How to use the gravity dilution and boil-off calculator
- Pick what you want to work out: the water to add, the volume to boil off, or the gravity after you add water or boil.
- Enter the wort volume now and its gravity as specific gravity, such as 1.065.
- Enter the target gravity, the water you will add, or the volume you will boil off, depending on your choice. For boiling, add your own boil-off rate per hour to see a time.
- Read the answer, the final volume and the gravity, then check the before and after table.
Formula
gravity points = (SG - 1) x 1000
points x volume stays the same when you add water or boil it off
water to add = volume x points now / target points - volume
volume to boil off = volume - volume x points now / target points
Sugar in the wort does not change when you add water or boil it down, so the total of points times volume is constant. The formula assumes volumes simply add.
A 5.5 gallon batch that came out strong
The wort measures 1.065, which is 65 points, and you want 1.050, which is 50 points. The total is 65 x 5.5 = 357.5 point-gallons. Divide by 50 to get 7.15 gallons, so you add 7.15 - 5.5 = 1.65 gallons of water.
Going the other way, 5.5 gallons at 1.040 that you want at 1.050 has 220 point-gallons. At 50 points the volume is 4.40 gallons, so 1.10 gallons must boil off. At a boil-off rate of 1.25 gallons per hour that takes 52.8 minutes, which the tool shows as 53 minutes. Adding 1.5 gallons of water to 5.5 gallons at 1.065 gives 1.051, and boiling 0.75 gallon off the same batch gives 1.075.
When to dilute and when to boil
Dilution is the simple fix when the gravity is too high: add clean water that has been boiled and cooled, or sanitized water, and mix well before you measure again. Boiling longer raises the gravity but also changes flavor, color and hop use, so add time for hops separately from this calculation.
To convert a refractometer or Plato reading before you plug it in, use the Brix, Plato and Specific Gravity Converter. If the reading was taken on warm wort, correct it first with the Hydrometer Temperature Correction Calculator.
Why your boil-off rate belongs to your kettle
The rate depends on the pot width, the burner, the lid and the room. Measure it once by boiling a known volume of water for an hour and noting how much is lost, then reuse that number. The tool fills in an example rate that you should replace.
Assumptions and limits
- Water and wort mix without any change in total volume.
- The hydrometer reading and the volume are taken at the same temperature, because hot liquid takes up more space.
- Boil-off is steady through the whole boil, so time is the volume divided by the hourly rate.
- Evaporation takes only water. Alcohol is not present in unfermented wort, so the formula does not apply to beer that is already fermenting.
Frequently asked questions
Can I add water after fermentation has started?
This tool treats the liquid as unfermented wort. Alcohol and fermentation change what a hydrometer reads, so a mid-fermentation dilution needs different math and a different plan.
Why does the final volume come out larger than my fermenter?
The answer is the volume you would need to reach the target gravity. If it is more than your vessel holds, choose a less ambitious target, split the batch, or boil off less and accept a lower gravity.
What should I enter if my gravity is in Plato?
Convert it to specific gravity first. The converter linked above does that and gives the gravity points too, which makes the arithmetic easy to check by hand.
Does boiling for longer change anything besides gravity?
Yes. Color tends to darken, hop bitterness changes with how long the hops boil, and flavor compounds shift. This tool only follows the sugar and the water.