Homebrewers measure fermentation progress with a hydrometer before and after fermentation. This estimates the resulting alcohol by volume from those two readings.
How it works
Simplified ABV formula
ABV % = (OG − FG) × 131.25
OG (original gravity) and FG (final gravity) are hydrometer readings taken before and after fermentation.
The simplified formula, and where it’s less accurate
This is the standard simplified homebrewing formula, widely used because it’s close enough for practical purposes across the specific-gravity range home fermentation actually produces — accurate to within roughly half a percentage point of ABV for typical beer and wine gravity ranges. It is an approximation: the true relationship between gravity change and alcohol content isn’t perfectly linear. More precise formulas exist for high-gravity brews or where precision genuinely matters; for an ordinary homebrew batch, the gap here is smaller than most hydrometers can even measure reliably.
How to use this calculator
- Take a hydrometer reading before fermentation starts (original gravity).
- Take another reading once fermentation has finished (final gravity).
- Enter both to get an estimated ABV.
Frequently asked questions
Why does a higher original gravity give more alcohol at the same final gravity?
Because original gravity reflects how much fermentable sugar was available — more sugar to ferment means more alcohol produced, all else equal, which is exactly what a bigger gap between OG and FG in the formula captures.
What if my final gravity reading is higher than my original?
That’s a measurement error, not a real result — fermentation only ever lowers gravity as sugar converts to alcohol, so an FG above OG means one of the readings (or the hydrometer’s temperature calibration) needs rechecking.
Why isn’t the simplified formula exactly right?
Because the relationship between the change in specific gravity and the resulting alcohol content isn’t perfectly linear across all gravity ranges — the simplified ×131.25 formula is a close linear approximation, good enough for the gravity range most home fermentation falls into, but a genuinely more precise (non-linear) formula exists for high-gravity brewing.
Does temperature affect a hydrometer reading?
Yes — most hydrometers are calibrated for a specific reference temperature, and a reading taken at a different temperature needs a small correction. Check your specific hydrometer’s calibration temperature.
Is this formula the same for beer and wine?
The same simplified formula is commonly used for both, since it’s based on the general relationship between gravity change and alcohol content rather than anything specific to one beverage type.
