Why is my refractometer reading wrong after fermentation?

Alcohol bends light differently than sugar does, so once fermentation begins, your refractometer misreads the sample. The reading appears higher than the beer's true gravity. A correction formula, such as Sean Terrill's, uses your original and final Brix readings together to calculate the true final gravity.

How do I convert Brix to specific gravity?

For unfermented wort, Brix converts to specific gravity using a standard polynomial formula. As a general reference, 10°Bx is approximately 1.040 SG and 12°Bx is approximately 1.048 SG. Once fermentation begins, a correction formula should be used instead of a direct conversion.

Can I use a refractometer instead of a hydrometer?

Yes. A refractometer is accurate for original gravity readings before fermentation. After fermentation, alcohol affects the reading, so a correction formula is required. A hydrometer provides accurate final gravity readings without correction, which is why many brewers use a refractometer during brewing and a hydrometer before bottling.

What is a wort correction factor?

A wort correction factor is a small adjustment, typically between 1.02 and 1.06, that accounts for the difference between how a refractometer measures wort and pure sugar solutions. Most brewers use a default value of 1.04 and divide the raw Brix reading by this factor to obtain a corrected reading.

Which refractometer correction formula is most accurate?

For beer brewing, Sean Terrill's correction formula is one of the most widely used because it has been validated against hydrometer measurements. For wine, formulas developed by Son et al. or Roesener generally provide better accuracy because they were created using wine fermentation data.

Is Brix the same as Plato?

Brix and Plato measure nearly the same concentration of dissolved solids and are effectively identical to three decimal places for brewing purposes. In most practical situations, a corrected Brix reading can be treated as degrees Plato (°P) without additional adjustment.

Refractometer Calculator

Calculator Inputs

Original Brix (°Bx) Reading before fermentation

Final Brix (°Bx) Reading after fermentation

Wort Correction Factor Default 1.04 if unknown

Terrill Correction Formula:

FG = 1.0000 − 0.0044993(OB) + 0.011774(FB) + 0.00027581(OB²) − 0.0012717(FB²) − 0.0000072800(OB³) + 0.000063293(FB³)

Raw Brix and Final Brix are both divided by the WCF before correction.
Raw post-fermentation Brix without correction will overestimate final gravity.
Works for beer, wine, cider, and mead — the math is the same wherever the sugar came from.

Output

Alcohol By Volume

—

Corrected Final Gravity

—

Corrected Original Gravity

—

Apparent Attenuation

—

Calories (5 oz serving)

—

Correction is applied using Sean Terrill’s formula, cross-checked against hydrometer readings. For final gravity before bottling or kegging, rely on this corrected value or a hydrometer rather than a raw refractometer reading.

If you have ever bottled a beer that came out weaker than you expected, or watched your refractometer show a Brix reading that made no sense once fermentation kicked in, you already know the problem. A refractometer is fast and needs only a couple of drops of liquid, but once alcohol enters the picture, it starts lying to you. This calculator fixes that. Enter your original and final Brix readings, apply your wort correction factor, and get a corrected final gravity and alcohol by volume you can actually trust.

Use it for homebrew beer, wine, cider, or mead. The math is the same wherever the sugar came from.

How a Refractometer Works

A refractometer measures how much light bends as it passes through a liquid sample. Dissolved sugar bends light more than water does, and that bend shows up as a shadow line on a scale you view through the eyepiece. The scale is read in degrees Brix (°Bx), which is roughly the percentage of dissolved solids by weight.

Before fermentation, this reading is reliable. Wort at that stage is mostly water and sugar, so a Brix reading converts to specific gravity with very little error. Once yeast starts working, though, alcohol appears in the sample, and alcohol bends light differently than sugar does. The refractometer cannot tell the difference, so it reports a Brix value that looks higher than the beer’s real remaining sugar content. Left uncorrected, this makes your beer look far less fermented than it actually is.

That is the entire reason a correction formula exists. It takes your original Brix reading (before fermentation) and your final Brix reading (after fermentation) and works backward to give you the true final gravity.

Brix to Specific Gravity Quick Reference

For pre-fermentation readings, the conversion from Brix to specific gravity is simple, no alcohol correction needed. Here are common values for quick reference.

Brix (°Bx)	Specific Gravity	Approx. Potential ABV
6	1.023	3.0%
8	1.031	4.1%
10	1.040	5.2%
12	1.048	6.3%
14	1.057	7.4%
16	1.065	8.5%
18	1.074	9.6%
20	1.083	10.8%

These figures assume the wort ferments all the way down to 0°Bx, which rarely happens in practice. Your actual ABV will come in lower once real attenuation is factored in, which is exactly what the correction formula below is for.

The Wort Correction Formula

Every refractometer needs a wort correction factor (WCF) before its readings mean anything. Most refractometers ship with a default WCF of 1.04, though the true value for your specific unit can range from 1.02 to 1.06. You divide your raw reading by the WCF to get the corrected Brix value.

For post-fermentation readings, the most widely trusted correction is the one developed by homebrewer Sean Terrill after comparing thousands of refractometer and hydrometer readings side by side. It is a cubic formula that takes your corrected original Brix (OB) and corrected final Brix (FB) and returns the true final gravity:

FGtrue = 1.0000 − 0.0044993(OB) + 0.011774(FB) + 0.00027581(OB)² − 0.0012717(FB)² − 0.0000072800(OB)³ + 0.000063293(FB)³

Once you have FGtrue, alcohol by volume is calculated the standard way:

ABV% = (OG − FG) × 131.25

Worked Example

Say you take a refractometer reading of 12.5°Bx before pitching your yeast, using a refractometer with a WCF of 1.04.

Corrected OB = 12.5 ÷ 1.04 = 12.0°Bx
This converts to an original gravity of about 1.048

After fermentation finishes, your refractometer reads 4.16°Bx.

Corrected FB = 4.16 ÷ 1.04 = 4.0°Bx
Plugging OB = 12.0 and FB = 4.0 into the Terrill formula gives FGtrue ≈ 1.004

Now calculate ABV:

ABV = (1.048 − 1.004) × 131.25
ABV = 0.044 × 131.25
ABV ≈ 5.8%

Without correction, that same 4.16°Bx reading would have been misread as a much higher final gravity, making the beer look far weaker and less attenuated than it really is. This is why bottling based on a raw, uncorrected refractometer reading is one of the most common mistakes new brewers make.

Refractometer vs Hydrometer: When to Use Each

Neither tool is strictly better. They are suited to different stages of the process.

A refractometer only needs two or three drops of sample, so it is ideal for checking gravity during the mash or partway through the boil, when you cannot spare a full hydrometer sample.
A hydrometer needs a larger sample and should be read at its calibration temperature, usually 20°C (68°F), but it stays accurate all the way through fermentation because alcohol does not distort its reading the way it distorts light refraction.
For pre-fermentation checks, either instrument works well and the two should agree closely if both are properly calibrated.
For your final gravity reading before bottling or kegging, a hydrometer reading (or a properly corrected refractometer reading) is what you want to rely on.

A good workflow is to use the refractometer for speed on brew day and switch to a hydrometer, or the corrected refractometer math above, once you are close to packaging.

Which Correction Formula Is Most Accurate?

Homebrewers often notice that different calculators give different final gravity numbers from the same pair of readings, and it is a fair question to ask why. The answer is that several correction formulas exist, each built from a different dataset and intended for a slightly different use case.

Formula

Terrill (cubic)

Novotny

Barth & Race

Rogerson & Symington

Son et al.

Roesener

Developed By

Sean Terrill, 2011

P. Novotny, Zymurgy 2017

G. Barth 1905, modified by J. Race 1908

Rogerson & Symington, 2006

H. S. Son & colleagues, 2009

Werner Roesener, 2001

Best Suited For

General homebrew beer, most widely adopted across brewing tools

Beer, refined handling of mid-fermentation readings

Beer above roughly 4.5% ABV by weight

Port and other fortified wines, not accurate for dry wines

Wine, based on 30 wine samples measured before and during fermentation

Wine, popular among home winemakers though its derivation was never formally published

For most homebrewers making beer, Terrill’s formula is the safest default since it has been cross checked against hydrometer readings the most extensively. If you are working with wine, mead, or a fortified beverage, one of the wine-specific formulas will generally track closer to reality than a beer formula would.

How to Calibrate Your Refractometer

A refractometer that has never been calibrated will quietly throw off every reading you take with it, so this is worth doing before your first use and rechecking periodically.

Place two to three drops of distilled water on the prism and close the cover plate.
Look through the eyepiece in good light and find the boundary line between the blue and white fields.
If the line does not sit exactly at 0°Bx, turn the small calibration screw until it does.
Recalibrate at the start of each brew day, and again if the room temperature has shifted significantly since your last use.
Keep the sample near room temperature when testing. A hot wort sample straight off the stove will give you a falsely low reading even on a calibrated unit.

Frequently Asked Questions (FAQs)

Alcohol bends light differently than sugar does, so once fermentation begins, your refractometer misreads the sample. The reading looks higher than the beer’s true gravity. A correction formula, like Terrill’s, uses your original and final Brix readings together to calculate the real final gravity.

For unfermented wort, Brix converts to specific gravity using a standard polynomial formula. As a quick reference, 10°Bx is about 1.040 SG, and 12°Bx is about 1.048 SG. Once fermentation starts, you need a correction formula rather than a direct conversion.

Yes, for original gravity readings before fermentation. For final gravity, a refractometer needs alcohol correction to be accurate, while a hydrometer reads correctly throughout. Many brewers use a refractometer during the mash and switch to a hydrometer near bottling time.

It is a small adjustment, usually between 1.02 and 1.06, that accounts for how your specific refractometer reads wort compared to pure sugar water. Most units default to 1.04. You divide your raw Brix reading by this factor to get your true corrected reading.

For beer, Sean Terrill’s formula is the most widely used and cross-checked against hydrometer data. For wine, formulas from Son et al. or Roesener tend to perform better since they were built from wine fermentation data rather than beer.

They are nominally the same measurement to three decimal places, so in practice, a corrected Brix reading can be treated as degrees Plato (°P) without further adjustment.