It grew out of a tool I developed for my own blog, petedrinks.com but after spending the time to track down the best formulae to use, I thought I'd share them!
ABV calculations when using a hydrometer are fairly simple; the devices are usually marked in SG (specific gravity), but may be marked in Plato as well / instead. Both units are handled.
Refractometers are complicated on a number of fronts. Firstly, the alcohol present in fermented liquor affects the reading, so it's necessary to use both the original and current readings to calculate the actual final gravity. This is why the calculator displays a "calculated final gravity" that is significantly lower than the one you read.
Refractometers are normally marked in Brix (which, for our purposes, is essentially the same as Plato), but may also be marked in SG (specific gravity). Readings in Brix will require a correction (because of the form of sugar present in wort), which is what the 'wort correction factor' is - if you don't know, then it's probably safest to leave it at the default value of 1.04.
Readings in SG probably don't require correction, because that's done by the manufacturer when they set it up. The calculator therefore sets a default 'wort correction factor' of 1 when using SG values. Again, if you aren't sure then leave it at the defaults.
ABVCalc uses jQuery and jQueryUI (which is required for the tab switching between hydrometer and refractometer usage).
To use the script in your own pages, simply include the
script and the
abv-calc.css stylesheet, and copy the
petedrinks-abv-calculator div from
abv-calc.html. This contains
two forms - one for hydrometer users, and one for refractometers.
The script will automatically add appropriate handlers to the form fields, and applies the jQueryUI handling to wrap them into nice tabs, so nothing else should be necessary.
You will also need to ensure that both jQuery and jQueryUI scripts
(along with an appropriate jQueryUI stylesheet) are also included -
abv-calc.html for an example.
There are two fundamental operations required for the ABV Calculator.
Firstly, we need to be able to calculate the ABV for a given original and final gravity pair. I initially planned to use HMRC's calculation (on the basis that the Government knows what it's doing) but it seems woefully inaccurate - I have therefore returned to the formula I've used before:
ABV = ( 76.08 * ( OG - FG ) / ( 1.775 - OG ) ) * ( FG / 0.794 )
This appears widely on the Internet; I seem to have first encountered it on [brewersfriend.com] (http://www.brewersfriend.com/2011/06/16/alcohol-by-volume-calculator-updated/).
Secondly, we need to be able to convert between Specific Gravity and Plato / Brix. After evaluating a surprisingly wide variety of available formulae, I've settled on the ones from [brewersfriend.com] (http://www.brewersfriend.com/plato-to-sg-conversion-chart/).
SG = 1.0 + ( P / ( 258.6 - ( ( P / 258.2 ) * 227.1 ) ) ) P = -616.868 + ( 1111.14 * SG ) - ( 630.272 * SG^2 ) + ( 135.997 * SG^3 )
They are relatively simple and, although not perfect, deliver the same results as more complex calculations up to gravities of 1.100 - more than adequate for 99+% of beers.
This code is licensed under the MIT license. This means that you're free
to use it as you wish, as long as you credit me. See
LICENSE for more
It would be great if you could drop me a note to firstname.lastname@example.org, or even better leave a comment on the ABV Calculator link above.
Share And Enjoy!