The tap drill for 3/4-10 UNC is 21/32" (0.6562 in, 16.67 mm), which gives about 72% of full thread depth. The exact 75% hole is 0.6526 in (16.58 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 |
|---|---|---|---|---|---|
| 41/64" | fraction | 0.6406 | 16.27 | 84% | deep, hard to tap |
| 16.4 mm | metric | 0.6457 | 16.40 | 80% | deep, hard to tap |
| 16.5 mm | metric | 0.6496 | 16.50 | 77% | |
| 16.6 mm | metric | 0.6535 | 16.60 | 74% | |
| 21/32" | fraction | 0.6562 | 16.67 | 72% | chart drill |
| 16.7 mm | metric | 0.6575 | 16.70 | 71% | |
| 16.8 mm | metric | 0.6614 | 16.80 | 68% | |
| 43/64" | fraction | 0.6719 | 17.07 | 60% | easier to tap |
About the 3/4-10 thread
3/4-10 is the coarse 3/4 in thread: large hex bolts, threaded rod, foundation anchor rods and hold-down rods, machine feet, heavy equipment and agricultural implements. A common way to ask for it is “what size drill bit do I need for a 3/4 in national coarse tap”: national coarse (NC) is the older name for UNC, so that is this thread.
21/32 in (0.656 in) is the chart drill. A 3/4-10 tap is a serious tool: it needs a tap wrench with long handles, a firmly clamped part and cutting oil, and in steel many shops would rather use a nut, a weld nut or a thread-cutting machine than tap by hand.
3/4-10 thread dimensions
| Dimension | Value | Also |
|---|---|---|
| Nominal diameter | 0.7500 in | 19.05 mm |
| Threads per inch | 10 | pitch 0.1000 in (2.540 mm) |
| Basic pitch diameter | 0.6850 in | 17.400 mm |
| Basic minor diameter (nut) | 0.6417 in | 16.300 mm |
| Tensile stress area | 0.3345 in² | 215.78 mm² |
| 3/4-16 UNF (same diameter): tap drill | 11/16" (0.6875 in) | 77% thread |
Clearance hole for 3/4
The tapped hole is in the part that holds the thread; the part being clamped gets a clearance hole the bolt passes through. For 3/4 a normal fit is 25/32" (0.7812 in). Never drill the clearance part with the tap drill: the screw will not pass.
| Fit | Drill | Diameter |
|---|---|---|
| Close fit | 49/64" | 0.7656 in |
| Normal fit | 25/32" | 0.7812 in |
| Loose fit | 13/16" | 0.8125 in |
How deep to drill a blind 3/4-10 hole
| Part material | Full thread wanted | Drill depth (full diameter) | Depth to drill tip (118°) |
|---|---|---|---|
| Steel or cast iron (1 × diameter) | 0.750 in | 1.450 in (about 1-7/16″) | 1.646 in (about 1-21/32″) |
| Aluminium or brass (2 × diameter) | 1.500 in | 2.200 in (about 2-3/16″) | 2.396 in (about 2-13/32″) |
Wrench size and torque for 3/4
- Hex cap screw: 1-1/8 in wrench; heavy hex: 1-1/4 in
- Socket head cap screw: 5/8 in hex key
| Bolt grade | Dry | Lubricated |
|---|---|---|
| Grade 2 | 172 ft-lb | 129 ft-lb |
| Grade 5 | 267 ft-lb | 200 ft-lb |
| Grade 8 | 376 ft-lb | 282 ft-lb |
Threads easily mistaken for 3/4-10
| Thread | Diameter (in) | Diameter (mm) | Pitch |
|---|---|---|---|
| 3/4-10 UNC (this page) | 0.7500 | 19.05 | 10 TPI (2.54 mm) |
| 3/4-16 UNF | 0.7500 | 19.05 | 16 TPI (1.59 mm) |
| M20 x 2.5 | 0.7874 | 20.00 | 2.5 mm (10.2 TPI) |
| M20 x 1.5 | 0.7874 | 20.00 | 1.5 mm (16.9 TPI) |
| M18 x 2.5 | 0.7087 | 18.00 | 2.5 mm (10.2 TPI) |
| M18 x 1.5 | 0.7087 | 18.00 | 1.5 mm (16.9 TPI) |
Tips for tapping 3/4-10
- Drill in stages (for example 3/8 in, then 9/16 in, then 21/32 in) so the large drill cuts cleanly and on centre.
- A 3/4 in bolt through timber goes in a hole 1/32 to 1/16 in larger than the bolt (the usual rule for bolted wood connections); wood is not tapped for a bolt this size.
- In aluminium, a 3/4-10 thread needs about 1-1/2 in of engagement for the bolt to break first.
- Large hex bolts of this size use a 1-1/8 in wrench; heavy hex nuts need 1-1/4 in.
Frequently asked questions
What size drill bit for a 3/4-10 tap?
21/32" (0.6562 in), which gives about 72% thread. The exact 75% size is 0.6526 in (16.58 mm).
How deep should a blind 3/4-10 hole be?
For a full thread of 1 × diameter in steel, drill about 1.450 in (about 1-7/16″) at full diameter with a plug tap, or 1.646 in (about 1-21/32″) to the drill tip; in aluminium, about 2.200 in (about 2-3/16″).
What is national coarse thread?
The older American (National Coarse, NC) name for the coarse series. NC and UNC threads of the same size and pitch interchange; 3/4 NC is 3/4-10.
What size drill for a 3/4-10 tap in mm?
16.5 mm (0.650 in) is the nearest common metric drill to 21/32 in (16.67 mm) and gives a slightly deeper thread; 16.7 mm is closer if you can find it.
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.