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Attenuation Calculator

Find how much sugar the yeast fermented from your gravities.

Result

78.57%
63.64%

How it is calculated

The tool first converts both gravities to points, the part above 1.000 times 1000: OG points = (OG − 1) × 1000, and the same for FG. Apparent attenuation is the share of those points that disappeared: AA = (OG points − FG points) ÷ OG points × 100. Real attenuation is estimated as RA = AA × 0.81, a fixed rule of thumb that allows for the alcohol which makes a hydrometer read too low. Both are shown as percentages with two decimals. Enter gravity as a decimal such as 1.056. OG must be above 1.000, FG must be between 0.980 and 1.200, and FG cannot be higher than OG; anything else, including gravities typed in points, shows no result. The tool does not correct for temperature, so adjust hydrometer readings to the calibration temperature first.

Worked example

A beer starts at an original gravity of 1.056 and finishes at 1.012. In points, OG is 56 and FG is 12, so the yeast removed 56 − 12 = 44 points. Apparent attenuation: 44 ÷ 56 × 100 = 78.57%. Real attenuation: 78.57 × 0.81 = 63.64%. A yeast with a published range of 73 to 77 % apparent attenuation would have finished slightly above its range here.

Try this example →

Common mistakes

  • Entering gravity in points (56 and 12) or without the decimal (1056 and 1012). The fields take decimal gravity such as 1.056, and anything else shows no result.
  • Comparing the real attenuation figure with a yeast's published number. Yeast labels normally quote apparent attenuation, so compare against the first result.
  • Swapping OG and FG. A final gravity above the original gives no result, so check which reading is which.
  • Reading the sample before it was degassed or brought to the right temperature, which skews FG and with it the attenuation.
  • Treating 0.81 as exact. It is an approximation, so real attenuation is an estimate that varies a little between beers.

About this calculator

Attenuation measures how much of the sugar in wort a yeast strain actually fermented into alcohol, calculated from the original gravity (OG) taken before fermentation and the final gravity (FG) taken after. Apparent attenuation — the simple version, (OG − FG) / OG × 100, worked out on gravity points (the digits after the decimal point, so 1.050 counts as 50) — is what most homebrewers check first, but it overstates the real number because dissolved alcohol reads lighter on a hydrometer than the sugar it replaced.

Real attenuation corrects for that alcohol effect; this calculator uses the common rule of thumb that real attenuation is about 81% of the apparent figure, giving a more realistic picture of how much fermentable sugar was actually consumed. Brewers compare their real attenuation against a yeast strain’s published range — commonly around 73–77% for many ale strains — to judge whether fermentation finished healthily or stalled early, and to fine-tune mash temperature or pitch rate on the next batch.

This calculator takes your OG and FG readings as specific gravity (for example 1.050 and 1.010) and returns both the apparent and real attenuation percentages. It shows no result if the FG is higher than the OG or the readings are outside a plausible range.

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