Reading Original and Final Gravity to Calculate ABV

Original gravity reflects available sugar before fermentation

Original gravity, often abbreviated as OG, is the density of your wort measured just before you pitch the yeast. It reflects the total amount of dissolved sugar the yeast will have available to ferment. A higher original gravity means more sugar, which generally leads to a stronger beer. You measure it with a hydrometer or a refractometer after the wort has cooled and been thoroughly mixed so the sample is representative.

Final gravity is measured after fermentation completes

Final gravity, or FG, is the density measured after fermentation has finished. By this point the yeast has converted most of the fermentable sugar into alcohol and carbon dioxide, so the liquid is much less dense than it was at the start. Final gravity is never as low as plain water, because beer always retains some unfermentable sugars and other compounds that give it body and mouthfeel.

Confirming fermentation is truly complete

To know that fermentation is truly complete, take a gravity reading and then take another two or three days later. If the number has not changed, the yeast has finished its work and the beer has reached its stable final gravity. Relying on this stability, rather than on how long the airlock has been quiet, prevents you from bottling too early and risking dangerous over-carbonation.

Calculating alcohol by volume from the two numbers

Calculating alcohol by volume from these two numbers is straightforward. Subtract the final gravity from the original gravity and multiply the difference by a standard conversion factor of about one hundred thirty-one point three. The result is your approximate alcohol by volume as a percentage. This is an estimate rather than a laboratory-grade measurement, but it is accurate enough for homebrewing and lets you compare batches.

The gap between OG and FG reveals attenuation

The gap between your original and final gravity also tells you about attenuation, which is the percentage of sugar the yeast fermented. High attenuation gives a drier, crisper beer, while lower attenuation leaves more residual sweetness and body. By recording OG and FG for every batch in a brewing log, you build a personal database that helps you predict and control the character of future beers.

Frequently asked questions

What is original gravity in brewing? The density of your wort measured just before you pitch the yeast, reflecting the total amount of dissolved sugar the yeast will have available to ferment.

Why is final gravity never as low as water? Because beer always retains some unfermentable sugars and other compounds that give it body and mouthfeel, even after the yeast has converted most of the fermentable sugar.

What is the formula for calculating ABV from gravity readings? Subtract the final gravity from the original gravity and multiply the difference by a standard conversion factor of about one hundred thirty-one point three.

What does attenuation mean in brewing? The percentage of sugar the yeast fermented, shown by the gap between original and final gravity; high attenuation gives a drier, crisper beer while lower attenuation leaves more residual sweetness.