Dimensions, drill sizes and wrench sizes for 5/16-18 UNC.
Designation
5/16-18 UNC
Major (outside) diameter
7.938 mm · 0.3125"
Pitch
1.411 mm · 18 TPI
Pitch diameter
7.021 mm · 0.2764"
Minor diameter (external)
6.206 mm · 0.2443"
Minor diameter (internal)
6.41 mm · 0.2524"
Recommended tap drill
F · 6.53 mm · ≈77% thread
Clearance hole
Close 21/64" · Normal 11/32" · Loose 3/8" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Hex head (across flats)
1/2"
Socket key (across flats)
1/4"
Close 21/64" · Normal 11/32" · Loose 3/8" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
5/16-18 UNC in numbers
5/16-18 UNC is 0.3125 in across the crests with 18 threads per inch, a 1.411 mm pitch. The basic pitch diameter is 7.021 mm and the bolt's minor diameter 6.206 mm, so the thread is 0.866 mm deep on the bolt. A class 2A bolt is made with its pitch diameter between 0.2712 and 0.2752 in, and a class 2B nut between 0.2764 and 0.2817 in, so even the largest bolt clears the smallest nut by 0.0012 in. The nearest metric size, M8 × 1.25, is 0.06 mm larger across the crests and its pitch is 0.161 mm finer, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 7.938 mm and P = 1.411 mm. A smaller hole leaves more thread; the recommended F drill above gives about 77%.
Tap-drill hole diameters for 5/16-18 UNC at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
6.84
0.2692
I (0.2720 in) gives 56%
65%
6.75
0.2656
17/64" (0.2656 in) gives 65%
70%
6.65
0.2620
G (0.2610 in) gives 71%
75%
6.56
0.2584
F (0.2570 in) gives 77%
Class 2A and 2B limits
The commercial fit for 5/16-18 UNC: class 2A for the bolt, class 2B for the nut or tapped hole. Inches.
Class 2A external and class 2B internal limits of size for 5/16-18 UNC.
Dimension
Min
Max
2A major diameter
0.3026
0.3113
2A pitch diameter
0.2712
0.2752
2B pitch diameter
0.2764
0.2817
Matches the ASME B1.1 class 2A/2B table for this size, as printed in the data sheets of American Fastener (Cap Screws & Bolts - External Threads Class 2A) and American Fastener (Nuts - Internal Threads - Class 2B). Basic pitch diameter E = D − 0.649519 P = 0.2764; the allowance between basic and the 2A limits is 0.0012. The B1.1 formula, class 2A pitch tolerance T = 0.0015 D^(1/3) + 0.0015 √D + 0.015 P^(2/3) with the allowance 0.3 T and the 2B tolerance 1.3 T, gives the same figures for this size. Minor-diameter limits are not given here.
Tightening torque estimate
Target preload is 75% of proof load; torque is estimated from T = K · F · d.
Proof-load preload and estimated tightening torque for 5/16-18 UNC by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.0524 in²
2175 lbf
11 lb·ft · 136 lb·in
SAE Grade 5
0.0524 in²
3338 lbf
17 lb·ft · 209 lb·in
SAE Grade 8
0.0524 in²
4725 lbf
25 lb·ft · 295 lb·in
Strengths are SAE J429 for this diameter band. J429 steps down with size: Grade 2 falls to 33 ksi proof above 3/4 in and Grade 5 to 74 ksi above 1 in, so these figures are not the small-size values. The Grade 2 row applies to bolts up to 6 in long; a longer Grade 2 bolt of the same diameter drops to 33 ksi proof.
These figures assume a nut factor of K = 0.20 (dry / as-received steel). K captures thread and head friction and can swing torque by 30% or more - a lubricated joint needs noticeably less. Change the lubrication assumption in the bolt-torque calculator.
Put 5/16-18 UNC on a job sheet
The bolt schedule collects the fasteners for one job onto a single sheet - tap drill, wrench size, grade and tightening torque on every line, taken straight from pages like this one. Free schedules hold 3 lines and are saved in this browser only.
For a 5/16-18 UNC thread, drill the hole with a F tap drill (6.53 mm) before tapping. That produces roughly 77% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 7.938 mm and a pitch of 1.411 mm (18 TPI).
To pass a 5/16-18 UNC bolt through a part rather than thread it, use a clearance hole: Close 21/64" · Normal 11/32" · Loose 3/8" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice)..
All values come from the standard thread formulas (ISO 68-1 / ASME B1.1) and published standard tables. See the full reference.
Frequently asked questions
What drill size do I need to tap a 5/16-18 UNC thread?
Drill F (6.53 mm) before tapping 5/16-18 UNC. That leaves roughly 77% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 5/16-18 UNC a coarse or fine thread?
5/16-18 UNC is a Unified coarse (UNC) thread at 18 threads per inch, the default inch series for general fastening in this diameter.
What size clearance hole does a 5/16-18 UNC bolt need?
Close 21/64" · Normal 11/32" · Loose 3/8" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice). A clearance hole lets the 5/16-18 UNC bolt pass through the part instead of threading into it.
How much torque should I use on a 5/16-18 UNC bolt?
Estimated tightening torque for 5/16-18 UNC runs from 11 lb·ft (SAE Grade 2) to 25 lb·ft (SAE Grade 8), targeting 75% of proof load with a dry nut factor of K = 0.20. Lubricated threads need noticeably less torque.
Nearest standard metric sizes by major diameter. These are substitutes by size only - the thread forms are not interchangeable, so never run one into the other.
Each grade is a row on the torque chart, worked out at four nut factors from the diameter and clamp load, with the figures the published charts print beside it.