How much weight do you gain on steroids?

Depends on how much you take and why.

Small amounts will not cause weight gain, especially if the person is sick. Steroids may be given to help maintain healthy weight in some cases.

In small doses steroids are simply adding enzymes, hormones and other substances which are already present in the human body. (or replacing hormones that should have been present but that are missing due to some other cause such as illness)

Larger amounts of steroids added to combat specific illness, or steroid abuse for the sake of body building may cause very large weight gains of either muscle or fat depending on the persons diet and exercise levels.

With large amounts of steroids it is possible to gain a 10 to 20% mass increase in a very short amount of time, but the doses required will cause vital organ damage, shrunken genitals and a variety of other potential health problems.

Large dose steroids are only given for medical reasons where the damage done by the steroids is less than the damage caused by the disease it is combating.

Steroid abuse for the sake of athletic performance should be avoided, the amount needed to achieve even moderate weight gain causes significant long term damage to a persons health and shortens their lifespan.