Hard water and dishwasher tablets

Hardness is dissolved mineral, mostly calcium and magnesium picked up as water passes through rock. The US Geological Survey classifies it by weight of calcium carbonate equivalent: 0 to 60 mg per litre is soft, 61 to 120 moderately hard, 121 to 180 hard, and anything above 180 very hard 1. Milligrams per litre and parts per million are the same figure under two names.

American utilities usually add a second unit. A grain per gallon is one grain, 64.8 milligrams, of calcium carbonate dissolved in one US gallon, which works out at 17.1 parts per million 2. German law uses a third: water suppliers must state hardness in millimoles of calcium carbonate per litre, with soft below 1.5, medium from 1.5 to 2.5 and hard above 2.5 3, and German utilities publish the same bands in degrees of German hardness, where 1.5 millimoles is 8.4 °dH and 2.5 is 14 °dH 4.

The words do not travel. German law calls anything below 150 parts per million soft 3 4, while the USGS calls 121 to 180 hard 1, and Thames Water, reporting in mg per litre of calcium carbonate, treats up to 100 as soft and above 300 as very hard 5. The same water can be soft under one authority and hard under another, so take the number and ignore the adjective.

In the United States the figure comes from the utility, and many publish it by area. Scottsdale, which prints both units, lists 16 to 18 grains per gallon north of Chaparral Road and 22 to 25 grains south of Indian School Road 2. The USGS map gives a regional impression: enough to know whether the question matters, not enough to set a machine 1.

In Germany the supplier has to tell you. Under the detergents act, water companies must inform consumers of the hardness range at least once a year, and again whenever it changes other than temporarily, by sticker or a similarly effective method 3. In Britain the water company publishes it, usually as mg per litre of calcium carbonate and usually by supply zone 5.

A test strip is the fallback, with limits worth knowing: it gives total hardness at the tap on the day you dipped it, not the split between calcium and magnesium, and not the carbonate hardness that governs scale.

Two mechanisms, and both are made worse by heat. The first is scale. Calcium hydrogen carbonate is readily soluble; heat the water and it converts to calcium carbonate, which is not, and the result is the hard white coating on the element and in the pipework 6. The USGS puts the same thing plainly: heat hard water and solid deposits of calcium carbonate can form 1. A dishwasher heats its water on every cycle by design.

The second is that the hardness ions get in the way of the chemistry before it can work. Calcium reacts with soap to make the scum familiar from a bath 1, and in a dishwasher the equivalent shows up as film on glass and spotting on cutlery. Every milligram of calcium and magnesium in the fill water is dealt with by something in the formulation, or it ends up on the load.

Machine makers treat this as a durability question, not a cosmetic one. Whirlpool states that extremely hard water mineral deposits, which it puts at 15 grains per US gallon or more, can damage the dishwasher, and strongly recommends a water softener at that level 7. That is roughly 257 parts per million, well inside the USGS very hard band.

A builder takes hardness out of play before the surfactants and enzymes meet the plate, by one of two routes. It can precipitate the calcium so it leaves solution somewhere harmless, which is what sodium carbonate does while also holding the wash alkaline. Or it can sequester it, wrapping the ion in a molecule that keeps it dissolved and unreactive, which is what a chelant such as sodium citrate does. BASF, which sells chelants into this category, describes them as softening water by sequestering and precipitating calcium and magnesium ions 8.

For decades the builder that did this best was sodium tripolyphosphate, and it has been legislated out of consumer product on both sides of the Atlantic. In the EU, a consumer automatic dishwasher detergent may not be placed on the market if total phosphorus reaches 0.3 grams in the standard dosage, in force from 01/01/2017 9. In the United States it went state by state: Washington banned the sale of dishwashing detergent containing 0.5 per cent or more phosphorus by weight from 01/07/2010, with commercial and industrial products exempt 10.

What replaced phosphate splits into two camps. The strong modern chelants, methylglycinediacetic acid and glutamic acid diacetate, bind calcium at low dose. Citrate and carbonate are milder, cheaper and easier to justify on an ingredient list. BASF’s own comparison of automatic dishwashing formulations makes the trade explicit: the formula relying on sodium citrate alone carries the most chelating agent by weight of any in the set 8. Gentler chemistry is not free, it is paid for in dose.

A built-in water softener is an ion exchange resin. Hard water passes through it, calcium and magnesium swap places with sodium held on the resin, and softened water goes to the spray arms. The resin fills up, and regenerating it takes a strong brine, which is the entire purpose of dishwasher salt. Machines that have one carry a screw-capped reservoir in the floor of the tub, under the lower rack 11.

This is the single most useful thing to know if you are shopping for tablets in the United States, because the compartment is standard on European machines and far from universal elsewhere. Whirlpool says it plainly: not all dishwashers have a salt reservoir, and only salt formulated for dishwashers should go in one, because table, sea, rock and de-icing salts carry anti-caking agents and impurities that can clog the softener 11.

We looked on 21/08/2026 for a manufacturer statement covering the whole US market and did not find one: some machines sold there do have a softener and a matching salt accessory, and many do not. So the check is physical rather than editorial. Pull the lower rack, look at the floor of the tub, and find the screw cap or fail to. If there is no cap, dishwasher salt has nowhere to go, and tipping it into the detergent compartment does not substitute.

Where there is no reservoir and the water is genuinely hard, the softening has to happen upstream of the machine, which is what Whirlpool’s 15 grain line is pointing at 7. That is a plumbing decision with a running cost, not a detergent decision, and it is the honest answer rather than a satisfying one.

Rinse aid is a surfactant released in the final rinse. Whirlpool describes it as changing the surface tension of water so it runs off the load rather than beading, which speeds evaporation during the dry cycle and limits the mineral deposits left on glass and stainless steel 12. On hard water that is a visible difference, and the dispenser is adjustable for a reason.

What it does not do is remove hardness. No calcium leaves the water because rinse aid is present; it changes where the calcium finishes, in the drain rather than in a drying droplet. It does nothing about scale on the element, because that forms in the wash, not the rinse. Rinse aid is a finishing step, not a substitute for a builder or a softener.

Say it straight: a tablet built on sodium carbonate and sodium citrate has less hardness-binding power per gram than one built on a strong chelant or, historically, on phosphate 8. In soft water that difference is invisible, because there is little to bind. In hard water it is the whole of the difference, and the reader most likely to be disappointed by a low-tox tablet is the one with the hardest supply.

None of which is a reason to avoid one, and there is no verdict here on cleaning: nothing on this site is ranked on how clean the dishes come out, because we have not washed any. It is a reason to change the conditions rather than the expectations. Fill the rinse aid and turn the setting up 12. Fill the salt if there is a reservoir, with dishwasher salt only 11. Use the hotter cycle where the machine offers one, descale on the schedule in the manual, and load so the spray reaches the glass.

If the supply sits at the top of the scale, treat the water rather than the symptom, because at that point every product in the category is fighting the same fill. The number from your utility is the first thing to find and the last thing to argue with.