Residual Alkalinity Calculator

Calculate the residual alkalinity of your brewing water from its alkalinity or bicarbonate, calcium and magnesium, in ppm as CaCO₃, mEq/L and °dH.

Residual alkalinity is the water's alkalinity minus its calcium divided by 1.4 and its magnesium divided by 1.7, all in ppm, after Kolbach. It shows which way the water pushes the mash pH. Water with 100 ppm of alkalinity, 50 ppm of calcium and 10 ppm of magnesium has about 58 ppm as CaCO₃.

Reviewed by Diego Santiago, Professional Brewer

Alkalinity as CaCO₃Bicarbonate (HCO₃)

Alkalinity (ppm as CaCO₃)

Total alkalinity from your water report

Calcium (ppm)

Ca from your water report, plus any salts you add

Magnesium (ppm)

Mg from your water report, plus any salts you add

Residual alkalinity58ppm as CaCO₃In milliequivalents1.17mEq/LIn German degrees3.3°dH

Results are estimates provided "as is," for informational and planning purposes only, without warranties of any kind. Check them before you rely on them: you are responsible for decisions about your beer and equipment, including pressure and carbonation levels, the pressure ratings of your kegs and fittings, and any alcohol content you declare. Use of this calculator is subject to our Terms and Conditions.

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How residual alkalinity is calculated

In the 1950s Paul Kolbach found that alkalinity raises the pH of wort, while calcium and magnesium lower it through their reaction with malt phosphates. About 3.5 milliequivalents of calcium, or 7 of magnesium, cancel one milliequivalent of alkalinity. His residual alkalinity is:

RA = Alk - \frac{Ca}{3.5} - \frac{Mg}{7}

with every value in milliequivalents per liter (mEq/L). In the units of most water reports, ppm as CaCO₃ for alkalinity and ppm (mg/L) for the ions, this becomes:

RA = Alk - \frac{Ca}{1.4} - \frac{Mg}{1.7}

The calculator converts with equivalent weights from the standard atomic weights: about 50 mg/mEq for alkalinity as CaCO₃, 20 for calcium and 12 for magnesium.

For example, water with 100 ppm of alkalinity, 50 ppm of calcium and 10 ppm of magnesium has a residual alkalinity of about 58 ppm as CaCO₃ (1.17 mEq/L, 3.3 °dH).

Inside Foambyte, water profiles keep the ions and alkalinity of your source water for every recipe.

Reference table

Residual alkalinity (ppm as CaCO₃) by alkalinity (rows, ppm as CaCO₃) and calcium (columns, ppm), without magnesium
Alkalinity \ Ca	0 ppm	25 ppm	50 ppm	75 ppm	100 ppm	150 ppm
0	0	-18	-36	-54	-71	-107
25	25	7	-11	-29	-46	-82
50	50	32	14	-4	-21	-57
75	75	57	39	21	4	-32
100	100	82	64	46	29	-7
150	150	132	114	96	79	43
200	200	182	164	146	129	93
250	250	232	214	196	179	143

Sources

Kai Troester. “The effect of brewing water and grist composition on the pH of the mash”, braukaiser.com, 2009. Kolbach's 1953 definition of residual alkalinity (3.5 mEq of calcium or 7 mEq of magnesium neutralize 1 mEq of alkalinity) as restated and tested in this paper, and 1 mEq/L = 50 ppm as CaCO₃ = 2.81 °dH. The paper is licensed CC BY-NC 3.0; none of its text, tables or figures are reproduced here.

Frequently asked questions

What is residual alkalinity?

The alkalinity of brewing water that is left over once calcium and magnesium have reacted with the malt. Alkalinity raises the mash pH; calcium and magnesium lower it. Residual alkalinity combines the three into one number that shows which way the water pushes the mash pH.

Is a high or a low residual alkalinity better?

It depends on the beer. Pale beers usually need a low or negative residual alkalinity, because the grist has little acidity. Dark beers get acidity from crystal and roasted malts and can take, or need, a higher one to keep the mash pH from falling too low.

My water report shows bicarbonate, not alkalinity. What do I enter?

Switch the field to bicarbonate and enter it as it is. For typical brewing water, bicarbonate makes up nearly all the alkalinity, and the calculator converts it: 61 ppm of bicarbonate is about 50 ppm of alkalinity as CaCO₃.

Does residual alkalinity give me the mash pH?

Not on its own. The mash pH also depends on the grist and the mash thickness. Use the mash pH calculator to estimate it from the water and the grain bill together.

Should I count the salts I add?

Yes. Calcium from gypsum or calcium chloride and magnesium from Epsom salt all lower the residual alkalinity. The brewing salt calculator shows the water after your additions, including its residual alkalinity.