The Bathmate Hydromax3 is for penises measuring 3 inches when erect.
Unlike air pumps, water pumps require water to create a vacuum. Water generally creates better pressure than air and is therefore somewhat more effective. The downside is that you need to be in the shower or bath to use the pump. For that reason, we recommend them solely for penile rehabilitation post prostatectomy rather than prior to sex.
Just fill your pump with water, seal it to your body and wait for 5 minutes (maximum 15 minutes use per day).
Hydromax3 will fit if your erect penis measures between 3-8 cm in length and less than 16.5 cm in girth.