How to read a hydrometer
Every calculator on this site runs on two numbers — OG and FG — and both come from this $10 piece of glass. Here's how to read it right.
What a hydrometer actually measures
A hydrometer measures density relative to water. Pure water at the calibration temperature reads 1.000. Wort full of dissolved sugar reads higher (a typical OG is 1.040–1.060); finished beer reads lower as the yeast has eaten the sugar (a typical FG is 1.008–1.015). That's it — density in, density out, and the ABV calculator does the rest.
Reading it correctly
- Sanitize your thief or turkey baster, pull a sample into the test jar — enough liquid that the hydrometer floats freely without touching the bottom.
- Spin it gently to shake off CO₂ bubbles clinging to the glass. Bubbles lift the hydrometer and inflate your reading.
- Get your eyes level with the liquid surface and read at the bottom of the meniscus — the lowest point of the curved surface where liquid meets glass. Reading the top of the meniscus adds ~0.002–0.004.
- Note the temperature of the sample. Most hydrometers are calibrated at 60°F (15.6°C); wort is usually warmer, which makes it read low.
Temperature correction
Warm liquid is less dense, so a hydrometer floats lower and under-reads. The rule of thumb: add ~0.001 for every 10°F above 60°F, subtract ~0.001 for every 10°F below. A 1.050 reading taken at 80°F is really about 1.052. That two-point error is a quarter-point of ABV — worth correcting on paper, though for most homebrew purposes the rule of thumb is plenty.
Returning the sample to the fermenter risks infecting the entire batch to save a few ounces of beer. Taste the sample instead — it's genuinely useful data about how fermentation is progressing.
Common mistakes
- Reading through foam or bubbles — degas the sample by pouring it back and forth between two glasses a few times.
- Hydrometer touching the jar wall or bottom — needs to float free.
- Ignoring temperature — a hot post-boil sample can read 0.006+ low.
- Trusting a single FG reading to bottle — take readings 2–3 days apart. Stable gravity means done; falling gravity means wait. Bottling early is how bottle bombs happen (see the bottling day checklist).
Frequently asked questions
Do you read a hydrometer from the top or bottom of the meniscus?
The bottom — the lowest point of the curved surface — read at eye level. The top of the meniscus reads ~0.002–0.004 high.
How do you temperature-correct a hydrometer reading?
Add about 0.001 for every 10°F above the 60°F calibration point; subtract below it. A 1.050 reading at 80°F corrects to roughly 1.052.
Should you put the hydrometer sample back in the fermenter?
No — infection risk isn't worth a few ounces. Taste it instead.