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
Diameter of Bolt Inches	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
6" and shorter	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
Longer than 6"	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
Bolt Diameter (inches)	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
Coarse Thread Series	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
Fine Thread Series	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.