About HomebrewNumbers

HomebrewNumbers is a free set of homebrewing calculators — gravity and alcohol, hop bitterness, and carbonation. Every page shows the formula it actually used, with the numbers from your own inputs substituted in, and says where that formula came from.

Everything runs in your browser. Nothing you type is sent anywhere.

Why the sources are shown

Brewing calculators disagree with each other more than most people realise. Several of the formulas in everyday use are empirical fits rather than derivations — someone fitted a curve to observations, and it works well across the range it was fitted to. Different tools pick different fits, usually without saying so, which is why two calculators can give you meaningfully different answers for the same beer.

The ABV page shows this directly: two published formulas side by side. At ordinary strength they agree to within a rounding error. At OG 1.092 they differ by nearly a full percent. Neither is wrong; they were fitted for different purposes. You should be able to see that rather than have a tool quietly choose for you.

How the formulas are checked

Every formula has tests with expected values worked out independently, by hand, before the code was written — never by running the function and recording whatever it produced, which would only prove the code agrees with itself. Where a published source includes its own worked example, that example is used as the test case. The ABV implementation reproduces its source's published figures of 9.32% and 10.17% exactly.

What is deliberately missing

Several calculators you might expect are not here, because their formulas could not be retrieved from a published source. Guessing at them would have been easy and would have looked identical to knowing:

• Rager bitterness. Its gravity adjustment is published; its utilisation lookup table is not, and the model cannot be implemented without it.
• Strike water and mash infusion. Palmer's How To Brew is the canonical reference and the site currently serves an expired security certificate; other calculators link to it rather than republishing the equation.
• Hydrometer temperature correction, keg carbonation pressure, and refractometer post-fermentation correction. Widely available as tools, but none of the ones checked publish the equation behind them.

If you know a citable published source for any of these, the contact page is the right place to send it and the calculator will get built.

Scope

These are calculation tools for people brewing at home. They deal with arithmetic — gravity, bitterness units, grams of sugar — and say nothing about consumption.

Who made this

Elbert Ballo built and maintains HomebrewNumbers, including the decision to leave out four calculators rather than guess at their formulas. If that changes, it'll be because a real published source turned up, not because a plausible-looking number was good enough.