Quick answer
- A hydrometer measures specific gravity (S.G.), which tells you how much sugar is left in your wine.
- A wine kit usually starts at 1.075 to 1.085 and finishes at 0.990 to 0.995.
- It is the only reliable way to know if fermentation is working, stuck or finished.
If you buy one piece of equipment, make it a hydrometer. It is the most valuable tool in winemaking. Watching the bubbles in the airlock does not tell you much. Some kits bubble like crazy, and others hardly bubble at all while fermenting perfectly well. The hydrometer tells you what is really happening.
What a Hydrometer Measures
A hydrometer is a sealed glass tube with a weighted bulb at the bottom, so it floats upright. It measures specific gravity (S.G.), the density of a liquid compared with water.
- Water has an S.G. of 1.000.
- Sugar makes juice denser, so the hydrometer floats higher. Wine kit juice usually reads 1.075 to 1.085.
- Yeast turns sugar into alcohol and carbon dioxide. Alcohol is lighter than water, so finished wine reads below 1.000, usually 0.990 to 0.995.
Your hydrometer may have more than one scale. The specific gravity scale is the one with “99” near the top. A triple scale hydrometer also shows potential alcohol and Brix.
How to Take a Reading
- Sanitize the hydrometer, the test jar and your wine thief.
- Draw a sample with a wine thief and fill a hydrometer test jar. Some wine thieves let you float the hydrometer right inside them.
- Lower the hydrometer in gently and give it a spin to knock off any bubbles clinging to it.
- Read it at eye level, where the surface of the wine crosses the scale.
- Write the reading down, with the date.
- Return the sample to the wine only if everything was sanitized. Otherwise discard it.
The Readings That Matter for a Wine Kit
| Stage | Typical S.G. | What to do |
|---|---|---|
| Day 1, before adding yeast | 1.075 to 1.085 | Record it. You need it to work out alcohol content. |
| Active fermentation | Falling every day | Leave it alone. |
| First racking | 1.020 or lower | Rack into the carboy. |
| Fermentation finished | 0.990 to 0.995, steady for 2 to 3 days | Degas and stabilize. |
| Before bottling | 0.990 to 0.995 | Clear, then bottle. |
When the Reading Won’t Drop
If the S.G. stops falling well above 0.995, the fermentation may be stuck. The first question any wine shop will ask is “What is your hydrometer reading?” Without it, nobody can tell whether a wine is stuck or finished. See Is My Wine Kit Fermenting? How to Fix a Stuck Fermentation.
Turn Your Readings Into Alcohol Content
With a starting and a finishing reading, you can work out the alcohol content of your wine. See How to Calculate the Alcohol Content of Homemade Wine.
Frequently Asked Questions
What should my hydrometer read at the start of a wine kit?
Most wine kits start between 1.075 and 1.085 after topping up to 23 L.
What should a finished wine read?
Usually between 0.990 and 0.995. When the reading stays the same for 2 or 3 days, fermentation is finished.
Does temperature affect the reading?
Yes, slightly. Hydrometers are calibrated for a set temperature, usually printed on the paper scale. For a wine kit at room temperature the difference is small.
Can I tell if fermentation is done by the airlock?
No. Some wines ferment with very few bubbles. Only the hydrometer tells you for sure.
Need a hand with your batch? Call us toll-free at 1-877-668-6160 (Monday to Friday, 8:00 a.m. to 3:00 p.m. Eastern) or send us a message.
This guide is adapted from the ABC Crafted Series booklet Wine From Kits: What the Instructions Don’t Always Tell You. Always follow the instructions that come with your kit.