Refractometer calculator
Enter your original Brix and, once fermentation has started, the current or final Brix reading. You get the alcohol-corrected gravity in your gravity unit (SG or °Plato, the other one alongside), plus ABV and apparent attenuation, adjusted with your wort correction factor.
Readings
Fermentation
Reading still dropping, or not sure. Uses Bonham’s formula.
Wort reading on brew day, before pitching the yeast.
The raw number from the refractometer.
Current gravity
1.009
- Original gravity (OG)
- 1.047 · 11.7 °P
- ABV
- 5.0%
- Apparent attenuation
- 79.3%
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Brix to SG: Brix vs. specific gravity
A refractometer measures how strongly a few drops of liquid bend light and shows it as degrees Brix: grams of sucrose per 100 g of solution. °Plato is the same idea for wort extract, so for unfermented wort Brix and Plato are close to one number. The calculator reports the corrected Brix as °Plato and converts it to specific gravity with the formula on the right.
Wort is not pure sucrose, so most refractometers read a little high on wort. That is what the wort correction factor fixes: the reading is divided by it before any conversion. The default is 1.03, which turns 12 °Bx into 11.7 °P or an OG of 1.047.
Bx = Bx(measured) / WCF
SG = 1 + Bx / (258.6 − (Bx / 258.2) × 227.1)
- Bx
- Brix after the wort correction (≈ °Plato)
- WCF
- Wort correction factor of your refractometer
| Reading | Gravity |
|---|---|
| 8 °Bx | 1.031 |
| 10 °Bx | 1.039 |
| 12 °Bx | 1.047 |
| 14 °Bx | 1.055 |
| 16 °Bx | 1.063 |
| 20 °Bx | 1.080 |
Why alcohol distorts readings after fermentation starts
Alcohol bends light more strongly than water, while sugar is being removed at the same time. A refractometer cannot tell the two apart, so once fermentation has started the raw reading overstates the remaining sugar by a lot. A wort that started at 12 °Bx and now reads 6 °Bx looks like 1.023 if you convert it naively. Corrected for alcohol it is about 1.009.
To correct the reading you need the original Brix as well, because it tells the formula how much alcohol can be in the sample. The calculator picks the correction by measurement stage: the Bonham polynomial while fermentation is still active, and Sean Terrill’s linear formula once it has finished. ABV then comes from the corrected original and final gravity with the same ABV formula as the ABV calculator.
SG = 1.001843 − 0.002318474·OB − 0.000007775·OB² − 0.000000034·OB³ + 0.00574·MB + 0.00003344·MB² + 0.000000086·MB³
SG = 1 − 0.00085683·OB + 0.0034941·MB
- OB
- Original Brix, divided by the WCF
- MB
- Current or final Brix as measured
Bonham (first formula) uses the raw current Brix. Terrill linear (second formula) divides the final Brix by the WCF too. Results are limited to 0.990–1.200.
Determining your wort correction factor
The factor depends on your refractometer and, to a lesser degree, on your wort. Typical values are 1.02 to 1.06. If you never measured yours, 1.03 is a reasonable start, but a few minutes of comparison on brew day make every later reading more accurate.
- Take a sample of unfermented wort, for example at the end of the boil, and cool it to room temperature.
- Measure it with a hydrometer, correct for temperature and convert the result to °Plato.
- Measure the same wort with the refractometer and divide the Brix reading by the hydrometer’s °Plato.
- Repeat over a few brews with different gravities and use the average.
WCF = Bx(refractometer) / °P(hydrometer)
Enter the result in the calculator’s WCF field. Use 1.00 if your refractometer matches your hydrometer exactly.
Getting accurate refractometer readings
Refractometers are popular because two drops are enough and you don’t lose beer to a hydrometer jar. They are also easy to misread, and every error carries into the alcohol correction.
- Zero the refractometer with distilled water before you start.
- Let hot wort drops cool for a moment. Automatic temperature compensation helps, but works best near room temperature.
- Use fresh, well-mixed wort and wipe the prism clean between samples.
- Note the original Brix on brew day: without it the fermented reading cannot be corrected.
- Added sugar, honey or fruit during fermentation? The correction assumes all sugar was in the original wort, so check the final gravity with a hydrometer instead.
Built into BrewForge fermentation tracking
In BrewForge the refractometer calculator sits in the calculators section of the app, next to the hydrometer, ABV and priming sugar calculators. Add the corrected gravity as a fermentation reading on your brew, or set it as the final gravity. Attenuation and ABV for the brew update from it, and manual readings sit on the same chart as those from WiFi hydrometers like the iSpindel, Tilt or RAPT Pill.
- The same calculator in the app, no extra tool needed
- Corrected gravity logged as a reading or set as final gravity
- Attenuation and ABV for the brew from your readings
Use this calculator via API
The same calculation is also a free JSON API: no account, no key, one GET request. Handy for scripts, spreadsheets and AI assistants that should use exact brewing math instead of estimating it.
Add units=imperial (or one unit parameter per value) for °F, gallons and pounds. Every answer lists the inputs, the units and the formula it used.
GET https://brewforge.sh/api/public/calc/refractometer?originalBrix=12&finalBrix=6
FAQ
Questions & answers
Can I use a refractometer to measure final gravity?
Yes, but only with an alcohol correction and your original Brix reading. Enter both in the calculator and choose “Fermentation finished”. For a check you can rely on, for example before bottling, compare it with a hydrometer reading.
Why does my refractometer still read high after fermentation?
Alcohol refracts light more than water, so a fermented sample shows a higher Brix value than the sugar left in it. A beer that started at 12 °Bx and reads 6 °Bx is actually at about 1.009, not 1.023.
Is Brix the same as Plato?
Nearly. Both express dissolved extract as a percentage by weight. Brix is defined for sucrose, Plato for wort extract, and the difference for wort is small. The calculator treats the corrected Brix as °Plato.
What is a wort correction factor and do I need one?
It corrects for the fact that wort bends light slightly differently than the sucrose solutions refractometers are calibrated with. Most fall between 1.02 and 1.06. Measure yours by dividing the refractometer Brix by the hydrometer’s °Plato for the same unfermented wort, or start with the default of 1.03.
Which correction formula does the calculator use?
It uses the Bonham polynomial while fermentation is still active and Sean Terrill’s linear formula once fermentation has finished. Choose “Fermentation finished” when the reading hasn’t changed for 2–3 days; while it is still dropping, or if you’re not sure, stay on “Still fermenting”.
Does the correction work if I added sugar during fermentation?
Not reliably. The formulas assume all sugar was already in the wort you measured on brew day. After adding sugar, honey or fruit, use a hydrometer for the final gravity.
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