The tap drill for 5/16-18 UNC is F (0.2570 in, 6.53 mm), which gives about 77% of full thread depth. The exact 75% hole is 0.2584 in (6.56 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 |
|---|---|---|---|---|---|
| E | letter | 0.2500 | 6.35 | 87% | deep, hard to tap |
| 1/4" | fraction | 0.2500 | 6.35 | 87% | deep, hard to tap |
| 6.4 mm | metric | 0.2520 | 6.40 | 84% | deep, hard to tap |
| 6.5 mm | metric | 0.2559 | 6.50 | 78% | |
| F | letter | 0.2570 | 6.53 | 77% | chart drill |
| 6.6 mm | metric | 0.2598 | 6.60 | 73% | |
| G | letter | 0.2610 | 6.63 | 71% | |
| 6.7 mm | metric | 0.2638 | 6.70 | 68% | easier to tap |
| 17/64" | fraction | 0.2656 | 6.75 | 65% | easier to tap |
| H | letter | 0.2660 | 6.76 | 64% | easier to tap |
| 6.8 mm | metric | 0.2677 | 6.80 | 62% | easier to tap |
About the 5/16-18 thread
5/16-18 is the coarse 5/16 in thread. It is common on lawn mower and small-engine parts, car and truck accessory brackets, bed-frame and furniture bolts, and the hanger bolts that hold table legs to corner brackets (hanger bolts are made in 1/4-20, 5/16-18 and 3/8-16). It is also one of the sizes most often mistaken for a metric one: an M8 x 1.25 bolt is only 0.0025 in larger in diameter and close in pitch, so it starts in a 5/16-18 hole and then jams.
Most charts give letter drill F (0.257 in) for 5/16-18, and the arithmetic below shows why: the 75% hole falls between letter drills, and F is the nearest. If you have only fractional drills, 17/64 in is the closest, but it leaves noticeably less thread.
5/16-18 thread dimensions
| Dimension | Value | Also |
|---|---|---|
| Nominal diameter | 0.3125 in | 7.94 mm |
| Threads per inch | 18 | pitch 0.0556 in (1.411 mm) |
| Basic pitch diameter | 0.2764 in | 7.021 mm |
| Basic minor diameter (nut) | 0.2524 in | 6.410 mm |
| Tensile stress area | 0.0524 in² | 33.83 mm² |
| 5/16-24 UNF (same diameter): tap drill | I (0.2720 in) | 75% thread |
Clearance hole for 5/16
The tapped hole is in the part that holds the thread; the part being clamped gets a clearance hole the bolt passes through. For 5/16 a normal fit is 11/32" (0.3438 in). Never drill the clearance part with the tap drill: the screw will not pass.
| Fit | Drill | Diameter |
|---|---|---|
| Close fit | 21/64" | 0.3281 in |
| Normal fit | 11/32" | 0.3438 in |
| Loose fit | 3/8" | 0.3750 in |
How deep to drill a blind 5/16-18 hole
| Part material | Full thread wanted | Drill depth (full diameter) | Depth to drill tip (118°) |
|---|---|---|---|
| Steel or cast iron (1 × diameter) | 0.312 in | 0.701 in (about 11/16″) | 0.779 in (about 25/32″) |
| Aluminium or brass (2 × diameter) | 0.625 in | 1.014 in (about 1″) | 1.092 in (about 1-3/32″) |
Wrench size and torque for 5/16
- Hex cap screw: 1/2 in wrench
- Socket head cap screw: 1/4 in hex key
| Bolt grade | Dry | Lubricated |
|---|---|---|
| Grade 2 | 11 ft-lb | 8 ft-lb |
| Grade 5 | 17 ft-lb | 13 ft-lb |
| Grade 8 | 25 ft-lb | 18 ft-lb |
Threads easily mistaken for 5/16-18
| Thread | Diameter (in) | Diameter (mm) | Pitch |
|---|---|---|---|
| 5/16-18 UNC (this page) | 0.3125 | 7.94 | 18 TPI (1.41 mm) |
| 5/16-24 UNF | 0.3125 | 7.94 | 24 TPI (1.06 mm) |
| M8 x 1.25 | 0.3150 | 8.00 | 1.25 mm (20.3 TPI) |
| M8 x 1 | 0.3150 | 8.00 | 1 mm (25.4 TPI) |
Tips for tapping 5/16-18
- Letter F is not in most fractional sets. A 6.5 mm drill (0.256 in) is almost the same size and easy to buy.
- If a bolt starts and then stops after two turns, check whether it is M8 x 1.25: 18 TPI is 1.41 mm pitch, so the threads drift out of step quickly.
- For hanger bolts in table legs, drill the wood end to the hanger bolt maker’s pilot size; the 5/16-18 machine end goes into the corner bracket or a nut.
- Some small-engine and automotive parts use the fine 5/16-24 thread instead; measure the pitch before buying a tap.
Frequently asked questions
What size drill bit for a 5/16-18 tap?
F (0.2570 in), which gives about 77% thread. The exact 75% size is 0.2584 in (6.56 mm).
How deep should a blind 5/16-18 hole be?
For a full thread of 1 × diameter in steel, drill about 0.701 in (about 11/16″) at full diameter with a plug tap, or 0.779 in (about 25/32″) to the drill tip; in aluminium, about 1.014 in (about 1″).
What is the 5/16-18 tap drill in mm?
6.6 mm is the nearest 0.1 mm drill to the 75% size and gives 73% thread.
What is the metric tap drill for 5/16-18?
6.5 mm (0.2559 in) is nearly identical to the letter F drill and gives about the same thread percentage.
Can I tap 5/16-18 with a 1/4 in drill?
A 1/4 in (0.250 in) hole gives a thread deeper than 80%, which makes the tap hard to turn and likely to break in steel. It can work in soft aluminium or plastic, but F or 6.5 mm is the safer choice.
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.