The tap drill for 3/8-16 UNC is 5/16" (0.3125 in, 7.94 mm), which gives about 77% of full thread depth. The exact 75% hole is 0.3141 in (7.98 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 |
|---|---|---|---|---|---|
| N | letter | 0.3020 | 7.67 | 90% | deep, hard to tap |
| 7.8 mm | metric | 0.3071 | 7.80 | 84% | deep, hard to tap |
| 7.9 mm | metric | 0.3110 | 7.90 | 79% | |
| 5/16" | fraction | 0.3125 | 7.94 | 77% | chart drill |
| 8 mm | metric | 0.3150 | 8.00 | 74% | |
| O | letter | 0.3160 | 8.03 | 73% | |
| 8.1 mm | metric | 0.3189 | 8.10 | 69% | |
| 8.2 mm | metric | 0.3228 | 8.20 | 64% | easier to tap |
| P | letter | 0.3230 | 8.20 | 64% | easier to tap |
| 21/64" | fraction | 0.3281 | 8.33 | 58% | easier to tap |
About the 3/8-16 thread
3/8-16 is the coarse 3/8 in thread used on general-purpose hex bolts, carriage bolts, lag-bolt alternatives in steel, threaded rod for hanging pipe and duct, and many machine feet and levelling pads. Photographers know it as the larger tripod thread: the head of most tripods screws on with 3/8-16, while the camera itself uses 1/4-20. In a woodshop, 3/8-16 threaded inserts and knobs show up on heavier jigs, router-table fences and workbench hold-downs, where a 1/4-20 knob would strip or bend.
The usual tap drill, 5/16 in, is an easy one to remember and to find: it is a fractional drill that every basic set includes, and it lands close to the 75% thread target, so there is rarely a reason to look for anything else.
3/8-16 thread dimensions
| Dimension | Value | Also |
|---|---|---|
| Nominal diameter | 0.3750 in | 9.52 mm |
| Threads per inch | 16 | pitch 0.0625 in (1.587 mm) |
| Basic pitch diameter | 0.3344 in | 8.494 mm |
| Basic minor diameter (nut) | 0.3073 in | 7.806 mm |
| Tensile stress area | 0.0775 in² | 49.99 mm² |
| 3/8-24 UNF (same diameter): tap drill | Q (0.3320 in) | 79% 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 3/8-16 hole
| Part material | Full thread wanted | Drill depth (full diameter) | Depth to drill tip (118°) |
|---|---|---|---|
| Steel or cast iron (1 × diameter) | 0.375 in | 0.812 in (about 13/16″) | 0.907 in (about 29/32″) |
| Aluminium or brass (2 × diameter) | 0.750 in | 1.188 in (about 1-3/16″) | 1.282 in (about 1-9/32″) |
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 | 20 ft-lb | 15 ft-lb |
| Grade 5 | 31 ft-lb | 23 ft-lb |
| Grade 8 | 44 ft-lb | 33 ft-lb |
Threads easily mistaken for 3/8-16
| Thread | Diameter (in) | Diameter (mm) | Pitch |
|---|---|---|---|
| 3/8-16 UNC (this page) | 0.3750 | 9.52 | 16 TPI (1.59 mm) |
| 3/8-24 UNF | 0.3750 | 9.52 | 24 TPI (1.06 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 3/8-16
- A 5/16 in drill is also the tap drill for 3/8-16; do not confuse it with the 5/16-18 thread, whose tap drill is letter F.
- For 3/8-16 threaded inserts in hardwood, drill the hole size the insert maker gives: the insert, not the bolt, sets the hole.
- Threaded rod hangers: the rod and coupling nut are 3/8-16; a 7/16 in hole lets the rod pass through a strut or angle.
- In aluminium, 3/8-16 needs about 3/4 in of thread for the bolt to break before the thread strips, so plan the boss length early.
Frequently asked questions
What size drill bit for a 3/8-16 tap?
5/16" (0.3125 in), which gives about 77% thread. The exact 75% size is 0.3141 in (7.98 mm).
How deep should a blind 3/8-16 hole be?
For a full thread of 1 × diameter in steel, drill about 0.812 in (about 13/16″) at full diameter with a plug tap, or 0.907 in (about 29/32″) to the drill tip; in aluminium, about 1.188 in (about 1-3/16″).
What drill for a 3/8-16 tap in mm?
An 8.0 mm drill is the nearest common metric size; it gives slightly less thread than 5/16 in (7.94 mm) but is fine for most work. 7.9 mm is closer still if you have it.
Is 3/8-16 the same as M10?
No. An M10 x 1.5 bolt is 0.394 in across against 0.375 in, and its pitch (1.5 mm) is close to but not the same as 16 TPI (1.59 mm). An M10 bolt will not thread into a 3/8-16 hole.
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.