This is something I’ve been meaning to do for a *long* time, but kept finding excuses to put off. It uses the simplified cubic polynomial derived in Refractometer FG Results. Please visit that post for more information. You can also download a spreadsheet to track OG, FG, and more for multiple batches.

Did you know this calculator is used, on average, once every four minutes? If you find it useful please consider a donation to support its development and maintenance.

Hi,

I’m confused by the results I’m getting from different calculators.

If I take some values from the sheet on this page…

http://www.homebrewstuff.com/refractometer-how-to

and work out some results.

I take

1. OG=1.053 (13 Brix) and FG=1.012 (3 Brix)

and

2. OG=1.088 (21 Brix) and FG=1.012 (3 Brix)

If I calculate ABV using the gravity readings I get:

1. 5.38% ABV

2. 9.98% ABV

If I use your calculator I get

1. 6.8% ABV

2. 12.8% ABV

Is the correction factor the reason? If not what?

If I use the calculator on Northern Brewer page…

http://www.northernbrewer.com/refractometer-calculator/

I get

1. 8.525% ABV

2. 15.474% ABV

Am I missing something here? I just made a Belgian Strong ale with the #2 measurements.

I was expecting about 10%ABV not 12.8% or 15.474%

Thanks.

Pat.

It seems like you’re using hydrometer readings instead of refractometer readings as the inputs for the calculator. A refractometer reading of 3°Bx would correspond to an FG of about 0.987, which seems unlikely even if you used a large quantity of simple sugars.

If those are in fact the hydrometer readings, then yes, the beer is about 10% ABV.

Sean

Thanks for your answer. What you say makes sense. So the chart I’m read on http://www.homebrewstuff.com/refractometer-how-to must not be correct?

It says that Brix 3 = 1.012

Can you point me to a Brix to SG chart that is correct?

Thanks.

Pat

Hi again,

I just looked up about a half dozen charts of Brix to SG and they all say that

3 Brix is about 1.012 SG?

Pat.

3°Bx is 1.012 SG, but refractometers don’t measure gravity. They measure the refractive index. Before fermentation, the two are directly related, but once there’s alcohol in solution they aren’t. Hence the need for an FG calculator.

http://seanterrill.com/2010/06/11/refractometer-estimates-of-final-gravity/

In principal, I think this table is great. However, I have a problem with a discrepancy. Using the downloaded spreadsheet, I put in the OB as 28 (Brix) which, according to the conversion sheet that came with the refractometer (and cross checked with my hydrometer) is 1.120 as an OG. Final Brix = 13 but my calibrated hydrometer said 1.005 at 64F. Going by hydro readings I make it about 16%. Your table dropped the orig Brix 28 to an OG of 1.115 which resulted in an ABV of 14.4%. Can you explain the descrepancy for me please?

Julian

Julian, that sounds right for a wort correction factor of 1.04 (28/1.04 = 26.9°Bx, or about 1.115 SG). If the WCF for your wort is significantly different then you’ll need to adjust that accordingly.

The discrepancy in ABV is probably simply due to your other software/spreadsheet using a different formula. Hope that helps.

Sean

Hi Sean,

I am so glad I found this…I was racking my brain trying to figure out why my barleywine’s fermentation seems to have stuck so soon, but after using your calculator, I see that it’s stuck about where I expected it to (once I reached 13% alcohol using White Labs California Ale yeast, rather than stuck at only 8% alcohol, which is where the refractometer is saying it is).

Question…would this same theory apply to wine? And if so, would the “wort correction factor” still be 1.040, or would it be something else?

Thanks,

Ron

Ron, the theory would apply to wine, but with the different mix of sugars I wouldn’t expect the same equation to do too well. And with a grape must being almost entirely simple sugars, you wouldn’t need a wort correction factor. I have heard anecdotally that the “old” refractometer correlation was developed by/for vintners, so it may give better results for wine than it does for beer.

Sean

[...] precisione degli strumenti che li hanno misurati. Per le densità uso il rifrattometro utilizzando questa formula per calcolare la SG, tenendo il fattore di correzione a 1.04. Non so dire se i numeri spacchino [...]

Sean

I received a lab instrument to measure the SG as well as serum protein and RI for urine(http://www.sperdirect.com/lab-digital-refractometer-clinical-149-prd1.htm).

Do you believe the RI for Wort or Wine RI would be similar enough for me to use ?

How would I approach a correction factor for urine vs wort ?

Thanks

You could use it in the sense that the nD range is correct (wort will range from about 1.33-1.37), but the tolerance is ±0.001. That won’t give you the precision you need to get meaningful results. If you did want to play around with it, though, the wort correction factor would remain unchanged. Just convert from nD to degrees Brix and proceed as usual.

Sean

Thanks for the reply.Could you explain where my precision would be too far off? Is it in the original conversion to Brix that I would have to do ? Would that give me a less precise Brix than using a cheaper optical refractometer that gives a Brix reading ?

Thanks

The issue is the precision of the refractometer itself. For example, if you measured a refractive index of 1.3525±0.001, that would correlate to a reading of 12.0-14.0°Bx. The instrument just isn’t as precise as a hand-held refractometer due to its larger range.

[...] my hydrometer. Rather quickly, I stumbled on Sean Terril’s killer website where he has a refractometer calculator that appeared to do exactly what I was looking for. His calculator requires brix, which was fine [...]

Sean, would this calculator be accurate for sour beers, considering the higher density of lactic acid? Some additional infos: http://www.themadfermentationist.com/2013/11/calculating-abv-for-sour-beers.html

I love your calculator BTW! Thanks!

At <1% lactic acid, I doubt it would make a measurable difference. That said, I have no data to back that up. The nD of lactic acid is ~1.462, which is quite high relative to either water or ethanol, so it could end up being what drives the refractive index of the beer. Sorry I can’t give you a definitive answer, but if even a single data point conforms, I would assume the equation gives overall good results for sour beers.

Sean

Thank you, but what is the wort correction factor?

Brian, this is copied from the original post on using a refractometer (linked above):

“For a simple sucrose solution (the refractometers common to homebrewers are “borrowed” from the wine industry) the refractive index depends only on the sugar content and the temperature. Automatic temperature correcting (ATC) refractometers use a bimetal strip to cancel out the temperature variable (within a given range), meaning that the reticle can be marked directly in units of sugar content. Brewers’ wort, however, is not a sucrose solution, and so a “wort correction factor” must be applied. Generally this is done by dividing the refractometer reading by 1.04.”

So the wort correction factor is simply the refractometer reading divided by a calibrated hydrometer reading. Obviously it will vary depending on the specific sugar makeup of the wort, but it’s generally 1.02—1.06.

Sean

Did your formula recently change? I’ve used this before with success, but tonight everything that I enter (Brix not SG) is giving me an estimated FG less than 1.000?!

No, there haven’t been any changes. I just checked the calculator on two machines and it seems to be working normally.

Hmm… I must be doing something wrong then. The Final Gravity just doesn’t seem right.

My inputs-

Original RI: 16.8

Final RI: 3.1

Wort Correction Factor: 1.040

Outputs-

Original Gravity: 1.0661 (16.15°P)

Final Gravity: 0.9941 (-1.53°P)

Alcohol by Volume: 9.3% (7.6% ABW)

Thanks for providing this useful tool!

That’s the correct result, so the calculator is working normally. It just doesn’t provide good results when the attenuation is very high or very low. Which doesn’t necessarily mean the result is wrong; I just wouldn’t have much confidence in it. Is this a beer?

Yeah, expected an OG of 1.066 and a FG close to 1.012, that’s why these results surprised me.

I am surprised as well with the results. I’ve been struggling with my Russian Imperial Stout which I was pretty much sure didn’t ferment enough and I started researching what could be wrong.

Original Brix: 25

Final Brix: 16

WCF: 1.04

Output FG with your formula: 14.9*P

I was shocked how huge the result is and I was sure something is wrong with the beer, but then I took the reading with a hydrometer and it showed 9*P, which is in line with popular formula from there:

http://primetab.com/formulas.html

The difference of 6*P is huge. What could be the reason of so big difference? I ultimately trusted the hydrometer and my taste – the beer turned out just fine. But the difference between results of your formula and popular software (Beersmith, Brewtarget and Northern Brewers calculator) got me really thinking and I have no clue what’s going on.

Thanks for your opinion!

[...] precisione degli strumenti che li hanno misurati. Per le densità uso il rifrattometro utilizzando questa formula per calcolare la SG, tenendo il fattore di correzione a 1.04. Non so dire se i numeri spacchino [...]

Hi Shaun, thank you very much for this calculator.

A question…. Is the chart in the link below correct as differs from your results e.g. 13 brix has a SG of 1.055. But in your calc with a OG of 13 and a FG of 7, the OG calculates to 1.051.

Thanks.

http://braukaiser.com/documents/Kaiser_Brix_Plato_SG_table.pdf

[...] precisione degli strumenti che li hanno misurati. Per le densità uso il rifrattometro utilizzando questa formula per calcolare la SG, tenendo il fattore di correzione a 1.04. Non so dire se i numeri spacchino [...]

Hi Sean,

I have a refractometer with Plato scale.

May I input the value I got from the refractometer directly in your calculator as Brix?

Plato and Brix Refractometer readings are quite the same aren’t they?

cheers

luis

[...] Refractometer showed just what Beersmith estimated…12.4 Brix…or 1.050 after converting. That’s 87% mash efficiency…about what I usually get. (Note: Refractometers measure refraction of light through a liquid, not specific gravity (SG). Hydrometers measure SG. The wort’s refraction index (Brix) does not have a linear relationship with its SG. In other words, your refractometer SG reading is off. That being said, with a little homework, you can keep using your refractometer with accuracy. You can even semi-accurately measure final gravity with a conversion calculator.) [...]

[...] cleaning up my mess, Sean Terrill (the awesome refractometer calculator guy) and Franklin Hess invited some of us to join them for lunch. Our initial plan to get a quick bite [...]

Hi Sean Terrill,

My results don’t make sense with your calculator.

I have an IPA with OG: 1.055. The Brix reading after 12 days is 7.8.

Using your calculator that is an estimated FG of 1.014. Took a hydrometer reading that came out to be 1.020.

Now that’s a pretty large difference.. I’m confused.

Refractometer is properly calibrated.

Does this refractometer method of measuring Brix and then calculating FG mathematically from a known OG work if sugar is added days/weeks later, after fermentation has already begun?

For example:

I use a refractometer to measure Brix of unfermented wort and calculate OG.

then

two weeks later, after fermentation is done, I measure Brix and calculate FG using your calculator

THEN, let’s assume I want more ABV:

So, I start a second fermentation cycle by adding more sugar to the already fermented wort.

Is it then possible, using a refractometer, to calculate my “second FG” in another few weeks after the second fermentation cycle has ended?

—————-

If the answer to the above question is “no you can’t add sugar after fermentation starts, and use a refractometer to calculate FG,” my second question is:

If I add the additional sugar, and then immediately measure Brix and also use a hydrometer to measure the gravity of my alcohol, water, sugar solution, can I, after the second fermentation stops, use a refractometer to calculate my “second FG”? (Remember, I cheated and measured the “Second OG” using a hydrometer)

What if your refractometer does not have a brix scale, only a gravity scale? I am using a clinical refractometer because I thought it would be more accurate than hydrometer, but apparently it’s not! My FG on refractometer was around 1.032, but the hydrometer read 1.015.

You can certainly use the Gravity scale, and convert to Brix using an online calculator. I would definitely calibrate your refractometer, though if it’s a medical model I’d think the gravity scale is probably more accurate than the home brewers’ models.