how do i mewasrue the grith of my penis
It's most easily done with a cloth tape measure. Starting at the zero inches end, wrap it once around the shaft of your penis (erect) (about halfway along), and note the reading where it meets itself.
If you don't have a cloth tape measure, you can use a strip of paper or a piece of string (not stretchy stuff). Mark where it meets itself, unroll it, then measure with a ruler.