Specific Gravity for Homebrewing

Specific gravity (SG) compares the density of your beer, mead, cider, or wine with water at 1.000. Sugar raises it, and as yeast converts sugar to alcohol it falls. Measuring it before and after fermentation gives your original gravity (OG) and final gravity (FG), which show how far fermentation has gone and how strong the result is.

Last updated October 15, 2026

Original gravity and final gravity

• Original gravity (OG) is measured after the sugars are dissolved and before yeast goes in. A typical beer is around 1.040–1.060; strong meads and wines can start well above 1.100.
• Final gravity (FG) is measured when fermentation has finished. Confirm it’s done by checking that gravity is unchanged across several readings over a few days.

Both numbers feed the ABV calculator. The difference between them tells you how much sugar was consumed.

Hydrometer readings

A hydrometer is a weighted glass float marked with a gravity scale. The denser the liquid, the higher it floats.

1. Fill a tall, narrow container with enough sample for the hydrometer to float freely.
2. Spin the hydrometer gently to dislodge bubbles, and make sure it doesn’t touch the sides.
3. Read the scale at eye level, at the surface of the liquid.
4. Record the sample temperature and compare it with your hydrometer’s calibration temperature.

Refractometer readings

A refractometer measures how much light bends through a few drops of liquid, reported in degrees Brix. Before fermentation, Brix converts to gravity after dividing by a wort correction factor (often about 1.04, but best measured for your own instrument). After fermentation, alcohol also bends light, so a raw Brix reading cannot be converted directly to FG. Use a correction that takes both original and final Brix into account, as the refractometer correction calculator does. To convert an unfermented reading, use the Brix to gravity converter. Calibrate with distilled water and let the sample reach room temperature.

Hydrometer vs. refractometer

Temperature and hydrometer readings

Most hydrometers are calibrated at 60 °F or 68 °F; check yours. A warm sample is less dense, so the hydrometer sinks and reads low. The table shows the corrected value for a sample that reads 1.050 on a hydrometer calibrated at 60 °F. To correct your own readings, use the hydrometer temperature correction calculator.

Common reading errors

• Dissolved CO₂ and bubbles cling to the hydrometer and read high. Degas the sample.
• Unmixed must or wort. Sugar can stratify, especially with honey. Mix well before OG.
• Sample temperature far from the calibration temperature.
• A hydrometer touching the container or a container that’s too narrow.
• Reading the top of the meniscus or reading from above or below eye level.
• Skipping the refractometer correction for fermented samples.
• Trusting one reading. Confirm final gravity with repeated readings over days.

Worked example

A beer is measured at OG 1.056 and, after fermentation, FG 1.012.

• ABV = (1.056 − 1.012) × 131.25 = 5.8%
• Apparent attenuation = (1.056 − 1.012) ÷ (1.056 − 1) = 79%

Try your own numbers in the ABV calculator. If you’re tracking a mead, see how to track mead fermentation.

References

• American Homebrewers Association — How to take an accurate hydrometer reading
• Homebrewers Association forum — Improved refractometer correction calculator
• John Palmer — How to Brew