What do dishwasher salt and rinse aid actually do?
Two completely different jobs, and neither is a cleaning agent — which is why leaving them out produces specific, recognisable faults rather than simply dirtier dishes.
Dishwasher salt. Not for cleaning at all. It regenerates the water softener built into the machine.
How it works. The dishwasher contains an ion-exchange resin that removes calcium and magnesium — the minerals causing hardness — from incoming water. The resin becomes saturated and must be flushed with a strong salt solution, which displaces the hardness minerals and restores it. The salt never touches your dishes; it goes into the softener compartment and is used to recharge the resin.
Why it matters: without softening, hard water leaves limescale on the heating element, reducing efficiency and eventually causing failure, and produces white deposits on glassware.
It must be dishwasher salt — coarse, very pure sodium chloride. Table salt is not a substitute: additives, anti-caking agents and iodine can damage the resin and the machine.
Set the hardness level on the machine to match your water supply, which most people never do. Your water company publishes the figure.
Rinse aid. A surfactant that reduces the surface tension of water.
What that achieves: water sheets off surfaces rather than forming droplets. Droplets dry in place and leave their dissolved minerals behind as spots and film; a sheeting film runs off, taking the minerals with it.
It also speeds drying, because less water remains — which matters considerably in machines relying on residual heat rather than a fan.
Why "all-in-one" tablets are not a complete substitute. They contain salt and rinse aid components, and in hard water areas the amounts are frequently insufficient. The common symptom is a machine using tablets alone producing cloudy glasses and scale — adding separate salt and rinse aid resolves it.
Machines with a water softener still need salt even when using tablets, and switching off the salt indicator because you use tablets is a common error.
In genuinely soft water areas, salt may be unnecessary — check your hardness figure rather than assuming.