One conserved quantity, three levers
Everything on this page falls out of a single fact: the boil removes water and creates no sugar. Gravity points times gallons is therefore constant from the end of the mash until you deliberately add something. Write the points as the third decimal multiplied out — 1.048 is 48 points — and 6.5 gallons of it is 312 points. Boil it down to five gallons and it is still 312 points, now at 62.4 points per gallon, which reads 1.062. Top it up to eight gallons and it is still 312 points, reading 1.039.
That gives you exactly three moves. Add water, which spreads the same points over more gallons. Remove water, which concentrates them into fewer. Or add sugar, which is the only move that changes the 312 at all. Everything else — stirring, waiting, chilling — changes nothing about the number.
The default case: 6.5 gallons measured at 1.048 against a 1.055 target. 312 points divided by the 55 you want is 5.67 gallons, so boiling away 0.83 gallons lands it exactly. At a measured boil-off rate of 1.25 gallons an hour that is about 40 minutes of extra boil, and it costs you 0.83 gallons of beer.
Why the extract weight is not the shortfall divided by the potential
Adding extract looks like simple arithmetic and is not, because the extract brings its own volume. A pound of dry malt extract dissolved in wort occupies something near 0.075 gallons, and that extra volume dilutes the very gravity you were raising. Solve it properly and the weight comes out higher than the naive division.
The relation is: with P points already in the kettle across V gallons, adding x pounds at potential p and volume factor k gives a gravity of (P + xp) / (V + xk). Set that equal to the target points t and rearrange:
x = (t × V − P) / (p − t × k)
For the default case that is (55 x 6.5 − 312) / (45 − 55 x 0.075), or 45.5 / 40.875, which is 1.11 pounds. The naive division would have said 45.5 / 45, or 1.01 pounds, which lands at 1.0544 — six tenths of a point short. At ordinary gravities the difference is small. At high gravity, where the target points are large and the denominator shrinks, it stops being small — and if the target points times the volume factor ever reach the potential, there is no weight of that extract that gets you there at all, because each pound dilutes as much as it contributes.
Which lever, and what each one costs
| Lever | Use when | What it costs |
|---|---|---|
| Add water | Gravity is above target | Nothing, except that the water was not boiled. You end up with more beer. |
| Boil longer | Gravity is below target and the shortfall is small | Time, colour, extra hop bitterness from the existing charge, and volume |
| Add extract | Gravity is below target and you want the volume | Money, and a small change in the character of the wort |
| Accept the number | Nearly always a defensible answer | The beer is a few points off the recipe and nobody can taste three points |
The last row is not a joke. Three gravity points on a 1.055 beer is about a fifth of a percent of alcohol, which is inside the error of the hydrometer reading that told you about the problem. Corrections are worth making when the miss is large enough to change what the beer is; below that, the correction introduces more variables than it removes.
Read a cooled sample before you do anything
A hot sample reads low. That matters here more than anywhere, because a low reading on a hot sample looks exactly like a weak wort and sends you toward a longer boil or an extract addition you did not need. The size of the error is a few thousandths across ordinary temperature ranges and more if you read straight out of the kettle.
The same applies to volume. Wort at boiling occupies about four percent more space than the same wort cold, so a dipstick reading taken while the kettle is going overstates the gallons. Both numbers on this page have to be on the same basis, and the honest way to do it is to pull a sample, cool it, read it, and measure the volume with the shrinkage taken off — the brewing water calculator handles that conversion.
Dilution water is unboiled water
Topping up after the boil is routine in extract brewing and unremarkable, but it is worth naming for what it is: water going into the fermenter that has not been through a boil. Whether that matters to you and what you do about it is your call and your source of water, and this page takes no position on it beyond saying it is a decision rather than a non-event. The arithmetic of the top-up is the same either way.
Questions people ask
How much water do I add to bring a gravity down?
Multiply the points by the gallons to get the total, divide by the points you want, and the answer is the volume you need to end up at. Subtract what you have and that is the water. Six and a half gallons at 1.062 is 403 points; at 1.055 you want 7.33 gallons, so you add 0.83. The thing to notice is that the answer is a final volume rather than a ratio, which is why rules of thumb about adding a quart per gravity point do not travel between batch sizes.
How long do I boil to raise the gravity by a set amount?
Work out the volume that lands the target, subtract it from what you have, and divide by your own measured boil-off rate in gallons per hour. The rate is the input that has to be yours: it depends on kettle diameter far more than on burner output, and published figures spanning roughly 0.8 to 1.5 gallons an hour are about somebody else. If you have not measured it, boil a known volume of water uncovered for an hour at your normal vigour and measure the drop, once, and stop guessing after that.
Can I add sugar instead of malt extract?
The arithmetic is identical — enter the potential for whatever you are adding and the calculator does not care what it is. Plain sucrose and dextrose are quoted a little higher than dry malt extract because they are essentially all fermentable solids, while liquid malt extract is lower because a meaningful fraction of its weight is water. What differs is not the gravity but what the addition leaves behind: highly fermentable sugar attenuates out almost completely and lightens the body, malt extract does not. That is a recipe decision rather than a calculation one.
Does the correction change my bitterness?
Diluting after the boil dilutes the bitterness with everything else, in proportion, so a batch topped up by ten percent is about ten percent less bitter than it read before. Boiling longer works the other way and less predictably: the hops already in the kettle keep isomerising, so a forty minute extra boil adds bitterness beyond what the schedule intended, and it concentrates the existing bitterness into a smaller volume as well. Neither effect is worth a precise figure at home, but the direction is worth knowing before you choose which lever to pull.
My gravity is high and I do not want more beer. What then?
Then you have a strong beer, and the honest options are to accept it or to split the difference by adding less water than the calculator says. There is no way to remove sugar from wort; the only lever that reduces points is dilution, and dilution necessarily increases volume. If the volume is genuinely fixed by your fermenter, dilute to the fermenter capacity and accept whatever gravity that lands on, which the calculator shows in the section that holds the volume you planned.