Coffee is 98 percent water. If your water is wrong, no grinder, machine or recipe will save the cup. Hardness and alkalinity are the two numbers worth knowing, and they do different things.
The two numbers
| Measure | What it is | Target | Too high | Too low |
|---|---|---|---|---|
| Hardness (CaCO3) | Calcium and magnesium | 50 to 100 mg/L | Scaling, muted flavour | Hollow, thin cup |
| Alkalinity (HCO3) | Bicarbonate buffering | 30 to 50 mg/L | Flat, dull, no acidity | Sharp, sour, unstable |
| pH | Acidity of the water | 6.5 to 7.5 | Rarely an issue | Corrosive |
| TDS | Total dissolved solids | 75 to 250 mg/L | Scaling, machine damage | Poor extraction |
The SCA spec uses 68 mg/L hardness and 40 mg/L alkalinity as its reference water. Those numbers are a good target, not a rule.
Hardness: body and extraction
Magnesium and calcium bind to flavour compounds during extraction and pull them into solution. More hardness means more body and sweetness, up to a point. Past roughly 150 mg/L the cup starts tasting chalky and the machine starts scaling.
Magnesium specifically improves perceived sweetness and fruit notes; calcium is more associated with body and is the one that scales. If you are building water, magnesium sulphate and calcium chloride are the two you reach for first.
Alkalinity: the flavour killer
Bicarbonate neutralises the acids in coffee. That sounds fine until you realise those acids are most of what makes a light roast taste like something.
- Above 100 mg/L alkalinity: bright coffees go flat and dusty. People compensate by grinding finer, which makes them bitter instead.
- 30 to 50 mg/L: the sweet spot. Acidity is present and readable without being sharp.
- Below 20 mg/L: acidity turns aggressive, and espresso can taste sour even at a normal ratio.
In most hard-water cities the alkalinity is the real problem, not the hardness. London tap water sits around 200 to 250 mg/L alkalinity, which is why coffee brewed there tastes muted no matter what you do.
Reading your water report
Your utility publishes one annually. Look for:
- Total hardness in mg/L as CaCO3, or in dH or grains per gallon. 1 dH = 17.8 mg/L; 1 gpg = 17.1 mg/L.
- Alkalinity or bicarbonate, in mg/L as CaCO3.
- Chlorine or chloramine. Both need removing before brewing. Chloramine needs a catalytic carbon filter, not a standard one.
If you cannot find it, a $10 aquarium test kit will get you within 20 mg/L on both numbers, which is close enough.
Fixing it
| Problem | Cheapest fix | Notes |
|---|---|---|
| Water too hard | Dilute 50/50 with distilled | Predictable, costs about $1 per litre |
| Alkalinity too high | Dilute with distilled, or add a drop of acid | Dilution is safer; acid is easy to overshoot |
| Water too soft | Add minerals (Epsom salt, baking soda) | Start at 0.1 g per litre of each |
| Chlorine taste | Carbon filter pitcher | Cheap, effective, needs regular cartridge changes |
| Chloramine | Catalytic carbon or RO | Standard carbon filters do not remove it |
Reverse osmosis and re-mineralising
RO gives you essentially pure water, which is too soft to brew with. The standard approach is to add minerals back: roughly 0.5 g/L Epsom salt for magnesium, 0.2 g/L baking soda for alkalinity, and a pinch of calcium chloride for body. Weigh the salts, do not eyeball them; a 0.1 g error in a litre is a meaningful swing.
For espresso machines, RO plus re-mineralisation is the best long-term answer because it eliminates scale entirely. For filter brewing, dilution is enough and much less work.
Test this before buying a new grinder
If your coffee tastes flat and you have already tried grinding finer, changing beans and adjusting temperature, check your alkalinity. It is a five minute test and it explains more disappointing cups than any equipment upgrade.