How to calculate the thread size of a MALE BSP fitting/hose.
To first find the thread size you must.
1) Measure the outside diameter (O.D) of the BSP thread.
2) Take the O.D measurement in 16th of an inch and subtract 4/16" or quarter of an inch.
For Example
A BSP O.D male thread measure's 10/16" across minus the 4/16" allowance and it equals 6/16" or a 3/8" thread size.