Use a rigid ruler or measuring tape
Measure when the penis is fully erect
Place the ruler on the top side of the penis
Press the ruler firmly into 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
Keep the penis parallel to the floor
Record the length in inches or centimeters
For flaccid length, measure the same way without erection
For consistency, measure under the same conditions each time