1 inch is equal to 8 minutes, such as 2 minutes is 2/8 inch, that is, 1/4, which is the nominal diameter; it corresponds to the number of millimeters to check the table. This table is easy to find on the internet, so search again, I've attached a portion of it.
Cylindrical Pipe Thread Size
Left 3 Columns Pipe Size 4 5 6 Columns Thread Size
Whitworth Thread (Whitworth Thread): BS 84 Designed by the British engineer Whitworth, the standard was introduced in the UK in 1841 and applies to manufacturing by the rolling method. Representation such as W 1/2 - 10, Whitworth coarse thread (B.S.W) and Whitworth fine thread (B.S.F). Inch thread W is not the same thread as Unified Standard thread
Nominal Diameter (inch) / Pipe O.D. (mm) / Pipe I.D. (mm) / O.D. D / I.D. d1 / Medium D.D. d2 / Number of Buckles per Inch (n)
1/8 " 9.729 8.567 9.148 28
1/4 " 13.5 8 13.158 11.446 12.302 19
3/8" 17 10 16.663 14.951 15.807 19
1/2" 21.25 15 20.956 18.632 19.794 14
5/8" 22.912 20.588 21.750 14
3/4" 26.75 20 26.442 24.119 25.281 14
7/8" 30.202 27.878 29.049 14
1" 33.5 25 33.250 30.293 31.771 11
11/8" 37.808 34.941 36.420 11
11/4" 42.35 32 41.912 38.954 40.133 11
1 3/8" 44.325 41.367 42.846 11
11/2" 48 40 47.805 44.847 46.326 11
13/4" 53.748 50.791 52.270 11
2" 60 50 59.616 56.659 58.137 11