Australian legal note: This article is for fermentation monitoring and for licensed distillation operations. In Australia, alcohol distillation requires the appropriate excise licence. TAB’s spirit calculators are designed for legally obtained spirits.

A gravity reading tells you how much dissolved sugar is in a fermentation wash. It is one of the best ways to check whether fermentation has started, whether it is progressing properly and whether it has finished.

For spirit-making ingredients, gravity readings are useful before fermentation and after fermentation. They help you understand how much sugar was available to ferment and how much alcohol may have been produced.

What does a hydrometer measure?

A hydrometer measures specific gravity, usually shown as SG.

Water has a specific gravity of 1.000. A sugar wash has a higher reading because dissolved sugar makes it denser. As yeast converts sugar into alcohol and carbon dioxide, the gravity drops.

A typical reading may look like this:

• Original Gravity: 1.070
• Final Gravity: 0.998

The difference between these readings can be used to estimate alcohol content.

Spirits Calculators

Original gravity vs final gravity

Original Gravity

Original Gravity, or OG, is measured before yeast is added. It tells you how much fermentable sugar is in the wash.

A higher OG usually means higher potential alcohol, provided the yeast can ferment all available sugar.

Final Gravity

Final Gravity, or FG, is measured once fermentation has finished. A stable final gravity helps confirm that the yeast has finished converting sugar.

For many dry washes, the final gravity will be close to 1.000 or slightly below.

How to take a wash gravity reading

1. Clean and sanitise your sample thief, trial jar and hydrometer.
2. Take a sample from the fermenter without disturbing excess sediment.
3. Fill the trial jar and gently lower in the hydrometer.
4. Spin the hydrometer to remove bubbles.
5. Read the scale at eye level, at the bottom of the liquid curve.
6. Record the reading and sample temperature.

Most brewing hydrometers are calibrated at 20°C. Hot or cold samples can affect accuracy.

When should you take readings?

For most washes, take at least two readings:

• One before fermentation begins
• One near the expected end of fermentation

To confirm fermentation has finished, take another reading two or three days later. If the reading remains unchanged, fermentation is usually complete.

Do not rely only on airlock activity. Temperature changes, leaks and pressure differences can all affect airlock movement.

What can cause an unexpected gravity reading?

A reading may be inaccurate if:

• The wash has not been mixed properly
• The sample is too warm or too cold
• Carbon dioxide bubbles are attached to the hydrometer
• The hydrometer is not calibrated correctly
• The sample contains excess sediment or foam

If the reading seems unusual, retest with a fresh sample.

Why wash gravity matters

Gravity readings help you make informed decisions. They can reveal a slow fermentation, an unexpectedly strong wash, incomplete fermentation or a recipe adjustment issue.

Use your starting and finishing readings in the [Link: Spirits Calculators] to estimate wash alcohol and plan the dilution, blending and bottling of legally obtained spirits.

Frequently asked questions

What gravity should a sugar wash finish at?

Many fully fermented sugar washes finish around 1.000 or slightly below, but the exact number depends on ingredients, yeast and fermentation conditions.

Can I use a refractometer for wash readings?

A refractometer works well before fermentation. Once alcohol is present, it needs a correction calculation. A hydrometer is usually simpler for final readings.

Does a lower final gravity mean more alcohol?

Usually, yes. A larger drop between original gravity and final gravity generally means more sugar was fermented into alcohol.