Dimensions, drill sizes and wrench sizes for 5/8-18 UNF.
Designation
5/8-18 UNF
Major (outside) diameter
15.875 mm · 0.625"
Pitch
1.411 mm · 18 TPI
Pitch diameter
14.958 mm · 0.5889"
Minor diameter (external)
14.144 mm · 0.5568"
Minor diameter (internal)
14.347 mm · 0.5649"
Recommended tap drill
37/64" · 14.68 mm · ≈65% 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).
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 5/8-18 UNF by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.256 in²
10558 lbf
110 lb·ft · 1320 lb·in
SAE Grade 5
0.256 in²
16317 lbf
170 lb·ft · 2040 lb·in
SAE Grade 8
0.256 in²
23036 lbf
240 lb·ft · 2880 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 5/8-18 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 5/8-18 UNF thread, drill the hole with a 37/64" tap drill (14.68 mm) before tapping. That produces roughly 65% 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 1.411 mm (18 TPI).
To pass a 5/8-18 UNF 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 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 5/8-18 UNF thread?
Drill 37/64" (14.68 mm) before tapping 5/8-18 UNF. That leaves roughly 65% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 5/8-18 UNF a coarse or fine thread?
5/8-18 UNF is a Unified fine (UNF) thread at 18 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 5/8-18 UNF 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 5/8-18 UNF bolt pass through the part instead of threading into it.
How much torque should I use on a 5/8-18 UNF bolt?
Estimated tightening torque for 5/8-18 UNF runs from 110 lb·ft (SAE Grade 2) to 240 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.