You need three numbers, and all of them are cheap to obtain.
Hardness (GH)
Aquarium GH test strips or drop kits. They measure general hardness — calcium plus magnesium — in degrees or ppm.
Target: 50 to 175 ppm as CaCO₃, ideally 70 to 100.
Alkalinity (KH)
Aquarium KH test kits. Carbonate hardness, which is what buffers acidity.
Target: around 40 ppm, below 70.
This is the number most often wrong and least often measured.
TDS
A handheld TDS meter. Reads conductivity and reports total dissolved solids.
Target: 75 to 250 ppm.
Useful for checking a filter is working and for comparing bottled waters quickly.
Why aquarium kits
They measure exactly the parameters coffee cares about, they are accurate enough, and they cost a fraction of anything sold for coffee.
The free option
Your water utility publishes an annual quality report with hardness, alkalinity and TDS by postcode. It is accurate for the supply and will not reflect your building’s plumbing or a softener.
Testing what matters
Test the water as it reaches your brewer — after any filter, from the tap you actually use.
What to do with the numbers
Hardness low, alkalinity low — add minerals, or use a harder bottled water. Hardness high, alkalinity high — filter or use RO plus minerals. Alkalinity high, hardness moderate — the commonest case. Filter.
Last reviewed 8 August 2026