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Brix to Specific Gravity: ICUMSA GS4-13 for Winemaking, Brewing, and Sugar Refining

Published July 4, 2026 · by Industrial Unit Converter Editorial Team

Brix to specific gravity: ICUMSA GS4-13 for winemaking, brewing, and sugar refining

On the morning of September 24, 2024, a Napa Valley winemaker walked a Stags Leap District Cabernet block with a digital refractometer (Atago PAL-1) and a notebook. The refractometer read 24.2°Bx at 21°C ambient. The winemaker needed to convert that Brix to specific gravity, apply the temperature correction to 20°C, and estimate the potential alcohol. The numbers came out to 24.0°Bx corrected, SG 1.1011, and roughly 12.7% ABV if the must fermented to dryness. He called the harvest crew for 5:30 the next morning. The decision rode on a single conversion.

This guide covers the ICUMSA Method GS4-13 polynomial, the temperature corrections that audit findings are made of, the difference between Plato and Brix, and three industry examples: a winery, a brewpub, and a Louisiana sugar refinery.

How Brix and specific gravity are measured

Brix (°Bx) is the percentage of sucrose by mass in a solution. A 12°Bx beer wort has 120 grams of sucrose per kilogram. The scale was calibrated for pure sucrose at 20°C, then adopted as a proxy for total fermentable sugar in must, wort, and juice. A digital refractometer gives a Brix reading in under five seconds.

Specific gravity (SG) is the ratio of solution density to water density at the same temperature. A hydrometer is the traditional tool: float it in a sample jar, read the meniscus, note the temperature. Modern labs use vibrating-tube density meters (Anton Paar DMA 35, Mettler Toledo Densito 30PX) for process and quality work, accurate to ±0.0001 SG.

The relationship between the two is not linear. The ICUMSA polynomial models this empirically and gives ±0.0001 SG accuracy for the 0-30°Bx range that covers nearly all winemaking, brewing, and juice work. Above 30°Bx the polynomial drifts by up to 0.005 SG; use a pycnometer or vibrating-tube densitometer for that range. See Brix to Specific Gravity Calculator and Specific Gravity to Brix Calculator for the math.

Brix by refractometer

A digital refractometer (Atago PAL-1, MISCO Palm Abbe, Bellingham + Stanley RFM) measures refractive index and converts to Brix using a built-in sucrose calibration. Most have ATC to 20°C. The catch: ATC assumes pure sucrose, and a 12% alcohol solution reads about 1.5°Bx high because alcohol has a different refractive index than water. The Terrill correction reverses this for fermenting wort.

Specific gravity by hydrometer

A hydrometer is calibrated at one temperature (20°C for Brix/SG work, 17.5°C for Plato). A reading off the calibration temperature requires correction per the printed chart or the ASBC table. A 1.058 reading at 18°C is 1.0576 after correction. That 0.0004 difference changes the calculated Brix by 0.1° and the predicted ABV by 0.05%.

Density by vibrating tube

A vibrating-tube densitometer (Anton Paar DMA 4500, Mettler Toledo Excellence) measures the natural frequency of a U-tube filled with sample, then converts frequency to density via calibration with air and water. Accuracy is ±0.00001 g/cm³. The instrument is temperature-controlled internally, usually to 20°C ±0.03°C.

The ICUMSA GS4-13 polynomial

ICUMSA Method GS4-13 (2009) is the international standard for Brix/SG conversion. The polynomial is calibrated for pure sucrose solutions at 20°C:

SG = 1 + (Bx / (258.6 - ((Bx / 258.2) × 227.1)))

For 0-30°Bx this matches laboratory density measurements to within ±0.0001 SG. The reverse:

Bx = 258.6 × ((SG - 1) / (258.6 - 227.1 × (SG - 1)))

gives ±0.05°Bx accuracy. A simpler approximation works in the 0-20°Bx range:

SG = 1 + 0.00373 × Bx + 0.0000012 × Bx²

The full polynomial is the one to cite in lab documentation and TTB reports. The simpler form is fine for in-process checks at a winery or brewery.

Reference data

Brix (°Bx) SG (20°C) Industry application
0 1.0000 Pure water
5 1.0198 Light beer, low-sugar beverage
10 1.0401 Standard beer wort
12 1.0484 Average beer original gravity
15 1.0608 Strong beer, Imperial styles
18 1.0736 Cider base, pommeau
20 1.0833 White wine (Chardonnay, Sauvignon Blanc)
22 1.0929 Late-harvest Riesling, rosé
25 1.1050 Red wine (Cabernet, Syrah)
28 1.1171 Ice wine, dessert wine
40 1.1773 Fruit juice concentrate
50 1.2317 Maple syrup base
65 1.3222 Corn syrup (HFCS-65)
75 1.3970 Refined sugar syrup

Source: ICUMSA Method GS4-13 (2009), 20°C reference. Cross-checked against the ASBC Beer-2B Plato table for the 10-15°Bx range.

Worked example 1: Napa Valley Cabernet harvest

A Stags Leap District vineyard, late September 2024. The winemaker samples berries, squeezes juice onto the Atago PAL-1 prism, and reads 24.2°Bx at 21°C ambient. The refractometer's ATC corrects to 20°C, giving 24.0°Bx.

Convert to SG:

SG = 1 + (24.0 / (258.6 - ((24.0 / 258.2) × 227.1)))

SG = 1 + 24.0 / (258.6 - 21.1) = 1 + 24.0 / 237.5 = 1.1011

Estimated potential alcohol (using the standard wine multiplier 0.55 with a 0.5% offset for non-sugar solids):

ABV ≈ 24.0 × 0.55 - 0.5 = 12.7%

For a 13.5% ABV target the winemaker needs about 24.5°Bx. The fruit accumulates 0.1-0.2°Bx per warm day. Decision: pick at dawn tomorrow, when sugars peak before the day's respiration loss. October 2023 dropped 1.8 inches of rain on this block and cost the winery about $18,000 in diluted lots. The Brix-to-SG conversion drives a $40,000-pick decision.

Worked example 2: Brewpub original gravity

A brewpub brewer pulls a cooled wort sample at 18°C. The hydrometer reads 1.058. Per ASBC Beer-2B, the temperature correction is -0.0002 SG per °C from 20°C:

Corrected SG = 1.058 - (0.0002 × 2) = 1.0576

Convert to Brix:

Bx = 258.6 × (0.0576 / (258.6 - 227.1 × 0.0576)) = 258.6 × (0.0576 / 245.5) = 13.6°Bx

The recipe target was 15°Bx (SG 1.061) for a West Coast IPA. Mash efficiency is 91% of target. The brewer calculates the deficit: 1.4°Bx × 10 g/L per °Bx × 2,400 L (20 barrels) = 336 g of additional sugar needed. The fix: 360 g of dried malt extract boiled into the kettle, re-verified at 20°C. Under-attended original gravity on 60 bbl/month at $180 per bbl is $5,800/month in lost revenue.

Worked example 3: Louisiana sugar refinery

At a Louisiana cane sugar refinery, the inline Brix analyzer (Endress+Hauser Liquiphant, calibrated against an Anton Paar DMA 35 reference) reads 56.4°Bx on the thick juice leaving the third evaporator effect, at 95°C process temperature. The instrument's ATC corrects to 20°C.

SG = 1 + (56.4 / (258.6 - ((56.4 / 258.2) × 227.1)))

SG = 1 + 56.4 / (258.6 - 49.6) = 1 + 56.4 / 209.0 = 1.2699

The setpoint for the vacuum pan seed stage is 70-72°Bx. A 0.5°Bx deviation corresponds to a 4-5% change in crystal yield. On a daily throughput of 8,500 tonnes of cane, that 0.5°Bx miss is roughly $90,000/day in recoverable sugar lost to molasses. The Brix signal feeds the DCS, which modulates the fifth-effect evaporator steam pressure to maintain the setpoint within ±0.2°Bx.

Older refineries in this region still report Baumé alongside Brix: °Bé = 145 - 145/SG. For SG 1.2699, that is 30.9°Bé.

Common mistakes

Mistake 1: Treating Plato and Brix as interchangeable

Plato and Brix differ by less than 0.05° for typical wort, so the two numbers are often used as if they were the same scale. They are not. Plato is calibrated to 17.5°C; Brix is calibrated to 20°C. A Plato reading of 12.4°P filed as 12.4°Bx is off by 0.05° in the wrong direction. Multiply that across a year of QC records and the reported original gravity trend drifts. TTB inspectors and EU brewing auditors check this.

Mistake 2: Skipping the temperature correction

A refractometer reading of 12.0°Bx at 25°C is actually 12.4°Bx corrected to 20°C. A 5°C error in sample temperature produces a 0.4°Bx error, which translates to a 0.4% potential ABV error in winemaking. Most digital refractometers have ATC, but the switch is sometimes disabled, the calibration is sometimes 17.5°C (Plato) instead of 20°C (Brix), and the instrument drifts over time. Verify ATC is on and zeroed with distilled water before trusting the reading.

Mistake 3: Using a refractometer for fermenting wort without the alcohol correction

A refractometer calibrated for sucrose reads high in the presence of ethanol. A 1.020 wort with 5% ABV reads about 1.030 on an uncorrected refractometer. Brewers who trust the refractometer through fermentation over-predict the final gravity and end up with under-attenuated beer. The fix: the Terrill correction or the ASBC Beer-2B post-fermentation formula.

Mistake 4: Applying the ICUMSA polynomial above 30°Bx

The polynomial is calibrated for 0-30°Bx. At 60°Bx, it under-predicts SG by about 0.004. At 70°Bx massecuite, the error is 0.008, a 3% error in the calculated dry substance, the basis for crystallization yield. Use a pycnometer or vibrating-tube densitometer (Anton Paar DMA 4500, Mettler Toledo Excellence) above 30°Bx.

Mistake 5: Forgetting that Brix is calibrated for sucrose, not fermentable sugar

A 20°Bx grape must is not exactly 200 g/L of fermentable sugar. The Brix scale was built on pure sucrose; grape must contains glucose, fructose, and organic acids. The relationship works for harvest decisions (the error is well within the 0.2-0.3°Bx sampling noise), but for research work the difference is real. Wine research labs measure glucose + fructose by HPLC or enzymatic assay.

Mistake 6: Using Brix for non-sugar density work

Brix is a sugar-specific scale. A glycol solution at the same density as a 10°Bx sugar solution will not have the same Brix reading on a refractometer, because glycol and sucrose have different refractive indices. For non-sugar process fluids, use actual density (kg/m³) or specific gravity. See kg/m³ to lb/ft³ Converter and Specific Gravity to Density Guide.

Sugar scale history: Balling, Brix, Plato, Baumé

Four scales appear in older process documentation. All four measure sugar concentration, with different calibrations and reference temperatures.

Scale Year Developer Reference T Industry
Baumé 1768 Antoine Baumé 12.5°C (later 15.6°C) Legacy sugar, honey
Balling 1850 Karl Josef Balling, Prague 17.5°C Original brewing scale
Brix 1854 Adolf Brix, Germany 20°C Winemaking, food, sugar
Plato 1918 Fritz Plato, Germany 17.5°C EU brewing (dominant)

The split comes at higher concentrations: for HFCS-65, Plato reads 64.8°P and Brix reads 65.0°Bx. EU brewing uses Plato; modern winemaking uses Brix; the US sugar industry still uses Baumé in some contracts. Conversion is documented in Plato, Brix, Balling: Sugar Scales History.

Standards and best practices

ICUMSA Method GS4-13 (2009)

The international standard for the Brix/SG polynomial, published by the International Commission for Uniform Methods of Sugar Analysis. Required for any regulatory submission, custody transfer, or inter-lab comparison that crosses borders. Polynomial coefficients (258.6, 258.2, 227.1) are fixed. Reference temperature: 20°C.

ASBC Methods of Analysis, Beer-2B

The American Society of Brewing Chemists standard for original gravity, final gravity, and apparent extract. Uses the Plato scale at 17.5°C. Required for US brewery QA documentation. Hydrometer temperature correction is defined here: -0.0002 SG/°C from 20°C.

TTB Ruling 2012-2

US Alcohol and Tobacco Tax and Trade Bureau. Defines the legal Brix measurement for US wine: refractometer or hydrometer at 20°C, ICUMSA polynomial for the conversion. Required for any wine produced in or imported to the US that crosses the 14% ABV threshold for tax classification.

ASTM E100

ASTM International standard for the Baumé scale. Still cited in older US sugar and honey contracts. The conversion to Brix: °Bx ≈ 145 × (1 - 1/SG) is approximate, accurate to ±0.2°Bx.

NIST SP 811

NIST Guide for the Use of SI Units. Defines the SI density unit (kg/m³) and the conversion factors for US customary units. Required for documentation that crosses the SI / US customary boundary.

When to use Brix vs specific gravity

Use Brix when reporting to winemakers, brewers, juice producers, or food scientists. A refractometer reads Brix directly. A Brix-calibrated hydrometer (20°C) reads Brix directly. US winemakers report to TTB in Brix per Ruling 2012-2. Use specific gravity when working from a hydrometer calibrated in SG, when communicating with lab analysts who think in density ratios, or when designing a process control loop. The DCS read from a vibrating-tube densitometer is SG, then converted to Brix in software.

For internal calculations, log both. A 12°Bx, SG 1.0484, 20°C record is unambiguous; a "1.05" record is not. For process control above 30°Bx, log density (kg/m³) directly. See Density Concentration Conversions for the full set of tools.

Frequently asked questions

What is the exact formula for Brix to specific gravity?

Per ICUMSA Method GS4-13 (2009): SG = 1 + (Bx / (258.6 - ((Bx / 258.2) × 227.1))). For 0-30°Bx this matches laboratory density measurements within ±0.0001 SG. Reverse: Bx = 258.6 × ((SG - 1) / (258.6 - 227.1 × (SG - 1))). All formulas reference 20°C.

Is Brix the same as specific gravity?

No. Brix (°Bx) is a sucrose concentration by weight (1°Bx = 1% sucrose by mass). Specific gravity is the ratio of solution density to water density at the same temperature. A 12°Bx beer wort has SG 1.0484, not 1.12. The polynomial is non-linear because dissolved sucrose has a non-linear effect on solution volume.

Why does my refractometer disagree with my hydrometer after fermentation starts?

Ethanol changes the refractive index. A refractometer calibrated for pure sucrose reads high once alcohol is present. For a 1.020 wort with 5% ABV, an uncorrected refractometer reads about 1.030. The fix is the Terrill correction formula or the ASBC Beer-2B post-fermentation correction.

What is the difference between Plato and Brix?

Both measure sugar concentration by weight. The numerical difference is less than 0.05° for typical wort. The reference temperatures differ: Plato is 17.5°C, Brix is 20°C. Plato dominates in EU brewing; Brix dominates in winemaking, food, and US brewing. A Plato reading should not be filed as a Brix reading without conversion.

How accurate is the ICUMSA polynomial for non-sucrose solutions?

The polynomial is calibrated for pure sucrose. For fruit juices, honey, and other sugar mixtures, the relationship is approximate because the dissolved solids include acids, salts, and other non-sugar material. A 20°Bx apple juice measures SG 1.082-1.085, slightly different from the polynomial's 1.0833. Error is usually under 0.5% for fresh fruit juices but can hit 2-3% for high-acid solutions.

Can I use a refractometer at high Brix (above 30°)?

You can, but the reading is approximate. Above 30°Bx, the polynomial under-predicts SG by 0.004-0.008. For process work above 30°Bx, use a pycnometer, a vibrating-tube density meter (Anton Paar DMA 4500, Mettler Toledo Excellence Densito), or an inline density probe.

References

  • ICUMSA Method GS4-13 (2009). International Commission for Uniform Methods of Sugar Analysis.
  • ASBC Methods of Analysis, Beer-2B. American Society of Brewing Chemists, current edition.
  • TTB Ruling 2012-2. US Alcohol and Tobacco Tax and Trade Bureau.
  • ASTM E100. ASTM International. Baumé hydrometer scale specification.
  • NIST SP 811. Guide for the Use of the International System of Units (SI).
  • Engineering Toolbox, Sugar Solutions Specific Gravity. https://www.engineeringtoolbox.com/sugar-solutions-specific-gravity-d_464.html
  • Terrill, S.W. et al. (2008). "Refractometer Calibration for Brewing." MBAA Technical Quarterly, 45(2).
  • White, F.M. (2016). Fluid Mechanics, 8th ed., McGraw-Hill.

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