Hard Water, Laundry and Dishwashers: The No-Guesswork Guide
How calcium and magnesium affect detergent, spotting and limescale — and how to adjust without automatically doubling the dose.
Founder & Editor, Mother's Toolkit
The short version
- Hard water uses up some cleaning capacity
- Check your local hardness and appliance settings before adding more product
- Salt, rinse aid, descaling and detergent solve different problems
If glasses emerge cloudy and towels feel stiff, detergent may not be the only suspect. Hard water contains more dissolved calcium and magnesium. These minerals can reduce detergent efficiency and form limescale when heated.
Start with your postcode, not a guess
UK water companies publish hardness information. Find the figure, then set your dishwasher softener and dose according to the appliance and product instructions. Randomly adding twice as much detergent can create residue without solving the mineral problem.
In the washing machine
- Use the hard-water dose on the pack
- Do not overload the drum
- Measure rather than pour
- Descale according to the machine maker
- Distinguish rough mineral feel from detergent residue by changing one thing at a time
In the dishwasher
Dishwasher salt regenerates the machine's built-in water softener; rinse aid helps water leave the surface; detergent removes soil. An all-in-one product may combine functions, but very hard water can still require separate salt and the correct machine setting.
Sources and further reading
This guide draws on the following medical, regulatory and technical sources.
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