Beer Gravity and ABV: The Complete Guide

Two hydrometer readings - one before fermentation, one after - describe almost everything about a beer: its strength, how completely it fermented and roughly how many calories it carries. This guide covers the gravity points behind the numbers, the ABV formula, the difference between apparent and real attenuation, and the temperature correction that keeps a warm sample honest. The ABV calculator and the attenuation calculator do the arithmetic as you read.

Gravity and gravity points

Specific gravity compares a liquid's density to water's: 1.050 means the wort is 5% denser than water. Brewers drop the leading 1 and speak in gravity points, the last three digits. That turns subtraction into ordinary arithmetic - 1.050 is 50 points, 1.010 is 10 points.

The ABV formula

Alcohol by volume comes straight from the gravity drop:

Worked example: an ale that starts at 1.050 and finishes at 1.010 dropped 40 points. 40 × 131.25 = 5,250, and dropping the last three digits gives 5.25% ABV. Alcohol by weight is a little lower - about 4.13%, since ABW = ABV × 0.794 ÷ FG.

The same 40-point drop also gives the apparent attenuation: the points fermented divided by the original points, 40 ÷ 50 = 80%. Ale yeasts commonly spec 65-80%, so this beer fermented out fully.

Apparent versus real attenuation

Apparent attenuation compares the readings as measured, but alcohol is lighter than water, so it flatters the number: the drop looks bigger than the sugar actually consumed. Real attenuation corrects for that, at about 81% of the apparent figure plus a small offset - the 80% above works out near 65% real attenuation. When comparing a ferment against a yeast strain's spec sheet, match like with like: strain data is quoted as apparent attenuation.

Correct the reading for temperature

Hydrometers are calibrated at roughly 59-60 °F (15 °C). Read a sample much warmer or colder and the number is off - a hydrometer is really a density float, and warmth expands both the liquid and the glass. A sample at 100 °F reads about 6 points low: a 1.044 reading is closer to 1.050. Two errors compound in ABV, because both readings shift the same way. Always cool a sample to near calibration temperature, or run the number through the hydrometer temperature correction tool.

Refractometers need different handling: they read Brix, and once alcohol is present they overstate the remaining sugar. Use a hydrometer for the final gravity, or correct the refractometer reading first with the Brix to SG calculator.

Where the calories come from

Beer energy is alcohol plus residual carbohydrate. The ASBC factors are 6.9 kcal per gram of alcohol and 3.87 kcal per gram of carbohydrate, with the carbohydrate part read from the real extract left after fermentation. A stronger beer carries more calories mostly because of its alcohol, which is why a dry 5% ale can read leaner than a sweet 4% one. The beer calorie calculator runs the full Balling balance for a serving size.

Priming: gravity after the finish

Bottle conditioning adds a final small gravity charge. Carbonation depends on the CO2 the beer already holds - which is set by the warmest temperature it reached after fermentation - plus the sugar you add to cover the rest. Corn sugar delivers about 4.49 g per liter per volume of CO2 and table sugar about 3.85, so sucrose needs roughly 14% less by weight. The priming sugar calculator subtracts the residual CO2 for your temperature before sizing the dose; the ABV calculator confirms the finished strength.

Common gravity mistakes

• Reading warm. A warm sample reads low and quietly corrupts ABV. Cool it or correct it.
• Trusting a refractometer after fermentation. Alcohol skews the reading; use a hydrometer or correct it.
• Entering an FG at or above the OG. That is a measurement error, not a strong beer - check the sample first.
• Comparing apparent and real attenuation across sources. They differ by around a fifth; keep the basis consistent.
• Assuming the recipe's final gravity is your final gravity. Mash temperature, yeast health and grain bill all move it; trust two stable readings a day apart.

Safety notes

Gravity and alcohol figures are planning estimates, not measurements of a specific bottle. Sanitation, pitching enough healthy yeast and fermenting in the yeast's temperature range matter more to the final beer than any rounding in the formulas. When you bottle, weigh the priming sugar and use pressure-rated bottles. This guide is for education, not food-safety advice.

Frequently asked questions