Four numbers that answer different questions
A clear glass of water can contain very different dissolved minerals. The SCA's explanation of water and acidity distinguishes the buffering that changes a coffee's perceived acidity from the minerals and equipment conditions that also matter.
| Measure | What it describes | Common confusion |
|---|---|---|
| Hardness | A measure associated mainly with dissolved calcium and magnesium, often expressed as mg/L as CaCO₃. | It is not the mass of calcium alone. |
| Alkalinity | The water's capacity to neutralize added acid, often expressed as mg/L as CaCO₃. | It is not the same as pH or necessarily identical to carbonate hardness. |
| TDS | Total dissolved material; a simple meter often estimates it from conductivity. | One number cannot identify the ions present. |
| pH | An acid/base measure at the time and conditions of measurement. | Two waters with similar pH can have different buffering. |
Evidence: SCA: water and coffee acidity
Start with your supply and your manual
For tap water, find a recent local water report and check the units. The report may cover a supply zone rather than your exact tap. A suitable test can help where plumbing, a private supply or treatment changes the picture. Test strips have limited resolution; do not report more precision than they support.
A carbon filter, softener and reverse-osmosis system do different jobs. Confirm the device's actual specifications and maintenance instructions. “Filtered,” “mineral” and “distilled” do not each describe one universally suitable composition.
Taste and equipment compatibility are separate from potable-water safety. A pleasant taste or a low TDS reading does not certify microbiological safety.
Evidence: La Marzocco home-water guidance · La Marzocco water specifications
A cupping standard is not a machine warranty
SCA Standard 102-2024, Table 2, gives water guidance for comparative cupping. It specifies calcium hardness of 50–175 ppm as CaCO₃, alkalinity at or near 40–70 ppm as CaCO₃, pH 6–8, and no chlorine. Its purpose is a tasting protocol, not a universal household recipe.
Notice the word calcium. Water made with magnesium sulfate alone does not thereby meet a calcium-hardness specification. A magnesium-based experiment can be described on its own terms without claiming SCA compliance.
For an espresso machine, consult its current water limits. La Marzocco, for example, lists separate limits for several properties. Meeting a flavor-oriented range does not establish protection against scale, corrosion, sensing problems or a warranty exclusion.
Evidence: SCA 102-2024: cupping water, Table 2 · La Marzocco water specifications
Two useful next checks
Scenario A: your kettle scales rapidly and you are filling an espresso machine from the same tap. Check the local hardness and alkalinity data against the exact machine manual before choosing treatment. Descaling more often is not a substitute for checking compatibility.
Scenario B: filter coffee has an unpleasant odor, and the cold tap water has that odor too. Check the supply and any filter's condition and instructions first. Compare a small brew using another known potable water with documented composition. Keep coffee and recipe constant, so the test tells you something about the water.
If the water tastes normal but the drink is flat, do not assume you have diagnosed high alkalinity by taste alone. Check coffee freshness, dose, recipe and measured/report values together.
After checking the water itself, use the taste troubleshooting workflow to choose one brewing change at a time. Connect any scale or residue concern to the equipment cleaning plan and the exact machine’s manual before selecting a treatment.
Evidence: La Marzocco home-water guidance · SCA: water and coffee acidity
Read a mineral calculation correctly
A recipe calculator must state the starting water, the final total volume and the exact salts. Food-grade magnesium sulfate heptahydrate is MgSO₄·7H₂O; sodium bicarbonate is NaHCO₃. Different hydration forms, baking powder, washing soda and scented bath products are not interchangeable.
An arithmetic example, not a recommended profile: dissolving 0.100 g magnesium sulfate heptahydrate in a final volume of 1.000 L contributes approximately 9.86 mg/L magnesium and 40.6 mg/L magnesium hardness expressed as CaCO₃. It also contributes sulfate and no calcium. Adding 0.100 g sodium bicarbonate per final liter contributes approximately 59.6 mg/L alkalinity as CaCO₃ and 27.4 mg/L sodium.
These figures assume the stated pure compounds and ignore any minerals already in the base water. Actual ingredient purity and measurement uncertainty matter. A concentrate is easier to weigh accurately than tiny doses only when its concentration, dispensing method and final dilution are controlled; the calculator must include its water in the final volume.
The water arithmetic tool makes these ingredient forms and final-volume assumptions explicit. Read the whole result and the stock-preparation instructions before treating its numbers as a recipe.
Evidence: FAO JECFA: magnesium sulfate hydrate specification · NIST Chemistry WebBook: sodium bicarbonate · USGS: hardness calculation · USGS: alkalinity reporting units
Write down enough to repeat the result
Record water source, treatment/filter and change date, measured hardness and alkalinity with units, and the machine model. For a mineral experiment, also retain salt identities, masses, stock concentration, final volume and any starting-water assumptions.
Use the result as a taste comparison for an appropriate brewer. A calculator's arithmetic cannot approve a water source as potable, predict a machine's corrosion behavior, or guarantee an ideal cup. If the manual and your measured composition conflict, resolve that with the manufacturer before using it.
Carry the starting water into the calculation
A mineral addition to tap or bottled water has two effects: it adds ions and displaces some of the existing water when the final volume is fixed. Start with measured total hardness and alkalinity, not a conductivity-derived TDS number. Keep the measurement date and method. Municipal averages can be useful context but do not establish today’s kitchen sample.
The water calculator solves both GH and alkalinity targets simultaneously. Worked example: for a final litre, starting at GH 30 / KH 20 and targeting GH 80 / KH 40 mg/L as CaCO₃, use 52.21 ml hardness stock, 21.47 ml buffer stock and 926.32 ml starting water. The stocks themselves dilute the starting mineral content. Retest before treating the result as a measurement.
If either stock amount would be negative, those targets cannot be reached through additions alone. Blend with a measured lower-mineral potable water first, then recalculate. Equal parts GH 200 / KH 100 water and nominally zero-mineral water predict GH 100 / KH 50; the ratio of those two quantities remains coupled.
Use each machine’s published water requirements as a separate check. Our mixing tool does not establish calcium concentration, pH, chloride content, scale saturation or corrosion protection. A plausible taste experiment in a manual brewer does not approve water for a boiler.
Evidence: Barista Hustle: existing-water dilution and recipes
Sources & limitations
This guide combines source-based explanations and practical starting points. Coffee Jacob has not performed hands-on product testing or controlled tasting trials for this guide. Source check: 13 October 2026.
- SCA: water and coffee acidity
- SCA 102-2024: cupping water, Table 2
- La Marzocco water specifications
- La Marzocco home-water guidance
- FAO JECFA: magnesium sulfate hydrate specification
- NIST Chemistry WebBook: sodium bicarbonate
- USGS: hardness calculation
- USGS: alkalinity reporting units
- Barista Hustle: existing-water dilution and recipes