5/16-18 UNC Thread Dimensions and Tap Drill
5/16-18 UNC is the Unified coarse thread for the 5/16 in (7.94 mm) diameter: 18 threads per inch, a pitch of 0.05556 in (1.411 mm), on a 60° profile. ASME B1.1 lists it in the UNC series. Finer threads on the same diameter are 5/16-24 UNF and 5/16-32 UNEF.
5/16-18 is a common size for small engine, bracket and furniture bolts.
- Tap drillF77% thread, 6.53 mm
- Pitch18 TPI1.411 mm
- Major diameter0.3125 in7.938 mm
- Pitch diameter0.2764 in7.021 mm
- Minor diameter (nut)0.2524 in6.410 mm
- Tensile stress area0.0524 in²33.8 mm²
- Clearance hole11/32 innormal fit, ASME B18.2.8
Thread profile
5/16-18 UNC basic dimensions
| Dimension | in | mm |
|---|---|---|
| Major diameter d = D | 0.3125 | 7.938 |
| Pitch P | 0.05556 | 1.411 |
| Pitch diameter d2 = D2 | 0.2764 | 7.021 |
| Minor diameter, internal thread D1 | 0.2524 | 6.410 |
| Minor diameter, external UNR thread d3 | 0.2463 | 6.257 |
| Height of fundamental triangle H | 0.0481 | 1.222 |
| Thread height, internal (5H/8) | 0.0301 | 0.764 |
| Thread height, external (11H/16) | 0.0331 | 0.840 |
| Crest flat of external thread (P/8) | 0.0069 | 0.176 |
| Crest flat of internal thread (P/4) | 0.0139 | 0.353 |
| Property | Value |
|---|---|
| Threads per inch | 18 |
| Flank angle | 60° (30° each side) |
| Lead angle at the pitch diameter | 3.66° |
| Tensile stress area As | 0.0524 in² (33.8 mm²) |
Basic (nominal) sizes per ASME B1.1, before tolerances. A UN external thread with a flat root has the same basic minor diameter as the nut (D1); d3 applies to the rounded UNR root.
5/16-18 UNC limits of size (class 2A bolt, 2B nut)
Classes 2A and 2B are the general-purpose fits for commercial bolts, screws and nuts. The 2A external thread has an allowance of 0.0012 in, so its maximum size is that much below basic; the 2B nut starts at the basic size.
| External thread 2A | Max (in) | Min (in) | Tolerance (in) |
|---|---|---|---|
| Major diameter | 0.3113 | 0.3026 | 0.0087 |
| Pitch diameter | 0.2752 | 0.2712 | 0.0040 |
| Internal thread 2B | Max (in) | Min (in) | Tolerance (in) |
|---|---|---|---|
| Minor diameter | 0.265 | 0.252 | 0.0130 |
| Pitch diameter | 0.2817 | 0.2764 | 0.0053 |
Limits of size from ASME B1.1 Table 2.
5/16-18 UNC tap drill size
Tap drill for 5/16-18 UNC: F (0.2570 in, 6.53 mm)
This leaves about 77% thread in the tapped hole.
F is the drill listed for 5/16-18 UNC in standard tap drill charts. It is 0.0555 in under the 0.3125 in major diameter.
The 2B minor diameter of the finished nut thread must lie between 0.252 and 0.265 in. The recommended drill falls inside that band.
Other drills that work for 5/16-18 UNC
A larger drill cuts less thread and taps more easily; a smaller one gives a fuller thread but loads the tap harder.
| Drill | mm | in | % thread | Within 2B limits | Thread engagement |
|---|---|---|---|---|---|
| 1/4" | 6.35 | 0.2500 | 87% | no | |
| E | 6.35 | 0.2500 | 87% | no | |
| 6.5 mm | 6.50 | 0.2559 | 78% | yes | |
| Frecommended | 6.53 | 0.2570 | 77% | yes | |
| G | 6.63 | 0.2610 | 71% | yes | |
| 17/64" | 6.75 | 0.2656 | 65% | no | |
| H | 6.76 | 0.2660 | 64% | no | |
| I | 6.91 | 0.2720 | 56% | no |
% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.
Clearance hole for 5/16 in
The clearance hole depends only on the 0.3125 in diameter, not on the thread series. The normal fit, a 11/32 in drill, is the usual choice for general assembly.
| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Clearance on Ø (in) |
|---|---|---|---|---|
| Close | 21/64 in | 0.3281 | 8.33 | 0.0156 |
| Normal | 11/32 in | 0.3438 | 8.73 | 0.0312 |
| Loose | 23/64 in | 0.3594 | 9.13 | 0.0469 |
Nominal drill sizes from ASME B18.2.8.
5/16-18 UNC proof and breaking loads
Loads for steel bolts by SAE grade, from the tensile stress area of 0.0524 in². The proof load is the highest load the bolt must carry without permanent set.
| SAE grade | Proof stress (psi) | Min tensile strength (psi) | Proof load (lbf) | Min breaking load (lbf) | Proof load (kN) |
|---|---|---|---|---|---|
| Grade 2 | 55,000 | 74,000 | 2,880 | 3,880 | 12.8 |
| Grade 5 | 85,000 | 120,000 | 4,450 | 6,290 | 19.8 |
| Grade 8 | 120,000 | 150,000 | 6,290 | 7,860 | 28.0 |
Strengths from SAE J429 for this diameter; stress area from ASME B1.1. Loads rounded to three significant figures.
5/16 in pitches compared
All standard pitches on the 5/16 in diameter. A finer pitch gives a shallower thread, a larger core and a smaller lead angle; a coarser pitch is faster to assemble and more tolerant of damage and plating.
| Thread | TPI | Pitch Ø (in) | Minor Ø D1 (in) | Stress area (in²) | Tap drill |
|---|---|---|---|---|---|
| 5/16-18 UNC | 18 | 0.2764 | 0.2524 | 0.0524 | F |
| 5/16-24 UNF | 24 | 0.2854 | 0.2674 | 0.0580 | I |
| 5/16-32 UNEF | 32 | 0.2922 | 0.2787 | 0.0625 | 9/32" |
Closest metric thread
No metric thread is interchangeable with 5/16-18 UNC. The nearest is M8×1.25, which is 0.06 mm larger in diameter with a pitch of 1.250 mm against 1.411 mm. A nut may start on the wrong thread and then bind or strip, so identify by both diameter and pitch.
How the 5/16-18 UNC dimensions are calculated
Every basic dimension follows from the major diameter and the pitch:
- P = 0.05556 in = 1 ÷ 18 threads per inch
- H = 0.866025 × P = 0.0481 in
- d2 = d − 0.649519 × P = 0.3125 − 0.0361 = 0.2764 in
- D1 = d − 1.082532 × P = 0.3125 − 0.0601 = 0.2524 in
- d3 = d − 1.190785 × P = 0.3125 − 0.0662 = 0.2463 in (external root, 11H/16 deep)
- As = 0.7854 × (d − 0.9743 × P)² = 0.7854 × 0.2584² = 0.05243 in²
- Lead angle = arctan(P ÷ (π × d2)) = 3.66°
Related sizes and tools
Sources: ASME B1.1 (profile, series, limits of size and stress area), ASME B18.2.8 (clearance holes), SAE J429 (mechanical properties) and the tap drill tables of Machinery’s Handbook. How this site checks its data. Last reviewed 29 September 2026.