Skip to content


Folders and files

Last commit message
Last commit date

Latest commit



3 Commits

Repository files navigation


A flexible Javascript ABV Calculator, for hydrometers and refractometers - primarily for beer brewers.

It grew out of a tool I developed for my own blog, but after spending the time to track down the best formulae to use, I thought I'd share them!

All of this work relies on the formulae derived from people far smarter than me, so thanks very much for all their work (and I hope I've now credited all my maths sources!)

Updated August 2023

The calculator has been rebuilt to be considerably smaller, and to remove any dependency on things like jQuery - as well as being a lot more responsive than it used to be.

The basic operation - and the underlying calculations - have not changed.

If you decide to upgrade, make sure you also take the updated css and js file(s), or behaviour may be ... unexpected!

User Guide

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 no longer relies on any external libraries; Javascript will need to be enabled on your browser.


This calculator is primarily built for brewing beer; the formulae I'm using become increasingly inaccurate as the level of sugar / alcohol increases. As such, gravity readings are capped at 1.125 (SG) / 30 (Brix).

This translates to maximum ABVs of around 18%; if you're fermenting higher than that then you're probably beyond the capabilities of this tool, and you are using some amazing yeast.


To use the script in your own pages, simply include the abv-calc.js script and the abv-calc.css and tiny-grid.min.css stylesheets, and copy the pdac-panel div from abv-calc.html. This form now accomodates both hydrometer and refractometer readings in one.

The script will automatically add appropriate handlers to the form fields. No other action should be necessary.

Formula - ABV

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 [] (

Formula - Unit Conversion

We potentially need to be able to convert between Specific Gravity and Brix (Plato). After evaluating a surprisingly wide variety of available formulae, I've settled on the ones from

SG = 1.0 + ( Bx / ( 258.6 - ( ( Bx / 258.2 ) * 227.1 ) ) )
Bx = -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.

Formula - Refractometer FG

As mentioned above, when using a refractometer it's necessary to derive the true final gravity from both the OG and FG readings; after a lot of poking around, I've ended up using the excellent work of Sean Terrill.

FG = 1.0000 - 0.0044993*RIi + 0.011774*RIf + 0.00027581*RIi^2 - 0.0012717*RIf^2 - 0.0000072800*RIi^3 + 0.000063293*RIf^3

Where RIi is the initial gravity reading, and RIf is the final gravity.


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 details.

It would be great if you could drop me a note to, or even better leave a comment on the ABV Calculator link above.


If you encounter any issue, feel free to raise an Issue (or even a PR!)

Share And Enjoy!


A flexible Javascript ABV Calculator, for hydrometers and refractometers - primarily for beer brewers.







No releases published


No packages published