Choose a taste profile, or set your own targets; say which water you start from; set the strength. The calculator subtracts what your base water already contains, gives the salts to weigh for a concentrate of the chosen size and dose, the resulting ionic composition, and places the water in the SCA and SCAE zones, next to the published recipes and the water of Paris.

The profiles, named by their effect in the cup, bright, floral, round, body, are not distinguished by their hardness but by the magnesium-to-calcium ratio and by the split between sulfate and chloride: these are the levers on which the published recipes really differ from one another. None goes beyond the hardness of the hardest recipe, 94 ppm for Rao 2013, and most stay within the acceptable SCA band, 17 to 85 ppm. The effects in the cup stated are those reported by practitioners, not measured results, and the calculator says so again under each profile.

Gagné 2021, pp. 33-35 (principle of the calculation); Apax Lab, Tonik, Jamm, Lylac and Konflux concentrates, product pages and declared compositions (accessed September 7, 2026); profiles and targets: the author of the site, not a standard

Tool · coffee water

Water calculator

Four stages. Say which water you start from; read what it is worth, hardness against alkalinity, on the graph; correct it with salts or by cutting it with pure water; measure it. Each figure carries its unit, its formula, which can be unfolded, and its source.

1. Your water
2. What it is worth
3. Making it
4. Cutting it
5. Measuring it

1Your water

Bottled waterLabel or analysisTap, TH and TAC

Label values, in mg/L, each with its source and date of reading. A brand may bottle several sources: read the name of the source on your bottle.

2What it is worth

Which zones

What it will do to the coffee

Zones: SCA, Standard 102-2024, table 2; SCAA, Water for Brewing Specialty Coffee, 2009, reproduced by Gagné 2018, p. 2; SCAE Water Chart (Wellinger, Smrke and Yeretzian, 2016), plate, sections 04 and 11. Diagonal: Gagné 2021, pp. 26-27.

3Making it: the salts to weigh

Start from a pure water, or from your own, and add ions: ions can never be removed Gagné 2021, pp. 33-35; plate, section 08

TargetStarting waterDistilled or demineralized, 0 mg/LReverse osmosis, about 10 mg/LMy water, from step 1

The target, adjustable

Total hardness GHppm CaCO₃Alkalinity KHppm CaCO₃Magnesium share of the hardness%of which supplied as sulfate%the rest as chlorideSodiummg/LStrength%100% = the target as is

The salts, and their form as read on the package

The rest of the bicarbonate carried byKHCO₃, potassium bicarbonateNaHCO₃, sodium bicarbonateCalcium chlorideMagnesium sulfateMagnesium chloride

Concentrate or direct dose

Concentrate, total massgsalts includedConcentrate doseg per literOr directly, forliters

How it calculates

1. The target in ppm CaCO₃ becomes ions: calcium = calcium hardness ÷ 2.497; magnesium = magnesium hardness ÷ 4.118; bicarbonate = alkalinity ÷ 0.8202, in mg/L.
2. The starting water is subtracted ion by ion; what it has in excess cannot be removed.
3. The magnesium comes from the sulfate for the share chosen, from the chloride for the rest; the calcium, from calcium chloride.
4. The bicarbonate is carried first by the sodium requested, then by the chosen salt; surplus sodium comes as sodium chloride.
5. Mass of a salt in mg/L = mass of the ion × molar mass of the salt ÷ molar mass of the ion. Molar masses: CaCl₂ 110.98, CaCl₂·2H₂O 147.01, CaCl₂·4H₂O 183.04, CaCl₂·6H₂O 219.08; MgSO₄ 120.37, MgSO₄·7H₂O 246.47; MgCl₂ 95.21, MgCl₂·6H₂O 203.30; NaHCO₃ 84.01; KHCO₃ 100.12; NaCl 58.44.
6. In a concentrate of total mass C dosed at D grams per liter, each salt weighs mg/L × C ÷ (1,000 × D) grams; the water to add is C minus the sum of the salts.

Gagné 2021, pp. 26-35; plate, sections 05, 07 and 08

4Cutting it: mixing with a softer water

Cutting divides all the ions by the same number: the hardness-to-alkalinity ratio does not change. A water too alkaline for its hardness therefore fits no zone by simple dilution.

Cutting waterShare of your water in the mix%the rest is the cutting water

Suggest a proportion

5Measuring it: from drops to ppm

A titration kit counts the drops of reagent until the color changes. Measure the alkalinity first: it weighs more on taste than hardness Gagné 2021, pp. 34-38; plate, section 10

The count

Drops at the color change, alkalinity (KH)dropsDrops at the color change, hardness (GH)dropsValue of one dropppm CaCO₃17.85 for an API kit (1 °dH)For a volume ofmL5 mL for an API kitVolume of water titratedmLtwice as much: stretched titrationOr a reading from an HI775 colorimetermg/L HCO₃⁻

Carry these measurements over to step 1

What to know about the measurements

• A TDS meter reads conductivity and translates it into total dissolved solids with a ratio of 0.5 or 0.7: about 30% error, 20% more without temperature correction, and nothing on alkalinity. It serves to see that a water has changed, or that it is properly distilled, at 25 °C.
• API kit: one drop equals one German degree, 17.85 ppm, in 5 mL. Doubling the water halves the drop; one never does better than 5 ppm or 5%.
• Beyond 400 ppm, reverse titration: a fixed volume of reagent, then drops of water until the color changes.
• HI775 colorimeter: it reads mg/L of bicarbonate; divide by 1.22 for ppm CaCO₃.

Gagné 2021, pp. 34-45; Wellinger, Smrke and Yeretzian 2017; plate, section 10

Chemistry and conversions: Gagné 2021, pp. 22-23 and 33-35; Coffee ad Astra 2018 spreadsheets; zones: SCA Water Quality Handbook, SCAE water chart; Paris water: Eau de Paris (2026)