This is an enhanced version of my beer ABV calculator, which now works for both hydrometer and refractometer homebrewers. Please note that the calculations are a little different to those used in the previous version; this is intentional and this new calculator should be more accurate.

All the code is available on Github, along with details on the exact formulae being used and where they came from.

The Wort Correction Factor is generally required for refractometers which measure in Brix; unless you know better, it is best to leave it at the default value.

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ABV a little high? Brix reading 14.8 (initial) and 6.8 (final)

ABV= 131.25*(1.058-1.011) = 6.17 (calculator shows 6.35).

When I use OG =1+0,004*B3/1,04= 1.057 (I get the same FG of 1.011 through =1-0,00085683*B3/1,04+0,0034941*F3/1,04) I end up with ABV=6.07 (even lower).

The formula used by this calculator for the final ABV sum is a more complex version of “131.25 * difference”, which is intended to be slightly more accurate – especially for higher gravities. It does usually result in a slightly higher ABV than with the simpler formula.

The conversion from Brix to SG is also a lot more complex than “1 + 0.004 * Brix” (see the details on the Github page, linked above) – again, those formulae were chosen to be (hopefully) slightly more accurate.

Hey Pete! I was wondering how you were calculating your values from refractometer readings, as I just found the Sean Terrill site, which seemed pretty thorough. The github page didn’t actually say, so you forced me to read the code. 🙁

Just kidding, looks well written, but I did notice that your first coefficient in the RefractFinal formula is wrong. 0.004493 instead of 0.0044993. Probably not much of a difference, but…

Thanks again for the calculator!

You’re right – I’ve got some of the formulae listed on that page, but somehow I’ve failed to define (or attribute!) the final refractometer calculation. I shall amend that now.

And well done, you’ve spotted my deliberate typo 🙂

I’m trying to learn the calculations to take 2 brix/refractometer readings and get an ABV. These values may seem weird, but I’m working on a rum ferment… So starting Brix was 36. I checked it after 1 week and it was 24. Plugging those values into your calculator gives me a higher final gravity than my original gravity. How is this?

Corrected Original Gravity: 34.6 1.152

Calculated Final Gravity: 52.0 1.245

ABV: -.-%

I suspect the major problem is your high Brix values – the formulae used are only really valid for gravities up to around 1.1 (around the 25 Brix mark).

Actually, those high values will both generate invalid ABV *and* a faulty final gravity; while their probably are formulae out there that will cover a wider range, they’re pretty horrendous!

Thanks for the online calculator. Have checked yours vs actual sp. gravity readings & found it to be the most accurate. Much appreciated tool!

Can I use this calculator if I checked the first gravity with a refractometer and I want to check where the gravity is after fermenting for a while and has alcohol within the wine at this point? I’d hate to waste the wine using a hydrometer at this point.

You can – the calculator exists for this purpose, because it takes into account the effect the alcohol content has on the refractometer reading. The only caveat is that if you’re making wine rather than beer, the higher alcohol content may result in slight inaccuracies.

Thank you very much for the reply. That’s exactly what I’m doing too as the starting gravity was 1.118 for a little over 6 gallons and if all of the sugar has been eaten the alcohol should be around 17%. I’m fine with slight inaccuracies in alcohol, but I’m just trying to see if my wine is finished or if it has stalled. I’ll try it out and let you know how it goes. Thanks again!