Boil-Off and Dilution Gravity Calculator

Adjusting wort volume and gravity on brew day relies on one principle: the total amount of dissolved sugar in the wort stays constant whether it is diluted with water or concentrated by boiling. That relationship is expressed as Volume 1 x Gravity Points 1 = Volume 2 x Gravity Points 2, where gravity points equal the specific gravity minus 1.000, multiplied by 1000. Two calculation modes cover the two directions a brewer runs into on brew day.

Find New Volume (Target Gravity Known) takes the current wort volume and current gravity plus a desired gravity, and returns the volume required to reach that target along with the volume difference. Find New Gravity (Target Volume Known) takes the current wort volume and gravity plus a target volume, and returns the resulting gravity along with the gravity point difference. A positive volume difference, where the new volume is larger than the current one, means adding clean water (bottled spring water or another source free of chlorine and contaminants) to dilute down to the target gravity. A negative volume difference means continuing the boil to evaporate water and concentrate the wort up to the target, with late hop addition timing adjusted to account for the extended boil.

Gravity values use standard specific gravity format, such as 1.048, 1.060, or 1.072. Settings holds the decimal precision for volume and gravity outputs and a step-by-step derivation showing the full calculation with substituted values. The relationship holds regardless of batch size, since gravity points scale with volume in the same proportion on both sides of the equation, which is why the same formula answers both a one-gallon test batch and a full ten-gallon brew day.

A brewer collects 5 gallons of wort at 1.060 but needs 1.050 for the recipe. Gravity points are 60 and 50. Applying V1 x GP1 = V2 x GP2 gives V2 = (5 x 60) / 50 = 6 gallons, a volume difference of 1 gallon, meaning 1 gallon of water needs to be added to hit the target gravity.

Brewers at every experience level run into a pre-boil or post-boil gravity that does not match the recipe target. The volume-gravity relationship gives a quick, accurate adjustment instead of a guess. The scenarios below are common situations where that adjustment matters.

• Homebrewing: After mashing and lautering, a pre-boil gravity of 1.055 needs to land at a post-boil original gravity of 1.052. Working from the current volume and gravity gives exactly how much water to add rather than overshooting the dilution.
• All-Grain Brewing: Boil-off rate varies between brew sessions with ambient temperature or kettle geometry. Finding the resulting gravity after boiling down to a known volume shows whether additional boiling or dilution is still needed.
• Extract Brewing: A gravity reading higher than expected after dissolving malt extract, from adding less water than planned, points to the exact volume of water needed to reach the target gravity without overshooting.
• High-Gravity Brewing: Strong ales and barleywines with original gravities above 1.080 turn small volume miscalculations into large gravity errors, which is where fine-tuning the relationship between actual volume and gravity target matters most.
• Brew Day Troubleshooting: A lower-than-expected pre-boil gravity of 1.038 instead of a planned 1.045, from poor mash efficiency, points to how much extra boiling time is needed to concentrate the wort, or whether adding dry malt extract instead makes more sense.
• Recipe Scaling: Scaling a 5-gallon recipe to 10 gallons calls for verifying that the collected wort volume and gravity align with the scaled targets before committing to the full boil.
• Competition Brewing: Hitting exact style parameters matters for competition entries. Checking both pre-boil and post-boil gravity targets against measured values confirms the finished beer will fall within style guidelines.