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).
1/4-28 UNF in numbers
1/4-28 UNF is 0.2500 in across the crests with 28 threads per inch, a 0.907 mm pitch. The basic pitch diameter is 5.761 mm and the bolt's minor diameter 5.237 mm, so the thread is 0.556 mm deep on the bolt. Its tensile stress area is 0.0364 in², 14% more than the coarser 1/4-20 UNC at the same diameter. A class 2A bolt is made with its pitch diameter between 0.2225 and 0.2258 in, and a class 2B nut between 0.2268 and 0.2311 in, so even the largest bolt clears the smallest nut by 0.0010 in. The nearest metric size, M6 × 1.0, is 0.35 mm smaller across the crests and its pitch is 0.093 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 = 0.907 mm. A smaller hole leaves more thread; the recommended #3 drill above gives about 80%.
Tap-drill hole diameters for 1/4-28 UNF at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
5.64
0.2222
#2 (0.2210 in) gives 63%
65%
5.58
0.2198
7/32" (0.2188 in) gives 67%
70%
5.53
0.2175
7/32" (0.2188 in) gives 67%
75%
5.47
0.2152
#3 (0.2130 in) gives 80%
Class 2A and 2B limits
The commercial fit for 1/4-28 UNF: 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 1/4-28 UNF.
Dimension
Min
Max
2A major diameter
0.2425
0.2490
2A pitch diameter
0.2225
0.2258
2B pitch diameter
0.2268
0.2311
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 = 0.2268; the allowance between basic and the 2A limits is 0.0010. 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 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
2325 lbf
9.7 lb·ft · 116 lb·in
SAE Grade 8
0.0364 in²
3263 lbf
14 lb·ft · 163 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 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 1/4-28 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.
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.