The tap drill for 3/8-24 UNF is Q (0.3320 in, 8.43 mm), which gives about 79% of full thread depth. The exact 75% hole is 0.3344 in (8.49 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 |
|---|---|---|---|---|---|
| 8.3 mm | metric | 0.3268 | 8.30 | 89% | deep, hard to tap |
| 21/64" | fraction | 0.3281 | 8.33 | 87% | deep, hard to tap |
| 8.4 mm | metric | 0.3307 | 8.40 | 82% | deep, hard to tap |
| Q | letter | 0.3320 | 8.43 | 79% | chart drill |
| 8.5 mm | metric | 0.3346 | 8.50 | 75% | |
| 8.6 mm | metric | 0.3386 | 8.60 | 67% | easier to tap |
| R | letter | 0.3390 | 8.61 | 67% | easier to tap |
| 8.7 mm | metric | 0.3425 | 8.70 | 60% | easier to tap |
| 11/32" | fraction | 0.3438 | 8.73 | 58% | easier to tap |
About the 3/8-24 thread
3/8-24 is the fine thread at 3/8 in: 24 threads per inch on a 0.375 in diameter, against 16 for the coarse 3/8-16. It is common in automotive and machinery work, where fine threads are often chosen for thin sections and fine adjustment, and it is one of the two usual mounting threads for the chuck on a portable drill, the other being 1/2-20.
Q (0.332 in) is the chart drill and gives about 79% thread. R (0.339 in) gives about two thirds and is easier to tap by hand. 21/64 in, the nearest fractional size below, is deeper again at about 87%, which makes the tap work much harder for little gain. If you have no letter drills, 8.5 mm is a close metric substitute.
3/8-24 thread dimensions
| Dimension | Value | Also |
|---|---|---|
| Nominal diameter | 0.3750 in | 9.52 mm |
| Threads per inch | 24 | pitch 0.0417 in (1.058 mm) |
| Basic pitch diameter | 0.3479 in | 8.838 mm |
| Basic minor diameter (nut) | 0.3299 in | 8.379 mm |
| Tensile stress area | 0.0878 in² | 56.66 mm² |
| 3/8-16 UNC (same diameter): tap drill | 5/16" (0.3125 in) | 77% thread |
Clearance hole for 3/8
The tapped hole is in the part that holds the thread; the part being clamped gets a clearance hole the bolt passes through. For 3/8 a normal fit is 13/32" (0.4062 in). Never drill the clearance part with the tap drill: the screw will not pass.
| Fit | Drill | Diameter |
|---|---|---|
| Close fit | 25/64" | 0.3906 in |
| Normal fit | 13/32" | 0.4062 in |
| Loose fit | 7/16" | 0.4375 in |
How deep to drill a blind 3/8-24 hole
| Part material | Full thread wanted | Drill depth (full diameter) | Depth to drill tip (118°) |
|---|---|---|---|
| Steel or cast iron (1 × diameter) | 0.375 in | 0.667 in (about 21/32″) | 0.767 in (about 25/32″) |
| Aluminium or brass (2 × diameter) | 0.750 in | 1.042 in (about 1-1/32″) | 1.142 in (about 1-5/32″) |
Wrench size and torque for 3/8
- Hex cap screw: 9/16 in wrench
- Socket head cap screw: 5/16 in hex key
| Bolt grade | Dry | Lubricated |
|---|---|---|
| Grade 2 | 23 ft-lb | 17 ft-lb |
| Grade 5 | 35 ft-lb | 26 ft-lb |
| Grade 8 | 49 ft-lb | 37 ft-lb |
Threads easily mistaken for 3/8-24
| Thread | Diameter (in) | Diameter (mm) | Pitch |
|---|---|---|---|
| 3/8-24 UNF (this page) | 0.3750 | 9.52 | 24 TPI (1.06 mm) |
| 3/8-16 UNC | 0.3750 | 9.52 | 16 TPI (1.59 mm) |
| M10 x 1.5 | 0.3937 | 10.00 | 1.5 mm (16.9 TPI) |
| M10 x 1.25 | 0.3937 | 10.00 | 1.25 mm (20.3 TPI) |
| M10 x 1 | 0.3937 | 10.00 | 1 mm (25.4 TPI) |
Tips for tapping 3/8-24
- 8.5 mm (0.335 in) gives about 75% thread, the easiest substitute if your set has no letter drills.
- A 3/8-24 nut on a 3/8-16 bolt, or the reverse, jams within a turn; if a nut gets tight early, stop and check the pitch.
- In aluminium, aim for at least 3/4 in of engagement (2 x diameter).
- Hex cap screws of this size take a 9/16 in wrench.
Frequently asked questions
What size drill bit for a 3/8-24 tap?
Q (0.3320 in), which gives about 79% thread. The exact 75% size is 0.3344 in (8.49 mm).
How deep should a blind 3/8-24 hole be?
For a full thread of 1 × diameter in steel, drill about 0.667 in (about 21/32″) at full diameter with a plug tap, or 0.767 in (about 25/32″) to the drill tip; in aluminium, about 1.042 in (about 1-1/32″).
What is the 3/8-24 tap drill in mm?
8.5 mm is the nearest 0.1 mm drill to the 75% size and gives 75% thread.
What drill for a 3/8-24 tap if I do not have a Q drill?
8.5 mm (0.3346 in) gives about 75% thread and is easy to find; R (0.339 in) gives about 67%.
Is a drill chuck thread 3/8-24?
Many portable drill chucks mount on 3/8-24 or 1/2-20 UNF threads. Check the chuck or the drill maker’s specification before buying a replacement, since other threads are used too.
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.