A hydrometer measures specific gravity — how dense your wort or beer is compared to water. Get the technique right and every OG, FG, and ABV calculation that follows will be trustworthy. Get it wrong and you're guessing. Here's how to take a clean reading every time.
Open BrewLog →Use the tall plastic or glass test jar that came with your hydrometer, not the fermenter itself — a hydrometer needs to float freely without touching the sides. Draw your sample with a sanitized wine thief or turkey baster and fill the jar most of the way. Don't pour it back into your beer if it touched anything unsanitized.
Lower the hydrometer in gently and give it a light spin as you let go. The spin knocks off CO₂ and air bubbles that cling to the bulb; even a few bubbles add buoyancy and make the reading falsely low. Wait until it settles and stops bobbing.
Bring your eye level with the surface of the liquid. The liquid curves up where it meets the glass stem — that curve is the meniscus. Read the number where the flat surface of the liquid crosses the scale, not at the top of the curve. Reading from above or below introduces a parallax error of several gravity points.
Hydrometers are calibrated to one temperature, usually 20 °C (68 °F). Warmer liquid is less dense and reads artificially low; colder liquid reads high. A hot wort sample can read 0.003–0.006 too low. A handy approximation near the calibration point:
So a sample reading 1.048 at 30 °C is really about 1.050. Better still, cool the sample to calibration temperature before reading, or use an online correction calculator with your hydrometer's exact calibration figure.
Don't read straight from a swirling fermenter, don't ignore clinging bubbles, and never trust a single reading to declare fermentation finished — take several over a few days. Calibrate your hydrometer in distilled water at 20 °C; it should read exactly 1.000, and if it's off by a consistent amount, apply that offset to every reading.
Once you have an accurate number, log it. BrewLog stores each dated reading per batch and instantly computes ABV and attenuation, so your careful measurement turns straight into useful data. Free, offline, and private to your browser.
Log a reading in BrewLog →