The tap drill for 8-32 UNC is #29 (0.1360 in, 3.45 mm), which gives about 69% of full thread depth. The exact 75% hole is 0.1336 in (3.39 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 |
|---|---|---|---|---|---|
| #30 | number | 0.1285 | 3.26 | 87% | deep, hard to tap |
| 3.3 mm | metric | 0.1299 | 3.30 | 84% | deep, hard to tap |
| 3.4 mm | metric | 0.1339 | 3.40 | 74% | |
| #29 | number | 0.1360 | 3.45 | 69% | chart drill |
| 3.5 mm | metric | 0.1378 | 3.50 | 65% | easier to tap |
| #28 | number | 0.1405 | 3.57 | 58% | easier to tap |
| 9/64" | fraction | 0.1406 | 3.57 | 58% | easier to tap |
About the 8-32 thread
8-32 is the standard (coarse) #8 machine screw thread, 0.164 in nominal. Electricians use it every day: the mounting ears and crossbars of ceiling and wall boxes for light fixtures are usually tapped 8-32, while switches and receptacles use the smaller 6-32. It is also common in sheet-metal enclosures, small hinges, appliance panels and many knobs and handles.
The usual tap drill is #29 (0.136 in), which gives close to 70% thread. Some charts list #29 for 8-32 and #29 again for the fine 8-36 thread, which shows how little the drill choice changes between nearby pitches; the computed percentages below show the actual difference.
8-32 thread dimensions
| Dimension | Value | Also |
|---|---|---|
| Nominal diameter | 0.1640 in | 4.17 mm |
| Threads per inch | 32 | pitch 0.0312 in (0.794 mm) |
| Basic pitch diameter | 0.1437 in | 3.650 mm |
| Basic minor diameter (nut) | 0.1302 in | 3.306 mm |
| Tensile stress area | 0.0140 in² | 9.04 mm² |
| #8-36 UNF (same diameter): tap drill | #29 (0.1360 in) | 78% thread |
Clearance hole for #8
The tapped hole is in the part that holds the thread; the part being clamped gets a clearance hole the bolt passes through. For #8 a normal fit is #15 (0.1800 in). Never drill the clearance part with the tap drill: the screw will not pass.
| Fit | Drill | Diameter |
|---|---|---|
| Close fit | #18 | 0.1695 in |
| Normal fit | #15 | 0.1800 in |
| Loose fit | #10 | 0.1935 in |
How deep to drill a blind 8-32 hole
| Part material | Full thread wanted | Drill depth (full diameter) | Depth to drill tip (118°) |
|---|---|---|---|
| Steel or cast iron (1 × diameter) | 0.164 in | 0.383 in (about 3/8″) | 0.423 in (about 7/16″) |
| Aluminium or brass (2 × diameter) | 0.328 in | 0.547 in (about 17/32″) | 0.587 in (about 19/32″) |
Wrench size for #8
- Machine screw hex nut: 11/32 in wrench
- Socket head cap screw: 9/64 in hex key
SAE bolt grades start at 1/4 in, so there is no graded torque for #8 machine screws; tighten to the maker’s figure or until snug. Over-tightening is the usual way small tapped threads strip.
Threads easily mistaken for 8-32
| Thread | Diameter (in) | Diameter (mm) | Pitch |
|---|---|---|---|
| #8-32 UNC (this page) | 0.1640 | 4.17 | 32 TPI (0.79 mm) |
| #8-36 UNF | 0.1640 | 4.17 | 36 TPI (0.71 mm) |
| M4 x 0.7 | 0.1575 | 4.00 | 0.7 mm (36.3 TPI) |
Tips for tapping 8-32
- Damaged 8-32 threads in a fixture box can often be cleaned up with an 8-32 tap; if the thread is gone, replace the box or the strap rather than forcing in a larger screw. Switch off the circuit first.
- An 8-32 screw is easy to confuse with M4 x 0.7 (0.157 in) in a mixed bin; the M4 is slightly smaller and finer.
- 9/64 in (0.1406 in) is the nearest fractional drill and gives a shallower thread, acceptable in aluminium and plastic.
- Use a tap guide or a drill press to keep small taps square; a tap that starts crooked breaks on the third or fourth turn.
Frequently asked questions
What size drill bit for an 8-32 tap?
#29 (0.1360 in), which gives about 69% thread. The exact 75% size is 0.1336 in (3.39 mm).
How deep should a blind 8-32 hole be?
For a full thread of 1 × diameter in steel, drill about 0.383 in (about 3/8″) at full diameter with a plug tap, or 0.423 in (about 7/16″) to the drill tip; in aluminium, about 0.547 in (about 17/32″).
What is the 8-32 tap drill in mm?
3.4 mm is the nearest 0.1 mm drill to the 75% size and gives 74% thread.
What is the clearance hole for an 8-32 screw?
A #15 drill (0.180 in) for a normal fit, #18 (0.1695 in) for a close fit and #10 (0.1935 in) for a loose one.
Do electrical boxes use 8-32 or 6-32?
Most switch and receptacle devices mount with 6-32 screws; light fixture straps and the ears of round ceiling boxes are usually 8-32. Fan-rated boxes may use larger screws supplied with the box. Check before you tap or re-tap anything in an electrical box.
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.