How much blood does it take to get hard?
Getting a hard penis during sexual intercourse, you need approx 130 milliliters (ml), or 4.4 ounces of blood.
This is a very small fraction compared to the 1.2 to 1.5 gallons (4,500 to 5,600 ml) of blood that a mature human body needs to run. These amounts of blood need to be circulating in the whole body.
