Top-Down Brew: Decreasing original gravity with water

Purpose: This calculator is for decreasing the gravity of your wort by adding water.

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This calculator determines the amount of water needed to lower the gravity of a wort to a given target. It begins by determining the difference between the actual gravity and target gravity. This difference is then used to determine the amount of water needed to reach the desired target gravity. If your starting specific gravity is lower than the target, the result will be a negative number that shows the gallons that need to be boiled off to reach the target gravity.

For example, let's say that you were aiming to brew 5.0 gallons of a 1.050 wort, but in reality you ended up with a wort of 1.055. This means that each gallon has five extra gravity points. Altogether, there are 25 extra gravity points (5 points per gallon * 5 gallons). These 25 points will be enough to make a half gallon of wort at 1.050. Therefore, adding a half gallon of water to five gallons will yield 5.5 gallons of 1.050 wort.

For further information, please refer to chapter 6 of Daniels (2001) on "Hitting Target Gravity".


Daniels, R. (2001). Designing Great Beers. Brewers Publications. Boulder, Colorado.

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