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ABV from Gravity Calculator

Get ABV from OG and FG.

How it is calculated

Enter the original gravity (OG, before fermentation) and final gravity (FG, after fermentation) as specific-gravity readings, the way a hydrometer shows them, for example 1.050. The tool uses the common homebrew approximation ABV = (OG − FG) × 131.25, where OG and FG are unitless densities relative to water and the answer is a percentage with 2 decimal places. The factor 131.25 is a rule-of-thumb constant, so treat the result as an estimate. The tool only accepts OG from 1.000 to 1.200 and FG from 0.980 upward, and OG must be greater than FG. Anything else, including readings typed as 1050 or in °Plato or Brix, gives no result. Equal OG and FG are also refused.

Worked example

A beer starts at OG = 1.050 and finishes at FG = 1.010. The gravity drop is 1.050 − 1.010 = 0.040. Multiplying by 131.25 gives 0.040 × 131.25 = 5.25, so the tool prints 5.25% alcohol by volume. A typical session beer or standard-strength ale falls in this area, and a drop of only 0.010 would give about 1.31 %.

Try this example →

Common mistakes

  • Typing the reading as 1050 or 50. The tool expects the decimal form 1.050; a four-digit value is outside the accepted range and returns nothing.
  • Swapping OG and FG. The original reading is the higher one; if FG is larger, the tool refuses to calculate.
  • Entering a Brix or Plato reading. Those are different scales and must be converted to specific gravity first, or the ABV will be wrong.
  • Reading the hydrometer at the wrong temperature. Most are calibrated for about 20 °C, and a warm sample reads lower and understates the result unless you correct it.
  • Taking FG too early. If fermentation is still running, FG is still falling, so the ABV shown will be lower than the finished beer; check the reading stays the same over a few days.

About this calculator

Homebrewers measure the sugar content of their wort before and after fermentation with a hydrometer, recording the original gravity (OG) — the density before yeast is pitched — and the final gravity (FG) after fermentation finishes. Because yeast converts sugar into alcohol and carbon dioxide, the drop in gravity is a direct proxy for how much sugar was fermented, and a simple formula, ABV ≈ (OG − FG) × 131.25, converts that drop into an approximate alcohol-by-volume percentage.

This calculation is a staple of home beer and wine making because commercial alcohol meters are expensive and destructive, while a hydrometer reading takes seconds and costs almost nothing. Brewers use it to confirm fermentation has actually finished (gravity should stabilize over consecutive days), to hit a target ABV for a recipe, and to compare batches made with different malts, sugars or yeast strains. The formula is an approximation rather than an exact chemical measurement — more precise methods account for the nonlinear relationship between gravity and alcohol at higher ABVs — but it is accurate enough for virtually all homebrew purposes.

Read the hydrometer at the start and at the end of fermentation and the drop between the two gives your batch's estimated ABV.

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