Reading a Hydrometer: OG, FG, and What They Tell You

The hydrometer is the most important measuring tool in winemaking. It tells you exactly how much sugar is in your must before fermentation, how fermentation is progressing, and when it's complete. Understanding how to use one accurately — and what the numbers mean — is foundational.

What a Hydrometer Measures

A hydrometer measures specific gravity (SG): the density of your liquid relative to pure water. Pure water = 1.000. Dissolved sugar increases density above 1.000. Alcohol is less dense than water, so a fully fermented dry wine reads below 1.000 (typically 0.995–0.998).

The hydrometer floats at a height determined by this density. The scale on the stem shows the reading where the liquid surface crosses the glass.

How to Take a Reading

Draw a sample into a hydrometer trial jar (a tall, narrow cylinder). Never place the hydrometer directly in your fermentation vessel — disturbing the yeast and contaminating the wine are both risks.
Sanitise the hydrometer and trial jar before every use.
Lower the hydrometer gently into the sample. Do not drop it. Give it a gentle spin to dislodge any clinging bubbles (especially in active fermentation — CO₂ bubbles attach to the stem and give artificially low readings).
Read at the meniscus bottom: the liquid surface curves up at the edge of the tube (the meniscus). Read the flat middle of the surface, not the curved edges.
Note the temperature of the sample. Hydrometers are calibrated at 20°C.

Temperature Correction

If your sample is not at 20°C, apply a correction to the raw reading:

Corrected SG = Measured SG + 0.00027 × (T°C − 20)

Examples:

Sample at 15°C, reads 1.090 → 1.090 + 0.00027 × (−5) = 1.090 − 0.00135 = 1.0886
Sample at 25°C, reads 1.045 → 1.045 + 0.00027 × 5 = 1.045 + 0.00135 = 1.0463

For most purposes, if your must is within 5°C of 20°C, the correction is less than 0.001 SG — small enough to ignore. At larger temperature differences it matters.

Original Gravity (OG)

OG is the specific gravity of your must before you pitch yeast. It tells you how much fermentable sugar is present.

Typical ranges:

Style	OG Range	Potential ABV
Light table wine	1.070–1.080	9–10.5%
Standard table wine	1.080–1.095	10.5–12.5%
Full-bodied wine	1.095–1.110	12.5–14.5%
Dessert wine	1.120–1.140	15–18%
Show mead	1.090–1.130	11–17%

Record OG immediately after adding all sugar and before pitching yeast. This is the reference point for all downstream calculations.

Final Gravity (FG)

FG is the specific gravity when fermentation is complete. For a fully dry wine:

Most wines: 0.995–1.000
High-alcohol wines (above 14% ABV): can end lower than 0.995

If you're targeting a slightly sweet wine (e.g. off-dry white), you'd stop fermentation early at a higher FG — perhaps 1.005–1.015 — and stabilise the wine.

How to confirm fermentation is complete:
Take two readings 48 hours apart. If the gravity has not changed, fermentation is done. A single reading isn't sufficient — a stuck or slow fermentation can look complete but still has residual sugar.

Calculating ABV

The simplified formula used by most home winemakers:

ABV% = (OG − FG) × 131.25

Example:
OG = 1.092, FG = 0.997
(1.092 − 0.997) = 0.095 × 131.25 = 12.5% ABV

This formula is accurate to within ±0.5% ABV for typical wine gravities. For precision (e.g. commercial compliance), an ebulliometer or distillation ABV test is more accurate.

Reading in Brix

Some winemakers — particularly grape growers — prefer Brix (°Bx) rather than SG. Brix measures grams of dissolved sugar per 100g of solution. 1 Brix ≈ 10g sugar per litre.

Converting SG to Brix (approximate):

Brix ≈ (SG − 1) × 250

So 1.095 SG ≈ 23.75 Brix. More precise conversion tables are available from refractometer manufacturers.

At harvest, grape growers use a refractometer (measures Brix via light refraction on a droplet of juice — fast, field-portable). However, alcohol distorts refractometer readings — so once fermentation starts, a hydrometer is more accurate.

Tracking Progress in MakeWine

Every gravity reading you take can be logged in MakeWine as a gravity reading event on your fermentation batch. The app plots your gravity curve over time and calculates ABV automatically from OG and current reading — so you can see fermentation progress visually without doing the maths yourself.

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Tip: Take gravity readings at pitching, day 2–3 (start of vigorous fermentation), when bubbling slows, and at the suspected end. Four readings tell a complete story.