TOOLS YOU WILL NEED: 
Compliments of Barebackjack.com

A flexible measuring tape is best One Subject Penis Less than a half hour, depending on how well your dick cooperates Write everything down. It's easy to forget
1) A Ruler or a Tape Measure, American Standard
2) Your Penis (or the penis of another)
3) Time
4) Paper & Pencil

INSTRUCTIONS

First, take your penis and begin stroking it gently until it becomes fully hard. You may wish to use any of the optional materials available to you to help you get to this stage.

Once you have achieved full hardness, take the ruler in one hand, and place the end marked "0" (or the end with the lowest number) against the point where the top of your penis makes a conjunction with your torso. (Point "A", Fig. A & B) Do not press the ruler into your torso. Simply lay it gently along the top of your penis
Using your pencil, carefully mark on the ruler where the tip of your penis (that's the part that goes in first) lines up with the fractional INCH markings on the ruler. (Point "B", Figs. A & B). Count to the nearest 1/4", and then write this measurement down on the piece of paper.

Point A to Point B... It's so easy you can't possibly make a mistake

Dangerous curves are best measured with something flexible

If your penis curves in any direction, it would be advisable to use a tape measure instead of a ruler so it may easily follow the curve of your penis and give you a more accurate measure.
(Fig. B)