Use a rigid ruler or measuring tape
Measure when the penis is fully erect for consistency
For length, place the ruler on the top side of the penis
Press the ruler gently against the pubic bone at the base
Measure from the pubic bone to the tip of the glans
Do not include the foreskin in the measurement
For girth, use a soft measuring tape or a string
Wrap the tape around the thickest part of the shaft
If using a string, mark the overlap and measure it with a ruler
Measure at the same time of day and under similar conditions if comparing results
Repeat the measurement a few times and use the average