Water Hardness for Coffee

Water hardness is the amount of calcium and magnesium dissolved in your water. For coffee it matters twice: the minerals change how flavour is extracted, and the alkalinity that usually comes with them changes how acidic the cup tastes. Hard water also leaves limescale in kettles and machines. Here’s what the numbers mean and how to find out how hard yours is.

The short version

• Hardness = calcium + magnesium, measured as ppm (mg/L) of calcium carbonate.
• Soft is under 60 ppm, very hard over 180, by the US Geological Survey’s scale.
• The SCA asks for 50–175 ppm calcium hardness and alkalinity near 40–70 ppm.
• Alkalinity is the bigger problem in hard tap water: it flattens acidity.
• London’s water is about 275 ppm; Manchester’s about 25.

What is water hardness?

Hardness is caused by compounds of calcium and magnesium, and by a few other metals, the US Geological Survey says. Water becomes hard as it percolates through limestone, chalk or gypsum, picking up calcium and magnesium carbonates, bicarbonates and sulphates; rainwater and distilled water are soft, Wikipedia says.

There are two kinds. Temporary hardness, from bicarbonates, drops out when water is boiled, as limescale. Permanent hardness, from sulphates and chlorides, doesn’t. Limescale is mostly calcium carbonate, and in boilers it slows heat transfer and lets parts overheat.

How hard is hard?

The US Geological Survey’s classification:

Classification	Hardness, mg/L (ppm) as CaCO3
Soft	0–60
Moderately hard	61–120
Hard	121–180
Very hard	Over 180

Source: US Geological Survey.

Water companies use several units. One English degree (°Clark) is 14.254 ppm, one German degree (°dH) 17.848 ppm, one French degree 10 ppm and one grain per US gallon 17.118 ppm, Wikipedia says.

How hard is the water where you live?

In England it varies by city, depending on the source:

City	Main source	Hardness
Manchester	Lake District and Pennine reservoirs	About 25 ppm
Birmingham	Elan Valley reservoirs, Wales	About 43 ppm
Bristol	Mendip Hills	About 229 ppm
Southampton	Bewl Water	About 268 ppm
London	Lee Valley reservoirs	About 275 ppm

Source: Wikipedia, citing the water suppliers.

Drinking Water Inspectorate data show most of England, particularly east of a line between the Severn and Tees estuaries, above 200 ppm, Wikipedia reports; Wales, Devon, Cornwall and parts of north-west England are softer. In the United States, hard water is common in the east-central and western states, the US Geological Survey says, reflecting carbonate aquifers.

For your own address, ask your water supplier: the US Geological Survey points to the local utility as the best source of local data.

How does hardness change coffee?

Two things are going on, and the second matters more.

Hardness helps extract flavour. Observations cite over-extracted flavours above about 250 ppm and under-extracted flavours below about 40 ppm, though lab measurements suggest hardness changes overall extraction very little between 20 and 250 ppm, Marco Wellinger writes in the SCA’s 25 Magazine.

Alkalinity, mostly bicarbonate, neutralises the coffee’s acids. The perceived acidity of a cup is roughly the acids extracted from the coffee minus the alkalinity in the water, Wellinger says, so water high in alkalinity tastes flat and dull. Natural water tends to have similar hardness and alkalinity because both come from dissolved limestone, Jonathan Gagné notes on his Coffee ad Astra blog, so hard tap water is usually high in both. Average US tap water has about 170 ppm hardness and 150 ppm alkalinity, Wellinger says, against the SCA’s 40–70 ppm alkalinity.

What does the SCA recommend?

Measure	SCA Standard 102-2024
Calcium hardness	50–175 ppm as CaCO3
Alkalinity	At or near 40–70 ppm as CaCO3
pH	6–8
Chlorine	None

Source: SCA Standard 102-2024. The SCA water standard page compares this with the older 2009 version.

How do you fix hard water for coffee?

Option	What it does	Catch
Carbon jug filter	Removes chlorine; lowers hardness and alkalinity	Rarely reaches the ideal zone on its own (Gagné)
Softening cartridge	Lowers hardness only	Leaves alkalinity high; useful for espresso machines (Wellinger)
Ion-exchange softener	Swaps calcium and magnesium for sodium or potassium	Adds sodium (Wikipedia)
Mix with distilled water	Dilutes everything in proportion	Tap water varies with the seasons (Gagné)
Build your own water	Full control	Needs minerals and a 0.1 g scale

Sources as listed. To build water from scratch, see DIY coffee water recipes.

To check what you’ve got, aquarium test kits measure general hardness (GH) and carbonate hardness, roughly alkalinity (KH), Gagné suggests.

Water and other guides

• The SCA water standard: the targets, version by version.
• DIY coffee water recipes: make your own brewing water.
• Espresso: where water can carry more alkalinity, but scale is a risk.