Why Sugar Matters: Brix, Specific Gravity, and Residual Sugar

Updated: February 2026 | Reading Time: 11 minutes

Sugar is the fuel that drives fermentation—the foundation of winemaking. Understanding how to measure and manage sugar is essential for producing wine with the right alcohol content, body, and sweetness level.

In this guide, we'll explore the three key measurements related to sugar in winemaking: Brix, specific gravity, and residual sugar. You'll learn what they mean, how to measure them, and how to use this information to make better wine.

Understanding Brix

Brix (°Bx) is a measure of the dissolved solids—primarily sugar—in grape juice. It's expressed as a percentage: 20° Brix means the juice contains 20% sugar by weight.

What Brix Tells You

Ripeness: Higher Brix means riper grapes
Alcohol potential: Multiply Brix by 0.55 to estimate potential alcohol
When to harvest: Winemakers often decide harvest timing based on Brix

Typical Brix Ranges

Unripe grapes: 15-17° Brix
Table grapes: 17-19° Brix
Wine grapes: 19-25° Brix
Late harvest/ dessert wines: 25-35° Brix

The Brix-Alcohol Relationship

Here's the key formula:

Potential Alcohol % = Brix × 0.55

Example: Grapes at 22° Brix will theoretically produce a wine with about 12% alcohol (22 × 0.55 = 12.1%).

⚠️ The 0.55 Factor

This is an approximation. The actual conversion varies based on yeast strain, fermentation conditions, and other factors. Some sources use 0.6; others use 0.55. The difference matters for precise calculations but the 0.55 factor is generally accurate enough for home winemaking.

Understanding Specific Gravity

Specific gravity (SG) measures the density of a liquid compared to water. Since sugar is denser than water, grape juice has a higher specific gravity than water.

What SG Tells You

Sugar content: Higher SG = more sugar
Fermentation progress: SG decreases as sugar is consumed
When fermentation ends: When SG reaches ~1.000 or below

Typical SG Ranges

Water: 1.000
Grape juice: 1.080-1.100
Fermenting wine: 1.040-1.080
Finished dry wine: 0.990-1.000

The Relationship Between Brix and SG

Brix and specific gravity are related. While you can't convert perfectly between them, a common approximation is:

SG ≈ 1 + (Brix / 260)

Example: 20° Brix → SG ≈ 1 + (20/260) ≈ 1.077

🔬 Why SG Decreases During Fermentation

As yeast consume sugar, they convert it to alcohol and carbon dioxide. Alcohol is less dense than water:

Water = 1.000 density
Sugar = 1.59 g/cm³ (much denser than water)
Ethanol = 0.789 g/cm³ (much less dense than water)

As sugar (dense) is replaced by alcohol (less dense), the overall density decreases. This is why SG drops during fermentation—it's a direct measurement of how much sugar has been converted to alcohol.

Understanding Residual Sugar

Residual sugar (RS) is the amount of sugar remaining in wine after fermentation. It's what determines whether wine is dry, off-dry, or sweet.

Wine Sweetness Levels

Bone dry: 0-1 g/L RS (usually 0)
Dry: 1-5 g/L RS
Off-dry (semi-dry): 5-15 g/L RS
Semi-sweet: 15-30 g/L RS
Sweet: 30+ g/L RS

What Determines Residual Sugar?

Initial sugar: More starting sugar = more potential RS
Yeast tolerance: Alcohol kills yeast; higher-alcohol wines tend to be drier
Fermentation temperature: Cooler = slower, often sweeter
Intentional stopping: Winemaker can stop fermentation early for sweetness

Perceiving Sweetness

The human threshold for detecting sugar in wine is about 4-5 g/L. However, other factors affect perception:

Acidity: High acidity can make sweetness less noticeable
Tannins: Tannins can mask sweetness
Alcohol: High alcohol can make wine taste sweeter

Measuring Sugar in Winemaking

Using a Hydrometer

A hydrometer is the essential tool for measuring sugar. It measures specific gravity, from which you can calculate Brix.

How to Use a Hydrometer

Fill a test jar with wine/juice
Gently lower the hydrometer into the liquid
Wait for it to settle
Read the specific gravity at the bottom of the meniscus
Note the temperature (hydrometers are calibrated to 60°F/15°C)

Calculating Brix from SG

Use this formula:

Brix = (SG - 1) × 258.6

Example: SG 1.085 → Brix = (0.085 × 258.6) ≈ 22° Brix

Managing Sugar in Winemaking

If Sugar Is Too Low

If your grapes have low Brix, you can add sugar:

Add sugar (cane, beet, or grape) before fermentation
Calculate carefully: 1.5 oz sugar per gallon raises Brix by about 1°
Don't over-add—yeast can only tolerate ~15% alcohol

If Sugar Is Too High

If grapes are very ripe:

Add water to dilute
Blend with lower-sugar juice
Accept higher alcohol

Creating Sweet Wine

To create sweet wine:

Use high-sugar grapes (late harvest, botrytized)
Stop fermentation early (add sulfites, chill)
Add sugar after fermentation (back-sweetening)

Monitoring Fermentation

Regular hydrometer readings tell you how fermentation is progressing:

Daily readings: Track SG decrease during active fermentation
Stuck fermentation: If SG stops decreasing before reaching target
Completion: When SG stays steady at ~0.990-1.000

📊 Example Fermentation Tracking

Day	SG	Brix	Notes
0	1.090	23.2°	Initial reading
3	1.070	18.1°	Active fermentation
7	1.030	7.8°	Slowing down
12	0.995	-1.3°	Complete (dry)

Conclusion

Understanding sugar is fundamental to winemaking. Brix tells you about your starting grapes, specific gravity tracks your fermentation progress, and residual sugar determines your wine's sweetness level.

With a simple hydrometer, you can measure all three and make informed decisions throughout the winemaking process. This knowledge gives you control over your wine's alcohol content, body, and sweetness—essential tools for any winemaker.

Ready to learn about adjusting acidity? Read Acid Adjustment in Wine: When and How to Do It