Homebrewing Beer ABV Calculator
Calculate beer ABV from original and final gravity for your homebrew. Get alcohol by volume, ABW and apparent attenuation instantly with the formula shown.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Show the formula & steps
How the Beer ABV Calculator Works
Every homebrewer needs to know how strong their beer turned out, and the answer comes from two hydrometer readings: the original gravity (OG) taken before fermentation and the final gravity (FG) taken after. As yeast eats sugar and produces alcohol, the wort gets less dense, so the size of that drop reveals the alcohol by volume (ABV). This calculator applies the standard homebrew formula and also reports ABW and apparent attenuation.
The Formula
The widely used homebrew approximation is:
ABV % = (Original Gravity − Final Gravity) × 131.25
The constant 131.25 converts the gravity difference into a percentage. The calculator also computes alcohol by weight as ABW = ABV × 0.79336, and apparent attenuation as (OG − FG) ÷ (OG − 1), which tells you what fraction of the sugar fermented.
Worked Example
For a typical pale ale with OG 1.050 and FG 1.010:
- ABV = (1.050 − 1.010) × 131.25 = 0.040 × 131.25 = 5.25%
- ABW = 5.25 × 0.79336 = 4.17%
- Attenuation = (1.050 − 1.010) ÷ (1.050 − 1) = 0.040 ÷ 0.050 = 80%
Typical Gravity and ABV Ranges
| Beer style | OG | FG | ABV |
|---|---|---|---|
| Session ale | 1.038 | 1.010 | 3.7% |
| Pale ale | 1.050 | 1.010 | 5.3% |
| IPA | 1.065 | 1.012 | 7.0% |
| Imperial stout | 1.090 | 1.022 | 8.9% |
Tips for Accurate Readings
- Take OG before pitching yeast and FG only once gravity is stable over two or three days, so fermentation is truly finished.
- Correct for temperature — hydrometers are calibrated to a specific temperature (often 20 °C / 68 °F), and warm samples read low.
- Use the same instrument for both readings to cancel out calibration error.
- For very high-gravity beers, the simple formula slightly under-reads; brewing software with a more advanced equation is more precise above about 8% ABV.
This calculator uses the standard 131.25 approximation, which is accurate for most homebrew strengths. Always sanitise your sampling gear and let fermentation fully complete before trusting your final gravity.
Frequently asked questions
How do you calculate beer ABV from gravity?+
Subtract the final gravity from the original gravity and multiply by 131.25. For a beer with OG 1.050 and FG 1.010, ABV = (1.050 − 1.010) × 131.25 = 5.25%. This is the standard homebrew approximation used by most brewing software.
What is original gravity and final gravity?+
Original gravity (OG) is the density of the wort before fermentation, measured with a hydrometer or refractometer, reflecting how much sugar is available. Final gravity (FG) is the density after fermentation, when yeast has converted sugar into alcohol. The drop between them indicates how much alcohol was produced.
Why is the ABV formula multiplied by 131.25?+
The factor 131.25 converts the gravity drop into a percentage of alcohol by volume. It is an empirical constant that accounts for the density change as sugar becomes ethanol and carbon dioxide. Some brewers use 131 or a more complex formula for very strong beers, but 131.25 is the standard for typical strengths.
What is apparent attenuation?+
Apparent attenuation is the percentage of the original sugar that the yeast fermented, calculated as (OG − FG) ÷ (OG − 1). Most ales finish around 70 to 80 percent attenuation; higher means a drier beer, lower means more residual sweetness.
What is the difference between ABV and ABW?+
ABV is alcohol by volume, the standard measure shown on labels. ABW is alcohol by weight, which is lower because alcohol is less dense than water; ABW is roughly ABV × 0.79336. A 5.25% ABV beer is about 4.17% ABW.