Dimensions, drill sizes and wrench sizes for 1/4-28 UNF.
Designation
1/4-28 UNF
Major (outside) diameter
6.35 mm · 0.25"
Pitch
0.907 mm · 28 TPI
Pitch diameter
5.761 mm · 0.2268"
Minor diameter (external)
5.237 mm · 0.2062"
Minor diameter (internal)
5.368 mm · 0.2113"
Recommended tap drill
#3 · 5.41 mm · ≈80% 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).
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 1/4-28 UNF by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.0364 in²
1500 lbf
6.3 lb·ft · 75 lb·in
SAE Grade 5
0.0364 in²
2319 lbf
9.7 lb·ft · 116 lb·in
SAE Grade 8
0.0364 in²
3274 lbf
14 lb·ft · 164 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 1/4-28 UNF 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 1/4-28 UNF thread, drill the hole with a #3 tap drill (5.41 mm) before tapping. That produces roughly 80% 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 0.907 mm (28 TPI).
To pass a 1/4-28 UNF 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 are generated from the standard thread formulas (ISO 68-1 / ASME B1.1) and published standard tables - not hand-entered. See the full reference.
Frequently asked questions
What drill size do I need to tap a 1/4-28 UNF thread?
Drill #3 (5.41 mm) before tapping 1/4-28 UNF. That leaves roughly 80% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1/4-28 UNF a coarse or fine thread?
1/4-28 UNF is a Unified fine (UNF) thread at 28 threads per inch. Fine pitches have a larger stress area than coarse at the same diameter and suit thin walls, fine adjustment and vibration-sensitive joints.
What size clearance hole does a 1/4-28 UNF 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 1/4-28 UNF bolt pass through the part instead of threading into it.
How much torque should I use on a 1/4-28 UNF bolt?
Estimated tightening torque for 1/4-28 UNF runs from 6.3 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.