Hydrometer ABV calculator

How to use it

1. Take the original gravity. Before adding the yeast, once everything is dissolved and mixed: fill a trial jar, spin the hydrometer gently to shed bubbles, and read it at eye level.
2. Take the final gravity. When fermentation has finished - the reading stays the same for two or three days in a row.
3. Enter both. Choose the scale your hydrometer reads in. If a sample was warm or cold, tick the temperature correction and enter its temperature and the hydrometer's calibration temperature (printed on it).
4. Read the strength. Use it to plan your runs - the stripping and spirit runs guide has a planner that starts from it.

How it works

Sugar makes water denser; alcohol makes it lighter. As yeast turns one into the other, the gravity falls, and the size of the fall tells you how much alcohol was made.

The familiar formula multiplies the drop in gravity by 131.25. It is good for beer-strength washes but reads low as washes get stronger, so the main figure uses a formula that also allows for the strength of the starting wash and for the alcohol's own effect on the final reading. Both are shown.

Temperature. Hydrometers are calibrated at 20 °C (68 °F) or 60 °F. A warm sample is less dense and reads low; the calculator corrects each reading using the density of water at the two temperatures and the expansion of the glass.

Attenuation is how much of the original gravity has gone. Sugar washes usually finish below 1.000 - over 100% - because the alcohol is lighter than water.

A worked example

A sugar wash starts at 1.070 and finishes at 0.995. The calculator gives 10.1% ABV; the simple formula gives 9.8%.

If the first sample had been taken warm - 1.070 at 30 °C on a hydrometer calibrated at 20 °C - its true gravity would be 1.072, and the wash about 10.5%.

Questions

How do I know when fermentation has finished?

When the gravity stays the same for two or three days in a row. Sugar washes usually finish around 0.990–0.996; molasses and grain washes often stop at 1.010–1.020, because they contain sugars and solids yeast can't use.

Why is my final gravity below 1.000?

Because alcohol is lighter than water. Once the sugar has gone, a wash with 10% alcohol is lighter than plain water, so the hydrometer reads below 1.000.

Can I use a refractometer instead?

For the original gravity, yes. For the final gravity, not directly: alcohol bends light too, so a refractometer reads a fermented wash wrongly without a special correction.

What if I forgot to take the original gravity?

You can estimate it from the recipe with the sugar wash calculator. The result is an estimate, not a measurement.

Does temperature really matter?

For a wash a few degrees from the calibration temperature, only in the third decimal place; for a hot sample straight from a mash, yes - let it cool or correct it.