizes by gauge
- Screw and nail size calculator
- Tap drill size calculator
- Clearance hole size calculator
- Tapped hole depth calculator
Last reviewed: 24 October 2026. How we build and check our data: methodology and corrections. Spotted an error? Please let us know.