Dimensions, drill sizes and wrench sizes for 5/8-20 UN.
Designation
5/8-20 UN
Major (outside) diameter
15.875 mm · 0.625"
Pitch
1.27 mm · 20 TPI
Pitch diameter
15.05 mm · 0.5925"
Minor diameter (external)
14.317 mm · 0.5637"
Minor diameter (internal)
14.5 mm · 0.5709"
Recommended tap drill
37/64" · 14.68 mm · ≈72% thread
Clearance hole
Close 41/64" · Normal 21/32" · Loose 11/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Hex head (across flats)
15/16"
Socket key (across flats)
1/2"
Close 41/64" · Normal 21/32" · Loose 11/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
5/8-20 UN in numbers
5/8-20 UN is 0.6250 in across the crests with 20 threads per inch, a 1.27 mm pitch. The basic pitch diameter is 15.05 mm and the bolt's minor diameter 14.317 mm, so the thread is 0.779 mm deep on the bolt. Its tensile stress area is 0.2608 in², 15% more than the coarser 5/8-11 UNC at the same diameter. The nearest metric size, M16 × 2.0, is 0.13 mm larger across the crests and its pitch is 0.730 mm coarser, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 15.875 mm and P = 1.27 mm. A smaller hole leaves more thread; the recommended 37/64" drill above gives about 72%.
Tap-drill hole diameters for 5/8-20 UN at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
14.89
0.5860
no standard drill within 1/64 in
65%
14.80
0.5828
37/64" (0.5781 in) gives 72%
70%
14.72
0.5795
37/64" (0.5781 in) gives 72%
75%
14.64
0.5763
37/64" (0.5781 in) gives 72%
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 5/8-20 UN by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.2608 in²
10759 lbf
112 lb·ft · 1345 lb·in
SAE Grade 5
0.2608 in²
16628 lbf
173 lb·ft · 2079 lb·in
SAE Grade 8
0.2608 in²
23475 lbf
245 lb·ft · 2934 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 5/8-20 UN 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 5/8-20 UN thread, drill the hole with a 37/64" tap drill (14.68 mm) before tapping. That produces roughly 72% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 15.875 mm and a pitch of 1.27 mm (20 TPI).
To pass a 5/8-20 UN bolt through a part rather than thread it, use a clearance hole: Close 41/64" · Normal 21/32" · Loose 11/16" - 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 5/8-20 UN thread?
Drill 37/64" (14.68 mm) before tapping 5/8-20 UN. That leaves roughly 72% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 5/8-20 UN a coarse or fine thread?
5/8-20 UN is in the UN constant-pitch series: it runs 20 threads per inch, a pitch shared across a range of diameters rather than scaled with the size.
What size clearance hole does a 5/8-20 UN bolt need?
Close 41/64" · Normal 21/32" · Loose 11/16" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice). A clearance hole lets the 5/8-20 UN bolt pass through the part instead of threading into it.
How much torque should I use on a 5/8-20 UN bolt?
Estimated tightening torque for 5/8-20 UN runs from 112 lb·ft (SAE Grade 2) to 245 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.