You've brewed a batch, and now you need the one number everyone asks about: how strong is it? Guessing the ABV means either under-selling a beer that's actually potent or, worse, overserving something stronger than you thought. For homebrewers, that gap between "about 5%" and the real figure matters for balance, labelling, and safety.
This ABV calculator gives you the exact strength. Enter your original and final gravity readings, and it returns your alcohol by volume in a moment — using the standard homebrew formula, so your number matches what the rest of the brewing world uses.
What Does "ABV" Mean?
ABV stands for Alcohol By Volume — the percentage of a drink's total volume that is pure alcohol. A 5% ABV beer is 5% alcohol by volume. It's the universal measure of a drink's strength, which is why it appears on every label and why brewers calculate it from how much sugar the yeast converted into alcohol.
How to Use the ABV Calculator
Step-by-step
- Enter your Original Gravity (OG) — the reading taken before fermentation.
- Enter your Final Gravity (FG) — the reading taken after fermentation finishes.
- Read your ABV percentage instantly, then copy it for your notes or label.
The Formula Behind It
The standard homebrew formula is:
ABV % = (OG − FG) × 131.25
OG is the density of your unfermented wort (usually around 1.040–1.060), FG is the density after fermentation (usually around 1.008–1.015), and 131.25 is the conversion constant that turns the drop in gravity into an alcohol percentage. Some brewers use ×131 for a slightly simpler estimate.
How it works
The calculation rests on a simple idea: yeast eats sugar and produces alcohol, so the amount of alcohol is tied to how much the liquid's density falls during fermentation. Before fermentation, dissolved sugars make the wort denser than water — that's your original gravity. As the yeast ferments those sugars into alcohol and carbon dioxide, the density drops to the final gravity. The difference between the two readings measures how much sugar was converted. The calculator multiplies that difference by the constant 131.25, which scales the gravity drop into a percentage of alcohol by volume. So the tool isn't measuring alcohol directly — it's inferring it from the change in density, which is exactly how brewers have estimated strength for generations. Accurate gravity readings are everything here: a hydrometer misread at either stage throws off the final number, so the tool assumes clean OG and FG inputs.
Worked Example
Say your original gravity was 1.055 and your final gravity was 1.012.
ABV = (1.055 − 1.012) × 131.25 = 0.043 × 131.25 ≈ 5.6%
So your batch is about 5.6% ABV — a standard-strength beer. A bigger gap, say 1.070 down to 1.010, gives (0.060) × 131.25 ≈ 7.9%, a notably stronger brew.
Quick Reference Table
| OG | FG | Approx. ABV |
|---|---|---|
| 1.040 | 1.010 | 3.9% |
| 1.050 | 1.012 | 5.0% |
| 1.055 | 1.012 | 5.6% |
| 1.065 | 1.014 | 6.7% |
| 1.075 | 1.012 | 8.3% |
Common Mistakes to Avoid
- Taking gravity readings at the wrong temperature. Hydrometers are calibrated to a set temperature; readings off-temperature need correcting first.
- Swapping OG and FG. Original gravity is always the higher, pre-fermentation reading; mixing them up gives a negative or nonsensical result.
- Forgetting temperature correction entirely. A warm sample reads lower than reality — correct it before calculating.
- Expecting lab-grade precision. This is an accurate estimate, not a certified measurement; small hydrometer errors carry through.
Frequently asked questions
How do I calculate ABV?
Subtract your final gravity from your original gravity and multiply by 131.25. For example, (1.055 − 1.012) × 131.25 ≈ 5.6% ABV.
What is OG and FG?
OG (Original Gravity) is the density of your wort before fermentation, and FG (Final Gravity) is the density after fermentation. The difference between them reflects how much sugar was converted to alcohol.
Why multiply by 131.25?
It's the standard conversion constant that turns the drop in gravity into an alcohol-by-volume percentage. Some brewers use 131 for a slightly simpler estimate.
Why is my ABV coming out negative?
You've likely entered the readings the wrong way round. Original gravity should be higher than final gravity — check you haven't swapped them.
Is this ABV figure exact?
It's an accurate estimate based on standard brewing math, but it depends on precise, temperature-corrected gravity readings. Treat it as reliable for homebrewing, not as a lab certification.