Tips & Tricks

Screw Identification

Example:
Socket head cap screw similar to DIN 912, 1/4-20 UNC × 1-1/2

The screw diameter is 1/4" (6.35 mm).
The thread count per inch is 20 TPI.
This means it is a UNC coarse thread; UNF would have 28 TPI.
The length is 1-1/2" (38.1 mm).

To determine the thread count, count the number of threads over one inch (25.4 mm), or for shorter threaded sections simply convert to a one-inch equivalent.

To convert the length to millimeters, multiply the inch measurement by 25.4 (for 1/4": 1 ÷ 4 × 25.4 = 6.35 mm).

Thread Gauges

Alternatively, you can use our thread gauges to determine the thread count.

Simply place the gauge against the screw thread and read the threads per inch. In the example on the left, the screw would have 13 threads per inch.

With a measured diameter of 12.7 mm, this indicates a UNC coarse thread.

Thread Types

The two most common American thread types are UNC and UNF.

UNC – Unified Coarse Thread Series – ANSI B1.1

American standard coarse thread. The flank angle is 60°.

ø	Threads per Inch	Millimeters	Inch Decimal
#2	56	2.1844	0.0860
#3	48	2.5146	0.0990
#4	40	2.8448	0.1120
#5	40	3.1750	0.1250
#6	32	3.5052	0.1380
#8	32	4.1656	0.1640
#10	24	4.7625	0.1875
#12	24	5.4864	0.2160
1/4	20	6.3500	0.2500
5/16	18	7.9375	0.3125
3/8	16	9.5250	0.3750
7/16	14	11.1125	0.4375
1/2	13	12.7000	0.5000
9/16	12	14.2875	0.5625
5/8	11	15.5750	0.6250
3/4	10	19.0500	0.7500
7/8	9	22.2250	0.8750
1	8	25.4000	1.0000

UNF – Unified Fine Thread Series – ANSI B1.1

American standard fine thread. The flank angle is 60°.

ø	Threads per Inch	Millimeters	Inch Decimal
#2	64	2.1844	0.0860
#3	56	2.5146	0.0990
#4	48	2.8448	0.1120
#5	44	3.1750	0.1250
#6	40	3.5052	0.1380
#8	36	4.1656	0.1640
#10	32	4.7625	0.1875
#12	28	5.4864	0.2160
1/4	28	6.3500	0.2500
5/16	24	7.9375	0.3125
3/8	24	9.5250	0.3750
7/16	20	11.1125	0.4375
1/2	20	12.7000	0.5000
9/16	18	14.2875	0.5625
5/8	18	15.5750	0.6250
3/4	16	19.0500	0.7500
7/8	14	22.2250	0.8750
1	12	25.4000	1.0000