Measuring original and final gravity
With the original gravity before fermentation and the final gravity afterwards you know how much alcohol your beer has and whether fermentation is finished.
What you need
- A hydrometer with a trial jar, or a refractometer
- A sanitised siphon or pipette
- A thermometer
Step by step
Step 1Take a sample
Use a sanitised siphon to take a sample of the wort or beer. Don’t pour the sample back into the beer afterwards.
Step 2Bring it to the right temperature
Most hydrometers are calibrated to 20 °C (68 °F). Cool the sample to that temperature or correct for the difference. With hot wort the error is large.
Step 3Read the hydrometer
Let the hydrometer float freely in the jar and give it a quick spin so no bubbles cling to it. Read it at eye level at the bottom of the liquid surface.
Step 4Use a refractometer
A refractometer needs only a few drops. Before fermentation, divide the °Brix reading by the wort correction factor, usually 1.04, to get the original gravity. Once fermentation has started, alcohol skews the reading and you have to convert it, for example in the final beer calculator.
Step 5Note the values
Note the original gravity on brew day and the final gravity later. If the final gravity stays the same for two to three days, fermentation is finished.
Step 6Work out the alcohol
Enter both values in the final beer calculator. It works out the alcohol content and the attenuation.
Tips and common mistakes
- Gravity in degrees Plato and specific gravity (SG) can be converted: 12 °P is about SG 1.048.
- Zero your refractometer with water regularly.
- Taste the samples after measuring. That way you learn how the flavour develops during fermentation.
Matching calculators
New to brewing? The “First beer” guide walks you through your first brew day. Your first beer