# Homebrew Numbers: ABV, Priming Sugar and Water Salts

How to get ABV from two gravity readings, why the priming sugar depends on the temperature your beer has been sitting at, and what gypsum and calcium chloride actually do.

---

- **Canonical URL:** https://dothecalculation.com/blog/hobby/homebrew-abv-priming-and-water
- **Category:** Hobbies & Leisure Utilities
- **Author:** Do The Calculation Team
- **Published:** 2026-09-20
- **Reading time:** 17 min read
- **Publisher:** Do The Calculation (https://dothecalculation.com)
- **Methodology:** https://dothecalculation.com/methodology

---

Three calculations cover most of what a homebrewer needs to work out: how strong the beer is, how much sugar to add at bottling, and what to put in the water. The first is easy and slightly approximate, the second has a step everyone forgets, and the third is where people either overthink or ignore it entirely.

## ABV from two gravity readings

Take a specific gravity reading before pitching yeast and another when fermentation is done. The difference is sugar that became alcohol.

**Alcohol by volume and apparent attenuation**

```
ABV % = (OG − FG) × 131.25
Apparent attenuation % = (OG − FG) ÷ (OG − 1) × 100
```
- Worked: OG 1.050, FG 1.010 gives (0.040) × 131.25 = 5.25% ABV and 80.0% apparent attenuation.
- The 131.25 is a fitted constant, not a physical one, and the formula is an approximation. It is accurate enough for normal-strength beer and drifts on very strong ones — treat a reading over about 8% as an estimate rather than a measurement.
- Attenuation is how much of the available sugar the yeast got through. It tells you whether the yeast did its job, which the ABV figure alone does not.

**Common gravity pairs**
| OG | FG | ABV | Apparent attenuation |
| --- | --- | --- | --- |
| 1.045 | 1.008 | 4.86% | 82.2% |
| 1.050 | 1.010 | 5.25% | 80.0% |
| 1.060 | 1.012 | 6.30% | 80.0% |
| 1.090 | 1.020 | 9.19% | 77.8% |

> **Attenuation is the number that diagnoses a problem** — If your FG is higher than expected, the ABV just comes out low and tells you nothing about why. Attenuation compares what happened against what was available: a beer that stopped at 60% attenuation when the yeast strain is rated for 75% has stalled, and that is a mash temperature, yeast health or fermentation temperature problem you can actually chase.

## Priming sugar, and the step everyone forgets

Bottle conditioning works by giving the remaining yeast a measured amount of sugar in a sealed bottle. The carbon dioxide they produce has nowhere to go, so it dissolves into the beer. Too little and the beer is flat; too much and you have bottle bombs, which is a genuine injury risk rather than a disappointment.

The step people miss is that the beer already contains dissolved carbon dioxide from fermentation, and how much depends on the temperature it has been sitting at. Warm beer holds less. So you do not prime for your target — you prime for the difference between your target and what is already in there.

**5 gallons to 2.4 volumes of CO₂, by the temperature the beer has been at**
| Conditioning temperature | CO₂ already dissolved | Still needed | Dextrose |
| --- | --- | --- | --- |
| 60°F | 0.99 vol | 1.41 vol | 107 g |
| 68°F | 0.86 vol | 1.54 vol | 116 g |
| 72°F | 0.81 vol | 1.59 vol | 120 g |
| 78°F | 0.75 vol | 1.65 vol | 125 g |

Eighteen grams between a cool cellar and a warm room on the same batch. Use the highest temperature the beer has reached since fermentation finished, not the temperature right now — the dissolved CO₂ came out at the warmest point and did not go back in.

**Sugar type, for the same 116 g of dextrose**
| Sugar | Amount | Why |
| --- | --- | --- |
| Dextrose (corn sugar) | 116 g | The reference; ferments cleanly and completely |
| Table sugar (sucrose) | 106 g | Slightly more fermentable by weight, so you need less |
| Dried malt extract | 155 g | Not fully fermentable, so you need noticeably more |

**Target carbonation by style, 5 gallons at 68°F**
| Roughly | Volumes of CO₂ | Dextrose |
| --- | --- | --- |
| British ale, cask-like | 1.8 | 71 g |
| American ale | 2.2 | 101 g |
| Lager, most styles | 2.4 | 116 g |
| Wheat beer | 2.8 | 146 g |
| Belgian, saison | 3.5 | 199 g |

> **Check your bottles before going high** — Standard crown-capped beer bottles are not rated for the pressures at the top of that table. Belgian styles at 3.5 volumes are traditionally bottled in heavy glass with a cage for a reason. If you are carbonating above about 3 volumes, use bottles made for it, and never use bottles that previously held a still drink.

Tool: [Work out ABV and priming sugar](https://dothecalculation.com/calculators/homebrew-abv-priming-calculator) — ABV and apparent attenuation from your gravity readings, plus the priming sugar for your batch size, target carbonation, conditioning temperature and sugar type.

## Water: two ions do most of the work

Brewing water chemistry has a reputation for complexity that mostly comes from people trying to replicate a historic city water profile. For a homebrewer the useful part is much smaller: two ions shape how the finished beer tastes, and the ratio between them is the lever.

- **Sulphate** accentuates hop bitterness and makes the finish drier and crisper. Gypsum (calcium sulphate) is how you add it.
- **Chloride** accentuates malt fullness and makes the beer taste rounder and sweeter. Calcium chloride is how you add it.
- **Calcium** matters for the mash rather than for flavour, and both of those salts bring it with them.

**20 litres of soft water, 6 g of salts added**
| Addition | Calcium | Sulphate | Chloride | Ratio | Reads as |
| --- | --- | --- | --- | --- | --- |
| Nothing | 10 ppm | 8 ppm | 6 ppm | 1.3 | Balanced but very low in everything |
| 6 g gypsum | 80 ppm | 175 ppm | 6 ppm | 29.2 | Hop-forward, dry, accentuated |
| 6 g calcium chloride | 92 ppm | 8 ppm | 151 ppm | 0.05 | Malt-forward, soft, rounded |
| 3 g of each | 86 ppm | 92 ppm | 78 ppm | 1.2 | Balanced |

The same six grams, split differently, moves the beer from dry and bitter to soft and malty. That is the whole technique for most purposes: pick a direction, pick a ratio, and stop there. Chasing a Burton-on-Trent profile ion by ion is a much larger project with much smaller returns.

> **Start from your actual water** — None of this means anything without knowing what you are starting with. Get a water report from your supplier, or test it — the additions are on top of what is already there, and a hard water supply may already have more sulphate than you intend to end up with. If your water is very hard or heavily treated, diluting with distilled or reverse-osmosis water gives you a known baseline to build on.

Tool: [Build a water profile](https://dothecalculation.com/calculators/home-brewing-water-chemistry-calculator) — Final ion concentrations and the sulphate to chloride ratio from your source water, dilution and salt additions.

## When the simple ABV formula drifts

The body of this guide warns that the 131.25 formula becomes an estimate above about 8%. It is worth seeing how large the drift is. Several online brewing calculators offer a second, more complex formula, generally attributed to an article by Michael L. Hall in Zymurgy in 1995. We could not obtain the original article to check it against, so treat it as a widely used alternative rather than a verified standard.

**The alternative ABV formula, as commonly reproduced**

```
ABV % = (76.08 × (OG − FG) ÷ (1.775 − OG)) × (FG ÷ 0.794)
```
- It accounts for the fact that the relationship between gravity drop and alcohol is not a straight line.
- For ordinary beers it agrees with the simple formula to within a tenth of a percent or so. The difference grows with strength.

**The two formulas side by side**
| OG | FG | Simple (× 131.25) | Alternative | Difference |
| --- | --- | --- | --- | --- |
| 1.040 | 1.008 | 4.20% | 4.21% | 0.01 |
| 1.050 | 1.010 | 5.25% | 5.34% | 0.09 |
| 1.060 | 1.012 | 6.30% | 6.51% | 0.21 |
| 1.075 | 1.015 | 7.88% | 8.34% | 0.46 |
| 1.090 | 1.020 | 9.19% | 9.99% | 0.80 |
| 1.110 | 1.025 | 11.16% | 12.55% | 1.40 |

For a normal-strength beer the choice of formula makes no practical difference. For a barleywine or imperial stout it can change the answer by more than a percentage point, with the alternative reading higher. Neither is a laboratory measurement. If a precise figure matters, such as for a label, it has to come from a laboratory, not from gravity readings.

## Gravity and degrees Plato

Recipes from professional brewers and from many countries quote sugar content in degrees Plato rather than specific gravity. Plato is the percentage of sugar by weight in the wort. A common approximation converts between the two.

**Specific gravity to degrees Plato (approximation)**

```
°P ≈ 259 − 259 ÷ SG
```
- Accurate enough for recipe conversion in the normal brewing range; exact conversions use published tables or a more detailed polynomial.
- A rough rule of thumb follows from it: at ordinary strengths, each 4 points of gravity is about 1 degree Plato.

**Gravity to Plato, approximate**
| Specific gravity | Degrees Plato |
| --- | --- |
| 1.040 | about 10.0 |
| 1.050 | about 12.3 |
| 1.060 | about 14.7 |
| 1.080 | about 19.2 |
| 1.100 | about 23.5 |

## Getting gravity readings you can trust

Every calculation in this guide is only as good as the two gravity readings behind it, and the readings have more ways to go wrong than the arithmetic does.

- **Correct for temperature.** A hydrometer is calibrated at a particular temperature, printed on it or in its instructions. Warmer liquid is less dense and reads low. Take the reading at the calibration temperature, or apply the correction the maker provides.
- **Read at the bottom of the curve.** Liquid climbs the hydrometer stem. Read where the flat surface of the liquid meets the scale, not the top of the curve.
- **Degas the sample.** Carbon dioxide bubbles cling to the hydrometer and push it up, reading high. Swirl or pour the sample back and forth first.
- **Confirm fermentation has finished.** Take two readings a few days apart. If they match, fermentation has stopped; if they differ, it has not, and bottling now risks over-carbonation.
- **Refractometers need a correction after fermentation starts.** Alcohol distorts their reading, so a refractometer's final gravity needs a correction formula before it is usable.

## Priming sugar for any batch size

Priming sugar scales directly with volume. At a fixed target and conditioning temperature the dose per bottle is the same whatever the batch size, which gives a useful sanity check: for 2.4 volumes of carbon dioxide at 68°F it works out to about 2.2 grams of dextrose per 12-ounce bottle.

**Dextrose for 2.4 volumes of CO₂, beer at 68°F**
| Batch size | Dextrose | In ounces |
| --- | --- | --- |
| 1 gal | 23 g | 0.82 oz |
| 2.5 gal | 58 g | 2.05 oz |
| 3 gal | 70 g | 2.46 oz |
| 5 gal | 116 g | 4.10 oz |
| 6 gal | 140 g | 4.92 oz |
| 10 gal | 233 g | 8.20 oz |

For a metric batch the arithmetic is the same: 20 litres at 20°C, which is 68°F, needs about 123 grams of dextrose for 2.4 volumes. Measure the batch you are actually bottling, after racking off the sediment, not the volume you started with. Several litres lost to trub and yeast is normal, and priming the original volume over-carbonates what is left.

> **Mix the sugar in, do not sprinkle it** — Dissolve the priming sugar in a small amount of boiled water, let it cool, and add it to the bottling bucket before racking the beer onto it so it mixes evenly. Uneven mixing is a common cause of some bottles being flat while others foam, even when the total dose was right.

## What one gram of each salt adds

The water chemistry calculator works from the ion contributions of each brewing salt, derived from the salts' formula masses rather than copied from a chart. Here they are for one gram dissolved in 20 litres, a common homebrew batch size, so you can see the scale of each addition at a glance.

**Ions added by 1 gram of salt in 20 litres of water**
| Salt | Adds | And |
| --- | --- | --- |
| Gypsum (calcium sulphate) | 11.6 ppm calcium | 27.9 ppm sulphate |
| Calcium chloride | 13.6 ppm calcium | 24.1 ppm chloride |
| Epsom salt (magnesium sulphate) | 4.9 ppm magnesium | 19.5 ppm sulphate |
| Table salt (sodium chloride) | 19.7 ppm sodium | 30.3 ppm chloride |

Scale by the grams you add and divide by your own batch volume. Two points are worth noting. Table salt adds sodium as well as chloride, so calcium chloride is usually the better way to raise chloride. And the figures for calcium chloride depend on which form you have, because it is sold in more than one hydration state. The calculator's figures, like its gypsum and Epsom salt figures, are for the hydrated forms (calcium chloride dihydrate, gypsum dihydrate, Epsom salt heptahydrate), so check your product's label.

## Residual alkalinity: the number behind mash pH

The sulphate to chloride ratio decides flavour balance. The other number the calculator reports, residual alkalinity, decides something more fundamental: where the mash pH lands. The calculator uses Kolbach's formulation, which weighs the water's alkalinity against the calcium and magnesium that counteract it.

**Residual alkalinity (Kolbach), all values as ppm CaCO₃**

```
RA = alkalinity − (calcium ÷ 3.5 + magnesium ÷ 7)
```
- Alkalinity comes mainly from bicarbonate in the water report.
- Calcium and magnesium are converted to their CaCO₃ equivalents before dividing.

In the soft-water example earlier in this guide, residual alkalinity is about +8 before any additions, and adding gypsum or calcium chloride pushes it well below zero, to about −42 and −50 respectively. A low or negative residual alkalinity suits pale beers, because pale malts do little to lower the mash pH on their own. Dark roasted malts are acidic, so they tolerate, and can even need, more alkaline water; that pairing is the usual explanation given for why some historic dark-beer brewing centres had hard, alkaline water.

## Common homebrew number mistakes

- **Reading a hydrometer at the wrong temperature.** Correct it, or cool the sample.
- **Bottling before fermentation has finished.** Two matching readings a few days apart, then bottle.
- **Priming the starting volume.** Prime the volume you are actually bottling.
- **Using the current temperature for priming.** Use the warmest the beer reached after fermentation.
- **Adding salts without a water report.** You are adding to what is already there, which may already be high.
- **Treating any ABV figure as exact.** Both formulas are estimates, and they diverge for strong beers.

## Diluting hard water, worked through

If your tap water is hard and alkaline, adding salts cannot fix it, because salts only add ions. Dilution is the tool that takes them away. Mixing tap water with distilled or reverse-osmosis water reduces every ion in proportion, and the calculator shows the effect directly. Here is an illustrative hard water, not any particular city's supply, diluted in stages.

**Illustrative hard water (110 ppm calcium, 280 ppm bicarbonate) diluted with reverse-osmosis water**
| Share of RO water | Calcium | Bicarbonate | Residual alkalinity | Calculator's reading |
| --- | --- | --- | --- | --- |
| 0% | 110 ppm | 280 ppm | about 140 | Very high: suits heavily roasted stouts, otherwise needs acidification |
| 25% | 82.5 ppm | 210 ppm | about 105 | High: suits brown and porter grists |
| 50% | 55 ppm | 140 ppm | about 70 | High: suits brown and porter grists |
| 75% | 27.5 ppm | 70 ppm | about 35 | Moderate: suits amber and lightly roasted grists |

Two things happen at once as you dilute. Residual alkalinity falls, which is usually the goal, but calcium falls with it, and calcium matters for the mash and for yeast. That is why a common pattern is to dilute first and then add back calcium with a salt that also moves the flavour balance in the direction you want. On the 50% dilution above, adding 3 grams of calcium chloride to 20 litres lifts calcium back to about 96 ppm, drops residual alkalinity from about 70 to about 41, and swings the sulphate to chloride ratio from 1.33 to 0.32, towards a softer, maltier beer.

## Gravity points: adjusting a recipe on brew day

Gravity readings can be treated as quantities of sugar, which makes brew-day adjustments simple arithmetic. Take the part after the decimal point as points, so 1.050 is 50 points, and multiply by the volume. The total stays the same when you add or remove water; only the concentration changes.

**Gravity points**

```
Total points = (SG − 1) × 1000 × volume
New gravity = 1 + total points ÷ new volume ÷ 1000
```
- Volume can be in gallons or litres, as long as you use the same unit throughout.
- Boiling removes water but not sugar, so gravity rises during the boil; adding water lowers it.

**Brew-day adjustments with gravity points**
| Situation | Working | Result |
| --- | --- | --- |
| 6.5 gal pre-boil at 1.040, boiled down to 5.5 gal | 260 points ÷ 5.5 | 1.047 after the boil |
| 5 gal of wort at 1.060, topped up with 0.5 gal of water | 300 points ÷ 5.5 | 1.055 |
| 5.5 gal at 1.045, target 1.050 | 247.5 points ÷ 50 points | Boil down to about 4.95 gal |
| Scaling a 5 gal, 1.050 recipe to 3 gal | 250 points × 3/5 | 150 points needed in total |

This is the arithmetic behind every correction on brew day. If your pre-boil gravity is low, you can boil longer to concentrate it, at the cost of a smaller batch, or add fermentable sugar or extract to add points. If it is high, you can top up with water, at the cost of a larger batch with the same total sugar. Knowing which lever you are pulling makes the decision quick.

## When the final gravity comes out high

The body of this guide points out that apparent attenuation is the number that diagnoses a stalled fermentation. Once you know the attenuation is low, these are the usual causes to work through, roughly in order of how often they turn out to be the answer.

- **The fermentation is not actually finished.** Some yeasts slow down long before they stop. Take two readings a few days apart before concluding anything.
- **The fermentation temperature dropped.** Yeast slows sharply when it gets cold. Moving the fermenter somewhere a little warmer and gently rousing the yeast often restarts it.
- **Too little yeast was pitched,** or the yeast was old or stressed. An underpitched fermentation can stall well above the expected final gravity.
- **The mash was hotter than intended,** which produces more sugars the yeast cannot ferment. The beer will finish higher and fuller-bodied, and no amount of waiting fixes it.
- **The recipe has a lot of unfermentable content,** such as crystal malts, lactose or some extracts. A high final gravity may simply be what that recipe produces.
- **The yeast strain's normal range is lower than you assumed.** Check the manufacturer's stated attenuation range for the strain, and compare with that rather than with a general figure.

Only the first three are fixable after the fact. The last three are information for the next batch, which is exactly why recording original gravity, final gravity, mash temperature and fermentation temperature for every brew pays off: after a few batches the numbers tell you which of these is your usual problem.

## Sources

- The ABV relation (OG − FG) × 131.25 and the apparent attenuation formula are the standard brewing approximations. The 131.25 is a fitted constant rather than a physical one, and the approximation drifts at high gravity.
- The residual CO₂ relation used for priming, 3.0378 − 0.05006 T + 0.0002655 T² with T in °F, is the standard brewing polynomial for dissolved carbon dioxide against temperature, as is the 15.113 g of dextrose per gallon per volume of CO₂.
- Ion contributions from each salt are computed from its formula mass using the IUPAC standard atomic weights: https://iupac.qmul.ac.uk/AtWt/

## Where to go next

Ratios run through every kind of brewing. [Coffee brew ratio](/blog/hobby/coffee-brew-ratio-guide) explains why holding a ratio constant matters more than any other variable when batch size changes, and [how to double or halve a recipe](/blog/hobby/scaling-recipes-math-guide) covers the same proportional reasoning in the kitchen, including the ingredients that refuse to take a multiplier.

## Common questions

**How do I calculate ABV from OG and FG?**

Multiply the gravity drop by 131.25. An OG of 1.050 finishing at 1.010 is (1.050 − 1.010) × 131.25 = 5.25% ABV. It is an approximation with a fitted constant, accurate enough for normal beer and increasingly rough above about 8%.

**How much priming sugar for 5 gallons?**

For 2.4 volumes of CO₂ it is about 116 g of dextrose if the beer has been at 68°F, 107 g at 60°F and 125 g at 78°F. The temperature matters because warm beer holds less dissolved CO₂ already, so it needs more sugar to reach the same target.

**Which temperature do I use for priming?**

The highest the beer has reached since fermentation finished, not its current temperature. Dissolved CO₂ came out of solution at the warmest point and did not go back in, so using a later cooler reading will under-prime the batch.

**Can I use table sugar instead of corn sugar?**

Yes. Table sugar is slightly more fermentable by weight, so you need about 9% less: 106 g where the recipe calls for 116 g of dextrose. Dried malt extract goes the other way — it is not fully fermentable, so you need about a third more, 155 g for the same result.

**What does the sulphate to chloride ratio do?**

It shifts the beer between hop-forward and malt-forward. Sulphate accentuates bitterness and dryness, chloride accentuates malt fullness. On 20 litres of soft water, 6 g of gypsum gives a ratio of about 29 and reads as dry and bitter; 6 g of calcium chloride gives 0.05 and reads as soft and malty.

**Why did my bottles over-carbonate?**

The usual causes are priming for the current temperature rather than the warmest the beer reached, bottling before fermentation had genuinely finished, or an infection continuing to ferment in the bottle. Take two gravity readings a few days apart and only bottle when they match.

**Why do ABV calculators give different answers for strong beers?**

Because they use different formulas. The simple (OG − FG) × 131.25 and a more complex alternative formula agree closely for normal beers but diverge as strength rises: for OG 1.090 and FG 1.020 they give 9.19% and 9.99%. Neither is a laboratory measurement.

**How do I convert specific gravity to Plato?**

A common approximation is °P ≈ 259 − 259 ÷ SG. A gravity of 1.050 is about 12.3 °P and 1.060 about 14.7 °P. At ordinary strengths, each 4 points of gravity is roughly one degree Plato.

**How much priming sugar per bottle?**

For 2.4 volumes of CO₂ with beer at 68°F, about 2.2 grams of dextrose per 12-ounce bottle. It is more reliable to dissolve the batch total in boiled water and mix it into the whole batch, so every bottle gets the same dose.

**What does residual alkalinity tell me?**

How strongly the water will resist the acidity of the malt in the mash. Low or negative residual alkalinity suits pale beers; higher values suit darker, more acidic grists. The calculator uses Kolbach's formula: alkalinity minus calcium ÷ 3.5 minus magnesium ÷ 7, all as CaCO₃.

**Why does my hydrometer reading seem wrong?**

The common causes are reading at the wrong temperature, reading the top of the meniscus instead of the flat surface, and carbon dioxide bubbles lifting the hydrometer. Correct for temperature, read at the liquid surface, and degas the sample first.

**How do I lower the alkalinity of hard brewing water?**

Dilute it with distilled or reverse-osmosis water, which reduces every ion in proportion. In an illustrative hard water, a 50% dilution halves bicarbonate and cuts residual alkalinity from about 140 to about 70. Calcium falls too, so many brewers add some back with calcium chloride or gypsum.

**How much will my gravity rise during the boil?**

Multiply the pre-boil gravity points by the pre-boil volume, then divide by the post-boil volume. 6.5 gallons at 1.040 is 260 points; boiled down to 5.5 gallons that is about 47 points, so 1.047.

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_Source: [Do The Calculation](https://dothecalculation.com/blog/hobby/homebrew-abv-priming-and-water). Quote freely with attribution and a link to this page._