What is residual alkalinity?

Residual alkalinity is the part of your water's alkalinity that calcium and magnesium do not neutralise during the mash. It is the net push on mash pH: positive RA raises pH and negative RA lowers it. Kolbach defined it in the 1950s and it remains the core brewing-water number.

RA as CaCO3 equals alkalinity as CaCO3 minus calcium ppm over 3.5 minus magnesium ppm over 7. The divisors come from how much each ion reacts with mash phosphates; every 3.5 ppm of calcium or 7 ppm of magnesium cancels 1 ppm of alkalinity.

My report gives bicarbonate, not CaCO3 — what do I do?

Select the bicarbonate option and the tool multiplies by 50.04 over 61.02 (about 0.82) to convert HCO3 to its CaCO3 equivalent before applying the Kolbach formula. This is the standard conversion based on equivalent weights.

What RA should I target for my beer?

Pale, hoppy beers like pilsners and pale ales want low or negative RA, roughly minus 60 to plus 30 ppm, to reach a mash pH near 5.3 to 5.4. Dark, malty beers tolerate or benefit from higher positive RA because roasted malts are acidic and pull pH back down.

Why is the mash pH only an estimate?

RA predicts the direction and rough magnitude of the pH shift, but the final mash pH also depends on grain bill colour, malt acidity, and water-to-grist ratio. Always confirm with a calibrated pH meter on the actual mash; the estimate here assumes a simple pale grist.

What is the Residual Alkalinity Calculator for Brewing?

Computes Kolbach residual alkalinity (RA) from your brewing water's alkalinity, calcium, and magnesium. Positive RA raises mash pH and suits dark beers; negative RA lowers it and suits pale beers. Guides water chemistry adjustments. It runs free in your browser on Gera Tools, with nothing uploaded.

Residual Alkalinity Calculator for Brewing

Name: Residual Alkalinity Calculator for Brewing
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Residual alkalinity (RA) is the single most useful number for brewing water. It tells you whether your water will push the mash pH up or down before you add a single salt or drop of acid. This calculator applies the classic Kolbach formula to your water report and reports RA along with its meaning and an estimated mash pH.

How it works

Alkalinity raises mash pH, while calcium and magnesium react with malt phosphates to lower it. Kolbach captured the net effect as:

RA (as CaCO₃) = Alkalinity (as CaCO₃) − (Ca / 3.5 + Mg / 7)

with calcium and magnesium in ppm. The divisors 3.5 and 7 come from the ions’ equivalent weights: each 3.5 ppm of calcium and each 7 ppm of magnesium offsets 1 ppm of alkalinity expressed as calcium carbonate.

If your water report lists alkalinity as bicarbonate (HCO₃⁻) rather than as CaCO₃, select that option and the tool converts it first by multiplying by 50.04 / 61.02 (about 0.82), the ratio of the two equivalent weights.

Reading the result

Positive RA raises the mash pH. It suits darker, malty beers whose roasted grains are acidic and pull pH back down.
Negative RA lowers the mash pH. It suits pale, delicate, hoppy beers that need a lower mash pH for a crisp profile.
Near-zero RA is a balanced baseline that works across many styles.

The tool also estimates a pale mash pH using the rule that each 10 ppm of RA shifts pH by roughly 0.03 from an RA-zero baseline near pH 5.7. This is a guide for a simple pale grist only.

Tips and adjustments

To lower RA, add calcium (gypsum or calcium chloride) or acidify the water; to raise it, add bicarbonate (baking soda or chalk). Calcium is more effective than magnesium per ppm because of the smaller divisor, so gypsum and calcium chloride are the usual levers. Always verify the actual mash pH with a calibrated meter, since malt colour and acidity also move the final number. All calculation runs locally in your browser.