The tap drill for 1/2-13 UNC is 27/64" (0.4219 in, 10.72 mm), which gives about 78% of full thread depth. The exact 75% hole is 0.4251 in (10.80 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 |
|---|---|---|---|---|---|
| Z | letter | 0.4130 | 10.49 | 87% | deep, hard to tap |
| 10.6 mm | metric | 0.4173 | 10.60 | 83% | deep, hard to tap |
| 10.7 mm | metric | 0.4213 | 10.70 | 79% | |
| 27/64" | fraction | 0.4219 | 10.72 | 78% | chart drill |
| 10.8 mm | metric | 0.4252 | 10.80 | 75% | |
| 10.9 mm | metric | 0.4291 | 10.90 | 71% | |
| 11 mm | metric | 0.4331 | 11.00 | 67% | easier to tap |
| 7/16" | fraction | 0.4375 | 11.11 | 63% | easier to tap |
About the 1/2-13 thread
1/2-13 is the coarse half-inch thread: the standard thread on 1/2 in hex bolts, carriage bolts, threaded rod, J-bolts and wedge anchors. In a house you meet it at the bottom of the walls. The IRC (section R403.1.6) asks for sill plates to be held down with anchor bolts at least 1/2 in in diameter, so most sill anchors, and the nuts and washers on them, are 1/2-13. In the shop it is the thread on vise and machine-base bolts, heavy brackets, trailer fittings and the larger T-slot hardware.
Because it is a big tap, the drill size matters for a different reason than on small threads. A 27/64 in hole leaves a lot of metal for the tap to cut, and the torque needed to turn a 1/2-13 tap in steel is enough to snap a cheap tap handle or twist a part out of a vise. A drilled hole that is a few thousandths oversize makes almost no difference to strength at this diameter, which is why some machinists use 7/16 in in tough steel.
1/2-13 thread dimensions
| Dimension | Value | Also |
|---|---|---|
| Nominal diameter | 0.5000 in | 12.70 mm |
| Threads per inch | 13 | pitch 0.0769 in (1.954 mm) |
| Basic pitch diameter | 0.4500 in | 11.431 mm |
| Basic minor diameter (nut) | 0.4167 in | 10.585 mm |
| Tensile stress area | 0.1419 in² | 91.55 mm² |
| 1/2-20 UNF (same diameter): tap drill | 29/64" (0.4531 in) | 72% 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 1/2-13 hole
| Part material | Full thread wanted | Drill depth (full diameter) | Depth to drill tip (118°) |
|---|---|---|---|
| Steel or cast iron (1 × diameter) | 0.500 in | 1.038 in (about 1-1/32″) | 1.166 in (about 1-5/32″) |
| Aluminium or brass (2 × diameter) | 1.000 in | 1.538 in (about 1-17/32″) | 1.666 in (about 1-21/32″) |
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 | 49 ft-lb | 37 ft-lb |
| Grade 5 | 75 ft-lb | 57 ft-lb |
| Grade 8 | 106 ft-lb | 80 ft-lb |
Threads easily mistaken for 1/2-13
| Thread | Diameter (in) | Diameter (mm) | Pitch |
|---|---|---|---|
| 1/2-13 UNC (this page) | 0.5000 | 12.70 | 13 TPI (1.95 mm) |
| 1/2-20 UNF | 0.5000 | 12.70 | 20 TPI (1.27 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 1/2-13
- Use a T-handle or a full-size tap wrench and a drop of cutting oil; back the tap off half a turn every turn in steel to break the chip.
- If the hole is through a plate thinner than about 1/2 in, consider a nut and a clearance hole instead: thin material holds only a few threads.
- For anchor bolts into concrete, you are not tapping at all: drill the hole size the anchor maker specifies, usually the anchor diameter (1/2 in for a 1/2 in wedge anchor).
- A 12.7 mm (1/2 in) bolt hole in timber wants a 9/16 in bit so the bolt can be driven without splitting.
Frequently asked questions
What size drill bit for a 1/2-13 tap?
27/64" (0.4219 in), which gives about 78% thread. The exact 75% size is 0.4251 in (10.80 mm).
How deep should a blind 1/2-13 hole be?
For a full thread of 1 × diameter in steel, drill about 1.038 in (about 1-1/32″) at full diameter with a plug tap, or 1.166 in (about 1-5/32″) to the drill tip; in aluminium, about 1.538 in (about 1-17/32″).
What is the 1/2-13 tap drill in mm?
10.8 mm is the nearest 0.1 mm drill to the 75% size and gives 75% thread.
Is 27/64 or 7/16 the right drill for 1/2-13?
27/64 in is the usual chart drill and gives a thread close to the 75% target. 7/16 in (0.4375 in) gives a shallower thread, still strong enough for most work at this size, and is easier to tap in hard steel or stainless.
What size are sill plate anchor bolts?
The IRC (R403.1.6) calls for anchor bolts at least 1/2 in in diameter, so 1/2-13 is the usual thread. Spacing and embedment rules are in the same section; check the edition your local building department uses.
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.
- International Residential Code (2021), section R403.1.6, foundation anchorage: paraphrased, not quoted. Check the edition adopted by your local building department.
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.