Bolt Thread Length and Threads per Inch

Thread Lengths
All Standard Bolts Except Lag Screws
Diameter of
Bolt Inches
No.
10
1/4 5/16 3/8 7/16 1/2 5/8 3/4 7/8
1/8 1-1/8 1-1/4 1-3/8 1-1/2 1-5/8 1-3/4 1-7/8 2
Thread Length in Inches
6" and
shorter
5/8 3/4 7/8 1 1-1/8 1-1/4 1-1/2 1-3/4 2
2-1/4 2-1/2 2-3/4 3 3-1/4 3-1/2 3-3/4 4 4-1/4
Longer
than 6"
7/8 1 1-1/8 1-1/4 1-3/8 1-1/2 1-3/4 2 2-1/4
2-1/2 2-3/4 3 3-1/4 3-1/2 3-3/4 4 4-1/4 4-1/2
Threaded Length Formula:
For bolts 6" and shorter–twice the diameter plus 1/4". (2D + 1/4").
Longer than 6"–twice the diameter plus 1/2" (2D + 1/2").
When bolts are short for formula thread length, thread will extend as close to head or shoulder as practical. In actual production, thread lengths may be longer than the formula thread lengths.
Bolt Threads
Number of Threads per Inch
Bolt
Diameter
(inches)
No.
10
No.
12
1/4 5/16 3/8 7/16 1/2 9/16 5/8 3/4 7/8
1 1-1/8 1-1/4 1-3/8 1-1/2 1-3/4 2 2-1/4 2-1/2 2-3/4 3
Coarse
Thread
Series
24 24 20 18 16 14 13 12 11 10 9
8 7 7 6 6 5 4-1/2 4-1/2 4 4 4
Fine
Thread
Series
32 38 28 24 24 20 20 18 18 16 14
14
12X
12 12 12 12            
X–Indicates number of threads per inch for Unified Fine
(1"–12 thread is U.N.F. standard). However 1"–14 threads is more popular and generally stocked.
Coarse Thread Series, Class 2A Fit.