These are a loose collection of notes about espresso brewing.

Water hardness is a function of two main components: calcium and magnesium. Hardness is typically measured in mg/L of equivalent calcium carbonate, which is found by adding together the concentration of calcium and magnesium ions with multipliers of 2.5 and 4.1, respectively. The formula is here. Typically, magnesium is only a small component, the dominant component to water hardness is calcium. The total is often called GH (general hardness).

Here is a scale of water hardness (from here.

Here are some data points of comparison.

• My area of Bath has water hardness of 300 mg/L (source

• Back in my hometown, Ottawa, Canada, the hardness is 30 mg/L (source)

• A recommended 'boiler-safe' water is Tesco Ashbeck, which has about 35-50mg/L hardness, dependent on the formula. One source is here and another is here. Current estimates are 11mg calcium and 3mg magnesium and that comes out to around 40 mg/L hardness.

Here are two clear sources:

• James Hoffman overview
• Hard water kills your coffee machine