Dimensions, drill sizes and wrench sizes for 1/4-32 UNEF.
Designation
1/4-32 UNEF
Major (outside) diameter
6.35 mm · 0.25"
Pitch
0.794 mm · 32 TPI
Pitch diameter
5.834 mm · 0.2297"
Minor diameter (external)
5.376 mm · 0.2117"
Minor diameter (internal)
5.491 mm · 0.2162"
Recommended tap drill
7/32" · 5.56 mm · ≈77% thread
Clearance hole
Close 17/64" · Normal 9/32" · Loose 5/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Hex head (across flats)
7/16"
Socket key (across flats)
3/16"
Close 17/64" · Normal 9/32" · Loose 5/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
1/4-32 UNEF in numbers
1/4-32 UNEF is 0.2500 in across the crests with 32 threads per inch, a 0.794 mm pitch. The basic pitch diameter is 5.834 mm and the bolt's minor diameter 5.376 mm, so the thread is 0.487 mm deep on the bolt. Its tensile stress area is 0.0379 in², 19% more than the coarser 1/4-20 UNC at the same diameter. The nearest metric size, M6 × 1.0, is 0.35 mm smaller across the crests and its pitch is 0.206 mm coarser, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 6.35 mm and P = 0.794 mm. A smaller hole leaves more thread; the recommended 7/32" drill above gives about 77%.
Tap-drill hole diameters for 1/4-32 UNEF at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
5.73
0.2256
#1 (0.2280 in) gives 54%
65%
5.68
0.2236
#2 (0.2210 in) gives 71%
70%
5.63
0.2216
#2 (0.2210 in) gives 71%
75%
5.58
0.2195
7/32" (0.2188 in) gives 77%
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 1/4-32 UNEF by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.0379 in²
1562 lbf
6.5 lb·ft · 78 lb·in
SAE Grade 5
0.0379 in²
2414 lbf
10 lb·ft · 121 lb·in
SAE Grade 8
0.0379 in²
3407 lbf
14 lb·ft · 170 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 1/4-32 UNEF 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 1/4-32 UNEF thread, drill the hole with a 7/32" tap drill (5.56 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 6.35 mm and a pitch of 0.794 mm (32 TPI).
To pass a 1/4-32 UNEF bolt through a part rather than thread it, use a clearance hole: Close 17/64" · Normal 9/32" · Loose 5/16" - 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 1/4-32 UNEF thread?
Drill 7/32" (5.56 mm) before tapping 1/4-32 UNEF. That leaves roughly 77% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1/4-32 UNEF a coarse or fine thread?
1/4-32 UNEF is a Unified extra-fine (UNEF) thread at 32 threads per inch, used for thin-wall parts, short thread engagement and fine adjustment.
What size clearance hole does a 1/4-32 UNEF bolt need?
Close 17/64" · Normal 9/32" · Loose 5/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice). A clearance hole lets the 1/4-32 UNEF bolt pass through the part instead of threading into it.
How much torque should I use on a 1/4-32 UNEF bolt?
Estimated tightening torque for 1/4-32 UNEF runs from 6.5 lb·ft (SAE Grade 2) to 14 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.