Rao Ideal Coffee

This isn't a city's water. It's a target profile developed by Scott Rao, one of the most influential voices in speciality coffee. His research into water chemistry and extraction led to a specific mineral composition designed to produce the best possible cup: enough hardness to extract flavour compounds effectively, with low enough alkalinity to let acidity come through.

Rao's ideal emphasises the ratio between hardness and alkalinity. The hardness (driven by calcium and magnesium) does the extracting. The alkalinity (bicarbonate) does the buffering. Too much of the latter relative to the former, and you lose brightness. His target keeps alkalinity low while maintaining enough total hardness to avoid under-extraction.

This profile is useful as a benchmark. Whether or not you agree with Rao's specific numbers, the principle behind them is sound: water for coffee needs enough minerals to extract well, but not so much buffering capacity that it kills the acids that make good coffee interesting. If you're experimenting with water for coffee, this is a well-reasoned place to start.