Hydrometer and refractometer
A hydrometer tells you how much sugar is left and, with two readings, how much alcohol there is. How to read it well, what to do about temperature and what a refractometer is for.
7 min read Getting good Reviewed October 26, 2026 20 facts, each with its source
What a hydrometer measures
A hydrometer measures the density of a liquid1. It's a weighted glass cylinder with a scale on its stem that sinks more or less depending on how dense the liquid is2. Pure water reads 1.0003. Dissolved sugar makes it denser, and the hydrometer floats higher.
That's why it's how you follow a mead or a cider. Yeast eats the sugar and turns it into alcohol, which is lighter than water. The gravity drops day by day, and when it's done it sits below 1.0004. The difference between the first reading and the last tells you how much sugar has become alcohol.
Scales and numbers
A winemaking hydrometer reads from 0.990 to 1.170. Some also carry a potential alcohol scale and a Brix scale5. The one that matters is specific gravity.
| Gravity | What it means |
|---|---|
| 1.000 | Pure water |
| 1.080 or higher | Where a wine or mead must starts6 |
| 1.040–1.080 | Where Fervida's cider starts, a house rule |
| 1.000 or lower | Dry: no sugar left. It can go as low as 0.9907 |
How to take a reading
- Draw a sample. With a clean tool, move a little liquid into a tall, narrow test jar where the hydrometer floats without touching the bottom8. Better not to drop the hydrometer into the carboy.
- Get the gas out. Bubbles push the hydrometer up. If the sample holds a lot of gas, remove it first9: stir it, or give the hydrometer a spin.
- Read at eye level. Take the reading where the liquid is flat, not at the edge that climbs up the stem10. If your hydrometer says it's read at the top of the meniscus, go by its scale11.
- Write down the temperature of the sample, not the room.
- Throw the sample away. Don't pour it back into the carboy: that's a house rule, so nothing from outside gets in.
Temperature
Density changes with temperature, so it's given at a reference temperature, which in winemaking is 20 °C (68 °F)12. Every hydrometer is graduated at one temperature, and says which on its scale or on a slip of paper inside the bulb11. Check it before you start.
To measure well, stir the sample with the thermometer in it and wait a minute before reading it13. If it isn't at the reference temperature, the reading is corrected with tables14: below 20 °C (68 °F) the correction is subtracted, and above it, added15. A warm sample reads lower than it really is.
Fervida's alcohol calculator and the app take the reading exactly as you type it. Correct it first or, easier, let the sample reach your hydrometer's temperature. That's a house rule.
The refractometer
A refractometer measures how much light bends as it passes through a few drops of liquid, and gives it in degrees Brix. It needs very little sample, which is why growers use it in the field for the sugar in grapes or apples. Before use, zero it with distilled water as its maker says. As a rough guide, each degree Brix is about 0.004 of gravity16.
It works for the starting reading, while there's no alcohol yet. After that, it doesn't: alcohol skews the reading, and it isn't recommended for a fermenting liquid17. Fermentation is followed with a hydrometer18. There are formulas that try to correct a refractometer, but Fervida doesn't use them: they're an estimate built on another estimate.
From gravity to alcohol
With two readings, the first one and today's, you can work out the approximate alcohol: (starting gravity − today's gravity) × 131.25. It's the brewers' and winemakers' formula, and no official guide gives it, so Fervida always presents it as an estimate.
| Start | End | Approximate alcohol |
|---|---|---|
| 1.100 (mead) | 1.000 | 13.1% |
| 1.050 (cider) | 1.000 | 6.6% |
The alcohol calculator does the math and tells you whether your starting gravity is in the usual range: 1.080–1.120 for mead and 1.040–1.080 for cider. In the app, every reading goes into the batch's log. It calls a batch finished when the gravity reads the same 3 times across 7 days, and only lets you bottle it still at 1.000 or lower. Both are house rules.
Where it doesn't work
- Kombucha and kefir. Their acids weigh about as much as alcohol and fool the hydrometer, and suspended particles fool the refractometer: neither gives you the alcohol in a kombucha19.
- Fermented vegetables. What tells you a sauerkraut or a batch of pickles has worked is pH dropping to 4.6 or lower20. For that, a pH meter or pH strips.
Where every number comes from
Every number in the text leads to its figure: the exact sentence from the official guide it comes from, in its own language.
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1
A hydrometer measures the density of the liquid
“One scale on the hydrometer measures specific gravity (S.G.), which is the weight or density of the liquid.”
Virginia Cooperative Extension (Virginia Tech) · Wine making for the home gardener (HORT-213) · read on October 26, 2026
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2
A weighted cylinder with a scale that sinks more or less with density
“A weighted cylinder equipped with a graduated stem is more or less immersed into the liquid sample whose density is to be determined.”
International Organisation of Vine and Wine (OIV) · Compendium of international methods of wine and must analysis, OIV-MA-AS2-01: Density and specific gravity at 20 °C (2021) · read on October 26, 2026
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3
Pure water reads 1.000
“Pure water has an SG of 1.000 (grams per milliliter).”
NDSU Extension (North Dakota State University) · From the Garden or Orchard to the Table: Getting Started With Home Winemaking (FN1638, reviewed August 2026) · read on October 25, 2026
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4
When it's done, below 1.000: alcohol is lighter than water
“It should be less than 1.000 (alcohol is lighter than water; the sugar has all been converted to alcohol and CO2).”
Virginia Cooperative Extension (Virginia Tech) · Wine making for the home gardener (HORT-213) · read on October 26, 2026
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5
Specific gravity from 0.990 to 1.170; some also show potential alcohol and Brix
“Hydrometer – Weighted glass tubing to measure specific gravity (0.990 to 1.170); some also measure potential alcohol (0% to 22%) and Brix sugar (0%-35%)”
Virginia Cooperative Extension (Virginia Tech) · Wine making for the home gardener (HORT-213) · read on October 26, 2026
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6
Must starts at 1.080 or higher
“Must has a starting SG of 1.080”
NDSU Extension (North Dakota State University) · From the Garden or Orchard to the Table: Getting Started With Home Winemaking (FN1638, reviewed August 2026) · read on October 25, 2026
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7
A dry wine can go as low as 0.990
“Finished dry wine can have an SG as low as 0.990”
NDSU Extension (North Dakota State University) · From the Garden or Orchard to the Table: Getting Started With Home Winemaking (FN1638, reviewed August 2026) · read on October 25, 2026
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8
A sample in a tall, narrow container the hydrometer can float in
“some of the liquid can be dipped out and placed into a tall thin plastic or glass container deep enough to float the hydrometer.”
Virginia Cooperative Extension (Virginia Tech) · Wine making for the home gardener (HORT-213) · read on October 26, 2026
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9
If it holds a lot of gas, remove it first
“If the wine or must contains notable quantities of carbon dioxide, remove the grand majority”
International Organisation of Vine and Wine (OIV) · Compendium of international methods of wine and must analysis, OIV-MA-AS2-01: Density and specific gravity at 20 °C (2021) · read on October 26, 2026
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10
Read where the liquid is flat, not at the edge that climbs
“The measurement (S.G. or P.A.) is taken at the meniscus, or bottom flat area of the liquid, not the raised area around the hydrometer edges of the container.”
Virginia Cooperative Extension (Virginia Tech) · Wine making for the home gardener (HORT-213) · read on October 26, 2026
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11
The reference temperature and how to read the meniscus are on the scale or a slip inside
“The indication of the graduation in density at 20 °C or specific gravity at 20 °C, and of the reading at the top of the meniscus, is to be given either on the graduated scale, or on a strip of paper attached to the bulb.”
International Organisation of Vine and Wine (OIV) · Compendium of international methods of wine and must analysis, OIV-MA-AS2-01: Density and specific gravity at 20 °C (2021) · read on October 26, 2026
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12
Reference density is at 20 °C (68 °F)
“Density is the quotient of the mass of a certain volume of wine or must at 20 °C by this volume.”
International Organisation of Vine and Wine (OIV) · Compendium of international methods of wine and must analysis, OIV-MA-AS2-01: Density and specific gravity at 20 °C (2021) · read on October 26, 2026
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13
Stir and wait 1 minute before reading the thermometer
“Stir the sample and wait 1 minute to allow temperature equilibration, then read the thermometer.”
International Organisation of Vine and Wine (OIV) · Compendium of international methods of wine and must analysis, OIV-MA-AS2-01: Density and specific gravity at 20 °C (2021) · read on October 26, 2026
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14
The reading is corrected for temperature with tables
“Correct the apparent density as read at t°C for the effect of the temperature, using the tables in Annex I”
International Organisation of Vine and Wine (OIV) · Compendium of international methods of wine and must analysis, OIV-MA-AS2-01: Density and specific gravity at 20 °C (2021) · read on October 26, 2026
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15
Below 20 °C (68 °F) you subtract; above, you add
“- if t°C is lower than 20°C + if t°C is higher than 20°C”
International Organisation of Vine and Wine (OIV) · Compendium of international methods of wine and must analysis, OIV-MA-AS2-01: Density and specific gravity at 20 °C (2021) · read on October 26, 2026
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16
Each degree Brix raises gravity by about 0.004
“Eleven grams of sugar per liter of must will raise the SG by 0.004 (or by 1 unit on the Brix scale).”
NDSU Extension (North Dakota State University) · From the Garden or Orchard to the Table: Getting Started With Home Winemaking (FN1638, reviewed August 2026) · read on October 25, 2026
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17
A refractometer isn't recommended mid-fermentation: alcohol skews the reading
“Note that refractometers are not recommended for a fermenting product as the ethanol will interfere with the reading produced by the refractometer.”
Penn State Extension · Wine analytical labs: how your winery can use them · read on October 26, 2026
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18
Fermentation is followed with a hydrometer
“Monitoring fermentation is typically done by using a hydrometer to watch the sugar concentration drop over time.”
Penn State Extension · Wine analytical labs: how your winery can use them · read on October 26, 2026
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19
In kombucha, neither a hydrometer nor a refractometer gives the alcohol
“testing using hydrometers (gravity) and refractometers (refractive index) are not reliable because organic acids produced may have the same weight as alcohols, i.e. interfering with hydrometer methods, and suspended particulates in kombucha may interfere with refractometer methods.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 2: Starter cultures and fermented food standards (August 2024, updated December 2024) · read on October 25, 2026
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20
In vegetables, what rules is pH dropping to 4.6 or lower
“The critical control point in vegetable fermentation is a pH drop to 4.6 or lower.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.1: Fermented Vegetables (August 2024, updated December 2024) · read on October 25, 2026