Dimensions, drill sizes and wrench sizes for 3/8-16 UNC.
Designation
3/8-16 UNC
Major (outside) diameter
9.525 mm · 0.375"
Pitch
1.588 mm · 16 TPI
Pitch diameter
8.494 mm · 0.3344"
Minor diameter (external)
7.577 mm · 0.2983"
Minor diameter (internal)
7.807 mm · 0.3073"
Recommended tap drill
5/16" · 7.94 mm · ≈77% thread
Clearance hole
Close 25/64" · Normal 13/32" · Loose 7/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Hex head (across flats)
9/16"
Socket key (across flats)
5/16"
Close 25/64" · Normal 13/32" · Loose 7/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
3/8-16 UNC in numbers
3/8-16 UNC is 0.3750 in across the crests with 16 threads per inch, a 1.588 mm pitch. The basic pitch diameter is 8.494 mm and the bolt's minor diameter 7.577 mm, so the thread is 0.974 mm deep on the bolt. A class 2A bolt is made with its pitch diameter between 0.3287 and 0.3331 in, and a class 2B nut between 0.3344 and 0.3401 in, so even the largest bolt clears the smallest nut by 0.0013 in. The nearest metric size, M10 × 1.5, is 0.47 mm larger across the crests and its pitch is 0.088 mm finer, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 9.525 mm and P = 1.588 mm. A smaller hole leaves more thread; the recommended 5/16" drill above gives about 77%.
Tap-drill hole diameters for 3/8-16 UNC at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
8.29
0.3263
21/64" (0.3281 in) gives 58%
65%
8.18
0.3222
P (0.3230 in) gives 64%
70%
8.08
0.3181
O (0.3160 in) gives 73%
75%
7.98
0.3141
5/16" (0.3125 in) gives 77%
Class 2A and 2B limits
The commercial fit for 3/8-16 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 3/8-16 UNC.
Dimension
Min
Max
2A major diameter
0.3643
0.3737
2A pitch diameter
0.3287
0.3331
2B pitch diameter
0.3344
0.3401
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 = 0.3344; the allowance between basic and the 2A limits is 0.0013. 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 3/8-16 UNC by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.0775 in²
3188 lbf
20 lb·ft · 239 lb·in
SAE Grade 5
0.0775 in²
4950 lbf
31 lb·ft · 371 lb·in
SAE Grade 8
0.0775 in²
6975 lbf
44 lb·ft · 523 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 3/8-16 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 3/8-16 UNC thread, drill the hole with a 5/16" tap drill (7.94 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 9.525 mm and a pitch of 1.588 mm (16 TPI).
To pass a 3/8-16 UNC bolt through a part rather than thread it, use a clearance hole: Close 25/64" · Normal 13/32" · Loose 7/16" - 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 3/8-16 UNC thread?
Drill 5/16" (7.94 mm) before tapping 3/8-16 UNC. That leaves roughly 77% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 3/8-16 UNC a coarse or fine thread?
3/8-16 UNC is a Unified coarse (UNC) thread at 16 threads per inch, the default inch series for general fastening in this diameter.
What size clearance hole does a 3/8-16 UNC bolt need?
Close 25/64" · Normal 13/32" · Loose 7/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice). A clearance hole lets the 3/8-16 UNC bolt pass through the part instead of threading into it.
How much torque should I use on a 3/8-16 UNC bolt?
Estimated tightening torque for 3/8-16 UNC runs from 20 lb·ft (SAE Grade 2) to 44 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.