Mash pH & Lactic Acid Calculator
Enter mash water, grain, mineral, and malt information to estimate the untreated mash pH and the lactic acid dose for a cooled target. The result is a planning estimate to check with a calibrated pH meter.
Lactic acid to add (mL)5.32
- Estimated distilled-water grist pH
- 5.65
- Initial residual alkalinity (mEq/L)
- 1.17
- Estimated untreated mash pH
- 5.75
- Target residual alkalinity (mEq/L)
- -2.75
- Lactic acid per gallon (mL/gal)
- 1.33
- Adjustment result
- Lactic acid is indicated to lower the estimated mash pH to the selected target.
The dose uses the selected lactic-acid concentration and estimated dissociation at the target pH.
How to use this calculator
- Enter only the water that will contact the grain during the mash, not sparge water.
- Use water-report alkalinity as ppm CaCO3 and include calcium and magnesium after planned salt additions.
- Break out acidulated, dark-roasted, and pale wheat malt, then enter the combined colored-malt percentage and weighted color.
- Choose a cooled mash-pH target and the labeled strength of the food-grade lactic acid.
- Use the displayed dose as a measured starting point, then verify the cooled mash with a calibrated pH meter.
How the mash pH and acid dose are calculated
This calculator applies the residual-alkalinity procedure published in PRO Tech Notes. It first estimates the mash pH of the grist in distilled water, then estimates how the mash water’s residual alkalinity shifts that pH. Finally, it works backward from the selected target to find the residual alkalinity and lactic-acid volume needed.
The grist estimate treats any unclassified remainder as Pilsner malt:
pHd = 5.65 − 0.1PA − 0.028PR + 0.003PW − 0.00027PO(CO − 1.8)
PA, PR, PW, and PO are the entered percentages of acidulated, dark-roasted, pale wheat, and other colored malt. CO is the weighted Lovibond color of the other-malt group. The calculator ignores that color when the other-malt percentage is zero.
Water alkalinity is converted from ppm as CaCO3 to milliequivalents per liter. Calcium and magnesium reduce its residual alkalinity:
RAi = Alk/50 − (Ca/20.039)/3.5 − (Mg/12.1525)/7
With VL as mash-water liters and Wg as grain pounds, the untreated prediction is pHu = pHd + 0.059(RAi − 0.05)VL/Wg. To reach a target pHt, the reverse calculation is RAt = (pHt − pHd)Wg/(0.059VL) + 0.05. Lactic acid neutralizes the positive difference between initial and target residual alkalinity. Its effective strength is adjusted for the labeled solution strength and the fraction dissociated at the target pH.
What moves the result most
Alkalinity and mash thickness have strong effects: more bicarbonate-bearing water per pound of grain pushes the estimate upward. Calcium and magnesium offset part of that effect, while acidulated and dark-roasted malt lower the distilled-water grist estimate. The concentration on the acid bottle changes the volume, not the milliequivalents the mash needs.
What this estimate leaves out
This is an empirical model, not a full malt titration or water-ion balance. It cannot account for a specific maltster, malt lot, mash thickness beyond the water-to-grain ratio, acid additions from all sources, or meter technique. It is intended for room-temperature measurements; take a representative sample, cool it, and adjust carefully from the measured pH. Phosphoric, sulfuric, hydrochloric, citric, and blended acids have different effective strengths and cannot use this lactic-acid volume.
Worked example
For 4.0 US gallons of mash water and 10.0 lb of grain, use water with 100 ppm alkalinity as CaCO3, 50 ppm calcium, and 10 ppm magnesium. With no specialty-malt adjustments, the distilled-water grist estimate is 5.65 and the initial residual alkalinity is about 1.17 mEq/L.
At a target pH of 5.40 and 88% lactic acid, the reverse calculation calls for about 5.3 mL of lactic acid, or about 1.3 mL per gallon. The untreated estimate is about 5.75, so acid is indicated. Measure that quantity accurately, mix thoroughly, and use a cooled reading to decide whether a small follow-up adjustment is needed.
Common questions
Should mash pH be measured hot or after cooling the sample?
Measure against a room-temperature target after cooling a representative mash sample. A pH electrode and the mash both respond to temperature, so a hot reading is not directly comparable with the usual 5.2 to 5.6 mash-pH targets. Calibrate the meter according to its instructions before brew day.
What does alkalinity as CaCO3 mean on a water report?
It is a reporting convention that expresses the water’s acid-neutralizing capacity as an equivalent amount of calcium carbonate. It does not mean the water literally contains that amount of dissolved chalk. This calculator divides ppm as CaCO3 by 50 to convert it to milliequivalents per liter.
Why do calcium and magnesium change residual alkalinity?
Calcium and magnesium ions affect the mash’s acid-base balance and are part of the residual-alkalinity model. Their concentrations are converted to charge equivalents and subtracted from total alkalinity. Enter the expected concentrations after gypsum, calcium chloride, or other planned salt additions.
Does acid go into the water before adding the grain?
Either method can work if the mash is mixed evenly, but adding a measured dose to the mash water before dough-in makes distribution easier to control. Add acid slowly, avoid splashing concentrated acid, and follow the product label. Check the cooled mash rather than assuming any prediction is exact.
Can phosphoric acid use the same calculated volume?
No. Different acids have different concentrations, densities, and dissociation behavior at mash pH, so the same volume does not provide the same neutralizing capacity. Use a calculation made for that product and its labeled strength.
Why can measured mash pH differ from the prediction?
Malt acidity varies with maltster, product, crop, and lot, and water reports can be incomplete or seasonal. Mash thickness, sample handling, temperature, and meter calibration also matter. A difference of 0.1 pH or more is possible, which is why a measured brew-day result is the better guide for the next batch.