What a hydrometer reading is actually worth.
Three calculators. No account, no email box. They run inside the page, they print the arithmetic they used underneath themselves, and they say plainly where that arithmetic gets thin.
A hydrometer floats lower the less sugar is dissolved under it. Fill the trial jar, spin the stem once so the bubbles let go of the glass, and read the flat surface of the liquid — not the curved lip climbing up the stem. Then write down the temperature next to the number, because you will need both of them.
Two readings, taken a few weeks apart, are the whole measurement. Everything below is what you do with them.
Three tools, and what each one quietly assumes.
One converts readings into alcohol. One works backwards from the strength you want. One tells you the range a starting gravity can land in.
The sugar figure ignores volume displacement. Dissolved sugar takes up room: 1KG stirred into 10L of water does not give you ten litres of sweet water, it gives you a little over ten. The calculator does not model that, so on high-gravity musts — anything heading past 1.100SG — it overstates the sugar you need by a few per cent. Hold back the last handful, take a reading, and add the rest only if the number asks for it.
Where these numbers stop being true.
All three tools run on the same homebrew approximation: gravity points times 131.25. That is a straight line drawn through a curve, and it fits best in the middle of the range. On a 5%ABV cider the error is small enough to ignore. On a 14%ABV mead it can be around 0.5%ABV out, and it errs low rather than high, which is the polite direction to be wrong in. Treat every figure on this page as an estimate to half a per cent, not a lab result.
Then there is temperature. A hydrometer is calibrated to be exactly right at one temperature — usually 20°C, printed on the stem — and a warm sample is less dense than a cold one, so a reading taken straight off an active ferment comes out low. Correct it before you type it in.
| Sample at | Correction | So 1.050 is really |
|---|---|---|
| 10°C | −0.0015SG | 1.0485SG |
| 15°C | −0.0009SG | 1.0491SG |
| 20°C | 0.0000SG | 1.0500SG |
| 25°C | +0.0011SG | 1.0511SG |
| 30°C | +0.0025SG | 1.0525SG |
| 35°C | +0.0040SG | 1.0540SG |
FOR A HYDROMETER CALIBRATED AT 20°C · CHECK THE STEM, SOME ARE 15°C
One more thing worth knowing: once there is alcohol in the sample, density stops being a clean measure of sugar. Alcohol is lighter than water, which is exactly why a dry mead finishes below 1.000SG. The formula already absorbs that, but it means the final gravity is an inference rather than a measurement, and it is the least trustworthy number here.
None of this asks whether the yeast can get there. A strain has an alcohol ceiling and a temperature range, and a 16%ABV target handed to a packet rated for 12%ABV gives you a stuck, sweet mess rather than a strong drink. Check the strain in the yeast guide first, then come back and do the arithmetic.