THREADS SIZES CHART

THREADS SIZES CHART

Useful British Thread Sizes
Size	Thread	TPI	Dia	Drill
BSC = British Standard Cycle Thread  -  Form 60°
1/8	BSC	40	0.125	0.098
5/32	BSC	32	0.156	0.123
3/16	BSC	32	0.188	0.154
7/32	BSC	26	0.219	0.178
1/4	BSC	26	0.250	0.209
9/32	BSC	26	0.281	0.240
5/16	BSC	26	0.313	0.272
3/8	BSC	26	0.375	0.334
7/16	BSC	26	0.438	0.397
1/2	BSC	26	0.500	0.459
9/16	BSC	26	0.563	0.522
5/8	BSC	26	0.625	0.584
11/16	BSC	26	0.688	0.647
3/4	BSC	26	0.750	0.709
7/16	BSC	20	0.438	0.384
1/2	BSC	20	0.500	0.447
9/16	BSC	20	0.563	0.509
5/8	BSC	20	0.625	0.572
11/16	BSC	20	0.688	0.634
3/4	BSC	20	0.750	0.697
BSF = British Standard Fine  -  Form 55°
3/16	BSF	32	0.188	0.148
7/32	BSF	28	0.219	0.173
1/4	BSF	26	0.250	0.201
5/16	BSF	22	0.313	0.254
3/8	BSF	20	0.375	0.311
7/16	BSF	18	0.438	0.366
1/2	BSF	16	0.500	0.420
9/16	BSF	16	0.563	0.483
5/8	BSF	14	0.625	0.534
3/4	BSF	12	0.750	0.643
7/8	BSF	11	0.875	0.759
SPARK = Spark Plug Thread
1/4	SPARK	24	0.250	0.196
3/8	SPARK	24	0.375	0.321
7/8	SPARK	18	0.875	0.803
UNF = Unified Fine  -  Form 60°
1/4	UNF	28	0.250	0.206
5/16	UNF	24	0.313	0.261
3/8	UNF	24	0.375	0.324
7/16	UNF	20	0.438	0.376
1/2	UNF	20	0.500	0.439
9/16	UNF	18	0.563	0.494
5/8	UNF	18	0.625	0.557
3/4	UNF	16	0.750	0.673
7/8	UNF	14	0.875	0.787

Nomenclature
Technical Terms:- Threads Per Inch, Pitch, Diameter, Form
Threads Per Inch (TPI), is the number of threads (Peaks) per inch length of thread. If you place a rule on a bolt and count the number of peaks (think of a thread as a mountain top or the top of a triangle) and you have the Threads per inch. The pitch is the distance between two of these peaks.
Coarse and Fine. Basically people invented threads to suit their manufacture needs. Bicycles and watch makers needed fine threads. Pipe fitters and Bridge makers needed coarse threads. You can therefore have a 1" diameter (25mm) bolt with a fine thread with 12  (TPI) or a coarse thread with 8 TPI. The coarse thread allows quicker assembly and will carry more pressure than the fine thread.
The other variable is the angle of the thread (Form).
There are numerous thread forms, named after by their inventors. BA (British Association), BSF (British Standard Fine), BSC (British Standard Cycle Thread), CEI (Cycle Engineers Institute), ISO (International Standards Organisation) Metric Coarse, Fine, and Special Pitch, UNF (Unified Fine Thread), UNC (Unified Coarse Thread), and Whitworth Threads.
You do not need to worry about it too much, unless you are a designer who has to determine the best thread to suit his need, or you are measuring a thread to find out what it is.