How to Read a Hydrometer: Original & Final Gravity
The hydrometer is homebrewing’s most useful tool. Learn to take accurate original and final gravity readings and calculate your beer’s alcohol content.
A hydrometer is the cheapest, most useful tool in homebrewing, and it answers two questions that matter more than almost anything else: how much sugar is in my wort, and is fermentation finished. If you learn to read one properly, you can calculate your beer's alcohol content and know exactly when it is safe to package. Here is how it works and how to use it.
What a Hydrometer Measures
A hydrometer measures specific gravity, the density of a liquid compared to water. Pure water reads 1.000. Dissolved sugar makes liquid denser, so unfermented wort might read 1.050. As yeast eats that sugar and turns it into alcohol, which is less dense than water, the reading drops. By comparing the reading before and after fermentation, you can tell how much sugar was consumed and estimate the alcohol produced.
Taking an Original Gravity Reading
Your original gravity, or OG, is the reading you take after cooling the wort but before pitching yeast. Fill a test jar with a sample, sanitize the hydrometer, and gently lower it in with a spin to knock off clinging bubbles. Let it settle, then read the number at the surface of the liquid.
Read at eye level. The liquid curves up against the glass in a shape called the meniscus; read the flat part of the surface, not the curved edge crawling up the stem. A typical session beer starts around 1.040 to 1.050, while bigger beers can start at 1.070 or higher.
Temperature Matters
Hydrometers are calibrated to a specific temperature, usually 60 or 68 degrees Fahrenheit, printed on the paper scale inside. A warm sample reads artificially low because warm liquid is less dense. If your sample is hot off the chiller, either let it cool or use a temperature-correction chart. For rough beginner purposes, cool the sample to near the calibration temperature and you will be close enough.
Taking a Final Gravity Reading
Your final gravity, or FG, is the reading once fermentation is complete, usually 1.008 to 1.015 for typical beers. The single best way to know fermentation is done is not the calendar and not the airlock; it is two identical gravity readings taken two or three days apart. If the number has not moved, the yeast is finished and the beer is stable enough to package.
Never rely on airlock bubbles alone. An airlock can go quiet while fermentation continues, or bubble from temperature swings when fermentation is done. The hydrometer is the only honest referee.
Calculating Alcohol Content
Once you have both readings, the math is simple. Subtract final gravity from original gravity and multiply by 131.25:
ABV = (OG − FG) × 131.25
So a beer that started at 1.050 and finished at 1.010 gives (1.050 − 1.010) × 131.25, which is about 5.25 percent alcohol by volume. This formula is an approximation, but it is accurate enough for every practical purpose.
Handling Samples
Every reading costs you a little beer, because you should not pour the sample back into the fermenter; doing so risks contamination. Take small samples, and consider tasting them, a flat, warm sample still tells you a lot about how your beer is developing. Sanitize your test jar and hydrometer every time.
Storing and Caring for Your Hydrometer
A hydrometer is glass and surprisingly fragile, so store it in its original tube or a padded spot where it cannot roll off the counter. Rinse it in warm water after each use and let it air dry; dried wort residue on the stem will throw off future readings. If yours ever reads noticeably off, test it in plain water at its calibration temperature, where it should read exactly 1.000, and note any consistent offset you can subtract from later readings.
Reading Between the Numbers
Over time, gravity readings tell a story. A beer that stalls far above its expected final gravity points to a fermentation problem, tired yeast, cold temperature, or unfermentable sugars. A beer that finishes lower than expected ferments dry and crisp. Logging OG and FG for every batch turns a guessing game into a repeatable craft. It is a two-dollar tool that makes you a measurably better brewer, one reading at a time.