The size of pipe that needs to fit a brass fitting will need to be determined by measuring the size or the diameter of the fitting. Depending on the connection type you may need the outside diameter or the inside diameter of the fitting. You can easily measure the diameter of a brass fitting by using a tape measure or simple ruler. However, the conversion process requires subtracting fractions.
Measure the inside or outside diameter of the brass pipe fitting using the tape measure. Make a note of the measurement on a piece of paper as a fraction, such as 3/8.
Subtract 1/4 from the inside or outside diameter of the brass pipe fitting. If the diameter of the fitting is 3/8 then the equation will look like 3/8 - 1/4.
Multiply 1/4 by 2 for a result of 2/8. The equation now looks like 3/8 - 2/8. Subtract 2 from 3 and leave the bottom number the same. The result is 1/8 which is the thread size of the brass fitting.
Repeat this process to measure other brass fittings.
The thread size of a pipe is typically 1/4 less than the diameter of the pipe. Subtracting fractions requires that the bottom number be the same. Simply a fraction if the bottom number is divisible by the top number, such as 2/8 or 1/4.