What it is
The Attenuation Estimator computes the expected final gravity (FG) of a beer after fermentation and the resulting alcohol by volume (%ABV) based on two inputs: the specific gravity of the wort before yeast inoculation (original gravity, OG) and the yeast attenuation percentage. Yeast attenuation is a measure of how much fermentable sugar a strain can convert into alcohol and CO₂; typical values range from 65 % to 95 %. The tool uses the standard formula FG = OG – ((OG‑1)×Attenuation÷100), which follows directly from conservation of mass and specific gravity units. Once FG is known, ABV is calculated with (OG‑FG)×131.25, a widely accepted factor that translates gravity points into alcohol percent.
This calculator matters because brewers need a quick way to hit a target ABV or final gravity without trial‑and‑error. By feeding the OG measured at 20 °C and the expected attenuation of the chosen yeast, the tool instantly tells you whether your recipe will be too weak, just right, or over‑strong. It also highlights how small changes in attenuation (e.g., using a different yeast strain) or wort density shift result in noticeable ABV differences—critical for style compliance, packaging decisions, and flavor balance.
How to use it
Enter the specific gravity of your wort in the ‘Original Gravity’ field; this is the hydrometer reading before fermentation. Specify the expected attenuation of the yeast you’ll use—most brew‑shop packs list a typical range like 76 – 84 %. The calculator will display the projected final gravity and alcohol content side‑by‑side. Use these numbers to tweak your recipe: if ABV is too low, increase OG by adding more fermentables; if it’s too high, lower OG or choose a strain with lower attenuation.
Worked example
To run the example, first note that the wort’s original gravity (OG) is 1.050 and we’re using a strain with an attenuation of 75 %. 1️⃣ Compute the drop in specific gravity: OG – 1 = 0.050. Multiply by the attenuation fraction (75 / 100 = 0.75) → 0.050 × 0.75 = 0.0375. 2️⃣ Subtract that drop from OG to get FG: 1.050 – 0.0375 = 1.0125. This is the predicted final gravity. 3️⃣ Translate the gravity drop into ABV: The gravity points lost (OG‑FG) equal 0.0375. Multiplying by 131.25 gives a 4.921875 % ABV. Rounding with typical 2‑digit display yields 4.92 %. Thus, for these inputs the calculator predicts a final gravity of 1.0125 and an alcohol content of ~4.92 %.
Inputs
- Original Gravity (OG): 1.05
- Yeast Attenuation (%): 75
Result
- Final Gravity (FG): 1.0125
- Alcohol by Volume (ABV % ): 4.921875
Frequently asked questions
What if my hydrometer reading is at a temperature other than 20 °C?
The tool assumes OG was corrected to roughly 20 °C (68 °F). If you measured at another temperature, adjust the reading first using the appropriate correction tables before entering it.
Can I use this calculator for specialty yeasts with very low or high attenuation?
Absolutely. Enter whatever numeric value your yeast’s specification gives; values outside 65‑95 % are accepted and will produce a FG and ABV consistent with that assumption.