Adding or removing water (or any liquid) during or after the boil directly affects the gravity of the batch of beer. Use the calculator below depending on whether you wish to reach a desired gravity or volume.
A negative volume is the amount of boil-off required to reach the desired gravity. A positive volume is the amount of water required to be added to reach the desired gravity.
During the boil, water is lost through steam and concentrates the sugars, increasing the gravity of the wort. When adding water to a batch, the sugars are diluted and the gravity of the wort decreases.