A cup of filter coffee is mostly water. The SCA’s brewing chart puts brewed coffee’s reference strength at 1.15–1.45% dissolved solids, which leaves 98.55–98.85% water (calculated). Even espresso, at roughly 8–12% dissolved solids by our Extraction Map’s rule of thumb, is about 90% water. So whatever is dissolved in your water is in your cup, and some of it changes how the coffee extracts.
Our video covers the same ground; it predates this revision, so where the two differ, follow the text.
Does water matter more than the beans?
We couldn’t find a study that ranks water against beans, so we don’t claim it. What the evidence does support is that water matters a lot. Jonathan Gagné, who studies coffee chemistry on his blog Coffee ad Astra, writes that water “has a significant impact on the taste profile of your cup,” in ways that don’t depend on how the water tastes by itself. In his own tasting, one coffee brewed with each of three mineral recipes tasted far better to him than the same coffee brewed with Montreal tap water.
Three numbers: hardness, alkalinity and TDS
Water reports and test kits give these three, and they measure different things.
Hardness (GH) is the amount of dissolved calcium and magnesium, as the US Geological Survey defines it. It’s reported in milligrams per liter (mg/L, the same as ppm) as calcium carbonate. The USGS calls 0–60 mg/L soft, 61–120 moderately hard, 121–180 hard, and anything above 180 very hard. In coffee, Gagné counts calcium and magnesium among the “potent mineral ions” that do the extracting, and wants enough of them to extract well, but not so much that they throw the flavor off balance. Exactly how the two differ is less settled than it sounds: he writes that they’re believed to bring out slightly different flavors, but lab tests hadn’t confirmed it when he wrote.
Alkalinity (KH) is water’s capacity to neutralize acid, mostly from bicarbonate. It’s also given in mg/L as calcium carbonate. It buffers the coffee’s acids: Gagné notes that high alkalinity reacts with the aromatic acids extracted from the beans and can mask some important flavors. As our Water Recipe Builder puts it, too little and a cup can taste sharp; too much and it can taste flat. Scott Rao found the effect isn’t a straight line: in one test with the same coffee, cups at 10 and 70 mg/L tasted better to him than those at 30 and 50.
Total dissolved solids (TDS) is everything dissolved in the water: calcium and magnesium, but also sodium, chloride, sulfate and more, which count toward TDS without being hardness or alkalinity. A handheld TDS meter doesn’t actually count any of them. It measures how well the water conducts electricity, which the USGS describes as a way to estimate dissolved solids, and converts that to a TDS figure using assumptions about what’s in the water. Gagné doesn’t trust the TDS readings themselves. A TDS number can’t tell you the hardness or the alkalinity.
An example. The Las Vegas Valley Water District’s 2026 water quality summary gives these averages:
| Measure | Las Vegas average | SCA target |
|---|---|---|
| Hardness | 280 mg/L (very hard) | 68 mg/L calcium hardness |
| Alkalinity | 137 mg/L as bicarbonate, about 112 as calcium carbonate | 40 mg/L |
| Total dissolved solids | 591 mg/L | 150 mg/L |
The alkalinity conversion is calculated (bicarbonate × 0.82). The TDS is about twice the hardness because the water also carries sodium (89 mg/L), chloride (99) and sulfate (209). All three numbers are well above the SCA’s targets, and the TDS is beyond the top of its acceptable range, 250 mg/L.
The SCA’s water standard
The Specialty Coffee Association’s Coffee Standards (revised 2018) sets these targets for brewing water:
| Characteristic | Target | Acceptable range |
|---|---|---|
| Calcium hardness | 68 mg/L | 17–85 mg/L |
| Alkalinity | 40 mg/L | At or near 40 mg/L |
| Total dissolved solids | 150 mg/L | 75–250 mg/L |
| pH | 7.0 | 6.5–7.5 |
| Sodium | 10 mg/L | At or near 10 mg/L |
| Chlorine | 0 mg/L |
It also asks for water that’s clear and free of odor. Few tap waters meet all of it, which is why people blend, filter or mix their own. Our guide to coffee water recipes compares the SCA standard with eight other recipes and shows how to mix them.
Chlorine and chloramine
US water utilities usually disinfect with either chlorine or chloramine, a compound of chlorine and ammonia, and some switch between them during the year, according to the CDC. Water with chlorine in it can taste and smell different, the CDC notes, and the SCA’s standard asks for no chlorine in brewing water.
The two behave differently. Chlorine leaves water that’s left to stand for a few days, the CDC notes; chloramine doesn’t. The EPA says boiling doesn’t remove chloramine, letting water sit at room temperature doesn’t, and reverse-osmosis filters don’t. Some home filters do; the EPA suggests looking for units certified under the NSF/ANSI 42 standard for chloramine reduction. The EPA also says chloramine-treated water that meets its standards is safe to drink and doesn’t need removing, so this is about taste.
Your utility’s annual water quality report says which disinfectant it uses. If the report is out of date or the utility switches, ask it.
Finding your water’s numbers
- Your water utility’s report. The annual water quality report lists hardness, often alkalinity and TDS, and the disinfectant. Gagné suggests watching for seasonal changes if you blend tap water.
- A test kit. Barista Hustle prefers measuring your own water, because a city’s water can vary from one area to another. Aquarium GH and KH test kits read in degrees (dGH, dKH); multiply by 17.9 for mg/L, as the Water Recipe Builder does.
- A bottle label. If it lists minerals in mg/L, hardness as calcium carbonate is about 2.5 × calcium + 4.1 × magnesium, and alkalinity about 0.82 × bicarbonate (calculated from the molar masses).
What to do about it
Start with what’s wrong, and change one thing.
- The water smells or tastes of chlorine. Use a filter certified under NSF/ANSI 42 for chlorine; if your utility uses chloramine, choose one certified for chloramine reduction, as above.
- The water is very hard or very alkaline, like the Las Vegas example. Blend it with distilled or reverse-osmosis water, or start from distilled water and add minerals to a recipe. Brewing-water recipes, including Barista Hustle’s and the ones Gagné publishes, start from distilled water for that reason.
- You use a filter pitcher. Gagné found that a Brita pitcher lowers both hardness and alkalinity, but rarely brings typical tap water into a good range for coffee.
- You want to know what your tap water needs. Enter its hardness and alkalinity in the Water Recipe Builder. It shows the nearest named recipe, and how much of your water to use with distilled water and two mineral stocks to reach a target.
Espresso machines
An espresso machine’s maker sets the water it needs: check its water specification before anything else. Hard water leaves scale when it’s heated. The USGS notes that calcium carbonate deposits form in heated hard water and can shorten the life of equipment. Gagné names scale and corrosion as the two concerns for espresso machines’ internal parts. Barista Hustle warns against putting remineralized water in an espresso machine unless you’re very sure what you’re doing. Our coffee water recipes guide covers chloride and the recipes made for machines, and our espresso machine cleaning guide covers descaling.
Water matters for more than coffee: jayarr.pizza has a guide to water for pizza dough.
Sources
- Jonathan Gagné, “Water for Coffee Extraction,” Coffee ad Astra, December 16, 2018, updated November 12, 2019: coffeeadastra.com
- Specialty Coffee Association, Coffee Standards, revised 2018, §4.1, “Water for Brewing Standards,” p. 12 (the printed table’s rows are misaligned, so its chlorine target of 0 mg/L is our reading of the layout); and the SCA Coffee Brewing Control Chart, revised March 2019 (reference strength 1.15–1.45%)
- US Geological Survey, Water Science School, “Hardness of Water” and the Water Science Glossary (“alkalinity,” “specific conductance”): usgs.gov, glossary
- Las Vegas Valley Water District, 2026 Water Quality Summary (averages), checked October 15, 2026: lvvwd.com (PDF)
- Centers for Disease Control and Prevention, “About Water Disinfection with Chlorine and Chloramine,” February 14, 2024: cdc.gov
- US Environmental Protection Agency, “Common Health Questions Related to Monochloramine,” question 29, “How can I remove monochloramine from my drinking water?” (February 2009): epa.gov (PDF)
- Scott Rao, “Lotus Water Drops: How to use them, and a great hack to find the best water for your coffee,” December 1, 2022 (the alkalinity test): scottrao.com
- Barista Hustle, “DIY Water Recipes Redux,” 2019, the espresso-machine warning and the staff reply on measuring tap water (June 28, 2023): baristahustle.com