Dilution & boil-off calculator
Original gravity too high or too low? Enter your current gravity, wort volume and target, and the calculator tells you how much water to add or how much longer to boil. Works in SG or °Plato, liters or gallons.
Wort now
From a cooled sample, with the volume it belongs to.
Target
Shows the gravity after adding exactly this much water.
Add water
1.20 gal
- Final volume
- 7.20 gal
- Final gravity
- 1.050
Runs in your browser. Nothing you enter is sent or stored.
Gravity points × volume stays the same
Adding water or boiling off steam changes the volume of your wort, but not the amount of sugar in it. Multiply the gravity points by the volume and you get a number that stays the same before and after the correction. Gravity points are the decimals of the specific gravity: 1.060 is 60 points.
So 6 gallons at 1.060 hold 360 point-gallons. To get down to 1.050, those 360 points have to spread over 360 / 50 = 7.2 gallons, so you add 1.2 gallons of water. In metric: 23 L at 1.060 become 27.6 L at 1.050, which means adding 4.6 L. Boiling off works the same way in reverse: the volume shrinks until the points reach your target.
GP = (SG − 1) × 1000
GP₁ × V₁ = GP₂ × V₂
V₂ = V₁ × GP₁ / GP₂
- GP
- Gravity points; ₁ is now, ₂ is your target
- V₁
- Current wort volume
- V₂
- Volume after the correction. Water to add = V₂ − V₁, volume to boil off = V₁ − V₂
Gravity in °Plato is converted to SG first.
OG too high: dilute with water
Diluting is the easy fix and gives you more beer. Where you add the water matters.
Pre-boil: if your pre-boil gravity is already high, add water to the kettle and boil as planned. Correct against your planned pre-boil gravity, not the OG, because the boil will still concentrate the wort. Your boil-off rate stays the same, so you end up with more wort after the boil.
Post-boil: add boiled, cooled water in the fermenter. This is the most precise way, because you measure the final wort first. It also lowers bitterness: hops that gave 40 IBU in 5 gallons give about 33 IBU in 6 gallons.
| Target OG | Per 20 L | Per 5 gal |
|---|---|---|
| 1.055 | 1.8 L | 0.45 gal |
| 1.050 | 4.0 L | 1.0 gal |
| 1.045 | 6.7 L | 1.7 gal |
| 1.040 | 10.0 L | 2.5 gal |
OG too low: boil longer or add extract
To raise the gravity you can remove water or add sugar. The calculator handles the first: it works out how much volume has to boil off and, from your boil-off rate, how many extra minutes that takes. 28 L at 1.040 need to lose 5.6 L to reach 1.050, which is about 84 minutes at 4 L per hour.
Boil-off rate depends on your kettle and heat source, not on the volume, so a longer boil does not go faster. Start the extra time before your first hop addition, so the hop schedule still counts down to flameout. A long boil darkens the wort a little and costs volume.
If you would rather keep your volume, add dry malt extract (DME) or sugar. The same points × volume logic applies, worked out by hand: 5 gallons that are 4 points short need 20 point-gallons, about 0.5 lb of DME. Sugar raises gravity without adding body and dries the beer out, so keep it to small corrections.
t = (V₁ − V₂) / R × 60
m = (GP₂ − GP₁) × V₁ / P
- t
- Extra boil time in minutes
- R
- Boil-off rate in volume per hour
- m
- Amount of extract to add
- P
- Extract potential: about 43 points per lb per gallon for DME, 46 for table sugar (about 360 and 385 per kg per liter)
| Ingredient | 500 g in 20 L | 1 lb in 5 gal |
|---|---|---|
| Dry malt extract (DME) | +9 | +8.6 |
| Liquid malt extract (LME) | +7.7 | +7.4 |
| Table sugar | +9.6 | +9.2 |
| Dextrose (corn sugar) | +8.8 | +8.4 |
When and how to correct
Correct your gravity before you pitch the yeast. Until then, adding water or extract is simple and the yeast sees the wort you planned. Once fermentation has started, the gravity no longer tells you how much sugar was there, so you lose the reading the calculation depends on.
A missed OG is usually off by a few points. Before you correct, make sure the reading itself is right.
- Measure the OG and fix it in the kettle or fermenter before the yeast goes in.
- Cool the sample or apply a temperature correction. A hot sample reads several points low.
- Stir well after adding water and take a fresh reading. Top-up water tends to settle in layers.
- Use the volume that belongs to the reading. Wort shrinks about 4% from boiling to room temperature.
Built into every BrewForge recipe
In BrewForge your equipment profile holds your boil-off rate, losses and fermenter top-up water, so every recipe plans the right pre-boil volume and gravity from the start. On brew day you log pre-boil gravity, volumes and OG next to the planned values, and the dilution and boil-off calculator is one click away when a reading is off.
- Boil-off rate per hour as volume or percentage in your equipment profile
- Planned pre-boil and pre-dilution gravity in every recipe
- Planned vs. measured gravity and volumes on brew day
Use this calculator via API
The same calculation is also a free JSON API: no account, no key, one GET request. Handy for scripts, spreadsheets and AI assistants that should use exact brewing math instead of estimating it.
Add units=imperial (or one unit parameter per value) for °F, gallons and pounds. Every answer lists the inputs, the units and the formula it used.
GET https://brewforge.sh/api/public/calc/dilution?gravity=1.060&volume=23&targetGravity=1.050
FAQ
Questions & answers
How much water do I add to lower my original gravity?
Multiply your current gravity points by your volume and divide by the target points. The difference to your current volume is the water to add. Example: 5 gallons at 1.060 to 1.050 is 5 × 60 / 50 = 6 gallons, so add 1 gallon.
I missed my OG and it is too low. What can I do?
Before pitching you can boil longer to concentrate the wort, or add dry malt extract or sugar to raise the gravity without losing volume. If the difference is only a few points, many brewers just accept a slightly lighter beer.
Does diluting change bitterness and ABV?
Yes. Water added after the boil spreads the same hop bitterness over more beer, so IBU drop in proportion to the volume. A lower OG also gives a lower ABV. Color gets a little lighter as well.
Can I correct the gravity after fermentation has started?
You can still add boiled, cooled water or sugar, but you can no longer measure the result, because the yeast has already turned part of the sugar into alcohol. Calculate from your OG reading taken before pitching.
How do I find my boil-off rate?
Measure the volume at the start of a rolling boil and again after exactly one hour, both hot. The difference is your boil-off rate per hour. Most homebrew kettles lose a few liters, or about a gallon, per hour.
Does the calculator work in °Plato?
Yes. Switch the gravity unit to °Plato. The calculator converts to SG and works with gravity points, which is more accurate than simply multiplying °Plato by volume.
Do the math once. BrewForge does it every brew.
Recipes, brew day and fermentation in one place, with these calculations built in. Free for up to 20 brews.