Original gravity (OG)

Original gravity, usually written OG, is a reading of how much sugar and other dissolved material is in your wort before yeast gets to work. Plain water reads 1.000 on a hydrometer, and wort reads higher because sugar makes liquid denser. A bigger, stronger beer starts with a higher OG than a light session beer, so the number is one of the first ways to gauge what you are brewing.

You take the OG reading right after cooling the wort and before pitching yeast, using a hydrometer in a sample tube or a refractometer on a few drops. That number becomes the baseline for two important calculations later: how much alcohol the beer will end up with, and whether fermentation actually finished. Recipes usually list a target OG range, and hitting it tells you your mash or extract additions went roughly to plan.

A common mistake is taking the reading from a hot or uneven sample, since temperature changes liquid density and skews the result, so let the sample cool close to the instrument's calibration temperature first. Another mix-up is confusing OG with final gravity: OG is the starting point, final gravity is where fermentation ends up, and the gap between the two is what actually tells you about alcohol and attenuation.