How to Calculate ABV from Original and Final Gravity

To estimate beer alcohol by volume from gravity readings, subtract final gravity from original gravity and multiply the result by 131.25. For example, a beer that starts at 1.050 and finishes at 1.012 has an estimated ABV of (1.050 − 1.012) × 131.25 = 4.99%, usually reported as about 5.0%.

This familiar homebrewing formula is an approximation, not a laboratory alcohol assay. It works only when you use the correct type of readings and collect representative samples. The arithmetic is easy; most errors come from the measurements, missing information, or entering numbers in the wrong format. This guide explains the calculation and the checks that make the result useful.

What original and final gravity mean

Specific gravity compares a liquid’s density with water. Dissolved extract raises the density of wort, while fermentation changes the mixture as yeast consumes fermentable sugars and produces alcohol and carbon dioxide. Beer also contains unfermented extract, so the final reading reflects a mixture rather than a direct alcohol concentration.

Original gravity, or OG, is measured before fermentation begins. For a typical extract batch, take it after adding and thoroughly mixing the final top-up water. Final gravity, or FG, is measured after fermentation reaches its endpoint. A reading collected in the middle of active fermentation is a progress reading, not necessarily the final number.

Keep both readings in specific-gravity format. Write 1.050 rather than 50 if the formula expects SG. Degrees Plato, Brix and gravity points are related ways to describe extract, but they are not interchangeable inputs. A calculator may accept several formats; confirm its selected units before entering your data.

Calculate the estimate step by step

1. Find the recorded original gravity for the batch.
2. Measure final gravity with a calibrated brewing hydrometer.
3. Apply appropriate temperature or instrument corrections.
4. Subtract FG from OG.
5. Multiply the difference by 131.25.
6. Round the result to a sensible level of precision.

Suppose OG is 1.055 and FG is 1.012. Subtraction gives 0.043. Multiplying 0.043 by 131.25 gives 5.64375. Reporting 5.6% ABV is more reasonable than printing every decimal from the calculator. Those extra digits imply a level of measurement certainty that a typical homebrewing setup does not provide.

If using gravity points, 1.055 becomes 55 points and 1.012 becomes 12 points. The difference is 43 points. Because a point equals 0.001 SG, you multiply 43 by 0.13125, not 131.25. Mixing the SG and points versions is a common source of results that are off by a factor of a thousand.

Compare three worked examples

Notice that original gravity alone does not determine the result. Two beers starting at the same OG can finish at different gravities because their recipes, mash conditions and yeast behavior differ. You need the observed final reading, rather than simply copying the expected FG from the recipe.

The difference also does not tell you whether the beer tastes dry, balanced or pleasant. Alcohol, residual extract, carbonation, acidity and ingredients all affect perception. Use the number to describe the batch, then evaluate the beer separately. A target percentage cannot substitute for a well-designed recipe and healthy fermentation.

Get reliable hydrometer readings

Check the instrument in distilled water at its stated calibration temperature. Use a test cylinder deep enough for it to float freely. Cool hot wort before measuring, remove bubbles from fermented samples, and read the appropriate meniscus at eye level. Our hydrometer instructions explain the complete procedure.

Record the sample temperature. If it differs from the calibration temperature, use a matching correction method. Do not assume that every instrument is calibrated at 60°F. A small systematic error in both readings can be hard to spot, particularly if the samples are measured at very different temperatures.

Mix top-up water and concentrated wort before collecting OG. A low reading from a watery surface layer can create a falsely low alcohol estimate. Once fermentation is underway, avoid unnecessary stirring or splashing of the entire batch. Collect samples consistently and discard them after testing to reduce contamination risk.

Handle refractometer readings differently

A refractometer measures refractive index and commonly reports Brix. It can be convenient before fermentation, but alcohol changes the relationship between refractive index and extract. An uncorrected post-fermentation refractometer reading is not a hydrometer FG and should not be dropped into this equation.

Use an appropriate brewing correction method that includes the original reading and your instrument’s wort correction factor. Compare corrected results with a hydrometer when possible. If you are uncertain about the device or calculator inputs, use a calibrated hydrometer for final gravity. See our instrument comparison before choosing a workflow.

What if original gravity is missing?

You cannot obtain a reliable batch-specific ABV from a single final hydrometer reading. A recipe’s predicted OG may provide a rough estimate, but brewing efficiency, final volume and ingredient changes affect the actual number. Clearly label a result based on predicted OG as an estimate based on assumptions.

For future batches, record OG immediately after the wort is prepared. Keep the recipe, final volume and measurement conditions together in the same notebook. If precise alcohol concentration is required for a commercial or regulatory purpose, use an appropriate validated analysis rather than reconstructing missing information from taste or a recipe calculator.

Know the formula’s limits

The 131.25 multiplier is a convenient approximation used in homebrewing. Other formulas account for the relationship differently and may give a somewhat different answer, particularly with stronger beers. Choosing a more complicated equation does not automatically improve an estimate if the input readings are poor.

Additional fermentable ingredients added after OG complicate the calculation. Fruit, sugar or dilution can change the batch after the starting measurement. Record the additions and use a recipe-aware calculation method instead of pretending that the first OG still represents the complete fermentation. Priming sugar can also add a small amount of alcohol during bottle conditioning.

Use ABV alongside fermentation records

Repeated stable gravity readings are part of deciding when fermentation has finished. A calculated ABV close to the recipe target does not prove the beer is ready for bottles. Unexpectedly high FG may signal incomplete fermentation or simply a less fermentable recipe; investigate it before adding priming sugar.

Keep OG, FG, estimated ABV, yeast strain, fermentation temperature and tasting notes together. Those records help you compare later batches and identify changes worth repeating. The aim is a dependable description of the beer, not an impressive number with unjustified precision.

Further reading: American Homebrewers Association measurement guidance. The worked examples above are illustrative calculations, not measurements of tested batches.