Dimensions, drill sizes and wrench sizes for 1/4-20 UNC.
Designation
1/4-20 UNC
Major (outside) diameter
6.35 mm · 0.25"
Pitch
1.27 mm · 20 TPI
Pitch diameter
5.525 mm · 0.2175"
Minor diameter (external)
4.792 mm · 0.1887"
Minor diameter (internal)
4.975 mm · 0.1959"
Recommended tap drill
#7 · 5.11 mm · ≈75% 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).
1/4-20 UNC in numbers
1/4-20 UNC is 0.2500 in across the crests with 20 threads per inch, a 1.27 mm pitch. The basic pitch diameter is 5.525 mm and the bolt's minor diameter 4.792 mm, so the thread is 0.779 mm deep on the bolt. A class 2A bolt is made with its pitch diameter between 0.2127 and 0.2164 in, and a class 2B nut between 0.2175 and 0.2224 in, so even the largest bolt clears the smallest nut by 0.0011 in. The nearest metric size, M6 × 1.0, is 0.35 mm smaller across the crests and its pitch is 0.270 mm finer, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 6.35 mm and P = 1.27 mm. A smaller hole leaves more thread; the recommended #7 drill above gives about 75%.
Tap-drill hole diameters for 1/4-20 UNC at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
5.36
0.2110
#3 (0.2130 in) gives 57%
65%
5.28
0.2078
#4 (0.2090 in) gives 63%
70%
5.20
0.2045
#6 (0.2040 in) gives 71%
75%
5.11
0.2013
#7 (0.2010 in) gives 75%
Class 2A and 2B limits
The commercial fit for 1/4-20 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 1/4-20 UNC.
Dimension
Min
Max
2A major diameter
0.2408
0.2489
2A pitch diameter
0.2127
0.2164
2B pitch diameter
0.2175
0.2224
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 = 0.2175; the allowance between basic and the 2A limits is 0.0011. The B1.1 formula lands 0.0001 in away from the table on one or more of these cells for this size, so the table's figures are printed. 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 1/4-20 UNC by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.0318 in²
1313 lbf
5.5 lb·ft · 66 lb·in
SAE Grade 5
0.0318 in²
2025 lbf
8.4 lb·ft · 101 lb·in
SAE Grade 8
0.0318 in²
2850 lbf
12 lb·ft · 143 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 1/4-20 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 1/4-20 UNC thread, drill the hole with a #7 tap drill (5.11 mm) before tapping. That produces roughly 75% 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 1.27 mm (20 TPI).
To pass a 1/4-20 UNC 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 1/4-20 UNC thread?
Drill #7 (5.11 mm) before tapping 1/4-20 UNC. That leaves roughly 75% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 1/4-20 UNC a coarse or fine thread?
1/4-20 UNC is a Unified coarse (UNC) thread at 20 threads per inch, the default inch series for general fastening in this diameter.
What size clearance hole does a 1/4-20 UNC 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 1/4-20 UNC bolt pass through the part instead of threading into it.
How much torque should I use on a 1/4-20 UNC bolt?
Estimated tightening torque for 1/4-20 UNC runs from 5.5 lb·ft (SAE Grade 2) to 12 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.