Dimensions, drill sizes and wrench sizes for 9/16-18 UNF.
Designation
9/16-18 UNF
Major (outside) diameter
14.288 mm · 0.5625"
Pitch
1.411 mm · 18 TPI
Pitch diameter
13.371 mm · 0.5264"
Minor diameter (external)
12.556 mm · 0.4943"
Minor diameter (internal)
12.76 mm · 0.5024"
Recommended tap drill
33/64" · 13.1 mm · ≈65% thread
Clearance hole
Close 37/64" · Normal 19/32" · Loose 5/8" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
Hex head (across flats)
13/16"
Close 37/64" · Normal 19/32" · Loose 5/8" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice).
9/16-18 UNF in numbers
9/16-18 UNF is 0.5625 in across the crests with 18 threads per inch, a 1.411 mm pitch. The basic pitch diameter is 13.371 mm and the bolt's minor diameter 12.556 mm, so the thread is 0.866 mm deep on the bolt. Its tensile stress area is 0.203 in², 12% more than the coarser 9/16-12 UNC at the same diameter. A class 2A bolt is made with its pitch diameter between 0.5205 and 0.5250 in, and a class 2B nut between 0.5264 and 0.5323 in, so even the largest bolt clears the smallest nut by 0.0014 in. The nearest metric size, M14 × 2.0, is 0.29 mm smaller across the crests and its pitch is 0.589 mm coarser, so the two do not interchange.
Tap drill by thread percentage
Hole = D − % × 0.0129904 × P, with D = 14.288 mm and P = 1.411 mm. A smaller hole leaves more thread; the recommended 33/64" drill above gives about 65%.
Tap-drill hole diameters for 9/16-18 UNF at several thread percentages.
Thread
Hole (mm)
Hole (in)
Nearest drill
60%
13.19
0.5192
33/64" (0.5156 in) gives 65%
65%
13.10
0.5156
33/64" (0.5156 in) gives 65%
70%
13.00
0.5120
33/64" (0.5156 in) gives 65%
75%
12.91
0.5084
33/64" (0.5156 in) gives 65%
Class 2A and 2B limits
The commercial fit for 9/16-18 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 9/16-18 UNF.
Dimension
Min
Max
2A major diameter
0.5524
0.5611
2A pitch diameter
0.5205
0.5250
2B pitch diameter
0.5264
0.5323
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 = 0.5264; the allowance between basic and the 2A limits is 0.0014. 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 9/16-18 UNF by fastener grade or property class.
Grade / class
Stress area
Preload
Torque
SAE Grade 2
0.203 in²
8373 lbf
78 lb·ft · 942 lb·in
SAE Grade 5
0.203 in²
12940 lbf
121 lb·ft · 1456 lb·in
SAE Grade 8
0.203 in²
18268 lbf
171 lb·ft · 2055 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 9/16-18 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 9/16-18 UNF thread, drill the hole with a 33/64" tap drill (13.1 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 14.288 mm and a pitch of 1.411 mm (18 TPI).
To pass a 9/16-18 UNF bolt through a part rather than thread it, use a clearance hole: Close 37/64" · Normal 19/32" · Loose 5/8" - 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 9/16-18 UNF thread?
Drill 33/64" (13.1 mm) before tapping 9/16-18 UNF. That leaves roughly 65% thread engagement, the usual target for a strong hole that still taps cleanly.
Is 9/16-18 UNF a coarse or fine thread?
9/16-18 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 9/16-18 UNF bolt need?
Close 37/64" · Normal 19/32" · Loose 5/8" - nominal + 1/64" / 1/32" / 1/16" to the nearest standard drill (common shop practice). A clearance hole lets the 9/16-18 UNF bolt pass through the part instead of threading into it.
How much torque should I use on a 9/16-18 UNF bolt?
Estimated tightening torque for 9/16-18 UNF runs from 78 lb·ft (SAE Grade 2) to 171 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.