In my limited experience in the several places I've lived in the US, only one had an issue with water that was hard enough to need addressing. Most residences there had a dedicated household water softener, so even there the dishwashers didn't need to do their own softening.

Here's a nice map [1]

[1] https://homewater101.com/articles/hard-water-across-us

Another anecdote from the USA: I've only ever had problems with hard water in a house that got water from a well. Never had a problem with municipal water (although there are other contaminants in that).

Nice to be #1 in something. But that explains a lot.

From that link [1 above]: "An overwhelming 85 percent of the USA has hard water, so the likelihood that you live in state or city with hard water is high."

For a random location. Comparing the water hardness map with a population density map it seems that most are living in areas with soft water.

I'll give you an upvote for that little dive into Wikipedia. For those who haven't already swum as deep, the article on ion exchange resins[0] is also worthwhile. Turns out they are used for reprocessing spent nuclear fuel as well as softening water. Plus a few other things.

[0] https://en.m.wikipedia.org/wiki/Ion-exchange_resin

I did bad in high school chemistry, I never could understand this stuff. First, the ion exchange resin beads are washed with Na+ which bind to the beads. Second, the hard water is run through the beads and bad stuff like Mg2+ has a stronger attraction to the beads than Na+, so the Na+ is pushed off and Mg2+ binds to beads. The resulting water out of tap is softer because Mg2+ is out, but saltier because Na+ is in. Then once in a while the beads are flushed with Na+ again with the Mg2+ being sent down the drain... but how does this happen if Na+ has a weaker bond? How can it push the Mg2+ off the beads??

The concentration matters. Concentrated Na will displace Mg, but in the Na concentration in the tap water Mg will displace Na. If you had a concentrated brine with large amounts of both sodium and magnesium, the Mg would win.

Ca²⁺ is the more important ion, since limescale is mainly calcium carbonate (CaCO₃). Otherwise, the explanation is fine.

The exchange of the ions is driven be the difference in concentration between water and resin surface. It works both ways, but Ca²⁺ is preferred (=stronger bonds) by the binding groups (e.g. carboxylate salts). That's why a concentrated Na⁺-brine is needed to flush out the calcium ions.

I believe the Mg+ Ca+ have a stronger attraction to the Cl- ions in the salt so when soaked in brine they trade places with the Na+

Beads prefer Mg over Na but Mg prefers Cl over beads. So you can recharge with salt brine.

Or it might just be a concentration thing where Na / Mg ratio wants to be the same in the brine and beads. So you just give a really strong brine and most of the Mg etc leaves the beads.

Imagine the bonding as reaching an equilibrium where Mg2+ bonds and then dissociates in a repeating cycle. If you flood that solution with Na+, the chances for Mg2+ to bond after dissociating are lower when an excess of Na+ is doing the same thing, and a new equilibrium is reached.

As explained, it's the concentration. If you don't have a dishwasher with a water softener: You don't sprinkle a little salt in some compartment, it's large and filled with 1-2 pounds of salt to make a saturated brine. The salt sold for dishwashers is purified NaCl (table salt has some other stuff like anti-caking agent). That's what the dishwasher uses to drive the Ca2 and Mg2 off the resin.

"It's not common in the United States."

If US folks are looking for this feature, it can be found on higher end Bosch dishwashers in the US.

I have very hard water and a Bosch benchmark series. The water softener makes a huge difference. I don't want a whole-house softener because I don't enjoy the feel of softened water in the shower.

We had a Bosch dishwasher with a built-in water softener when we lived in Lake in the Hills, IL, a suburb of Chicago. The municipal water there (from wells) was extremely hard and it made a big difference.

We now have a Bosch dishwasher in another house in NY state. The water is not hard (it's from one of the Finger Lakes), and the dishwasher does not have a softener, but we like the brand. After reading this story we looked up how to tweak the rinse aid dispenser and turned it down from "5" to "1".

Why don't you like the softener water? I thought it was better for the skin.

Softened water feels slimy to me - similar to seawater.

I actually do have a whole-house softener which came with my house. I installed a bypass valve so it does not feed the pipes in my home.

It may be over-softening. Our water felt slimy until we replaced our old unit. The new unit uses way less salt (at least 10x less) and the water feels normal.

I believe the old ones had an adjustment, but I can confirm new units definitely do. We hardly use any salt at all.

It's easier to rinse soap off with hard water.

Everyone I've ever known with water hard enough to be a bother just installs a whole-house water softener, so all the water inside the house is already softened. Seems inefficient to soften only the dishwasher when a whole house system is pretty cheap.

How could it be less efficient to soften less water?

Why haven't United States sellers of dishwashers taken advantage of a new revenue-boosting opportunity (dishwasher salts) is the bigger question.

Fisher & Paykel dish drawers have a salt-dispenser, at least my 2004 models do. They are (were?) designed in New Zealand but sold in the U.S. For a while they were sold rebranded by KitchenAid. My water isn't hard though so I haven't had to use any salt.

(Aside, and I don't know if they are still any good, but these dish drawers are really well engineered. Mine had some problems early on that were resolved by small changes in the design, but FP provided me the parts to retrofit my washers. They've been nearly trouble free since then.)

They were bought out by Haier a few years ago, they still exist as a brand in NZ, not sure about elsewhere. Anecdotally not as good as they used to be.

Available in the UK too. I don't have experience with the drawers (that's the dream though - fridge/freezer especially, drawers for everything (below waist height) makes so much more sense to me) but I'm familiar with a couple of more recent (last couple of years, definitely post-Haier) appliances that are pretty nice. You pay for it though... Seems roughly like Skoda/VW/Audi in terms of top-end F&P branded units with nicer fit and finish than fairly similar but much cheaper Haier branded ones.

I was looking for a fridge-freezer specifically, considered Haier but went with Liebherr in the end - I like that (in terms of white goods/kitchen appliances) it only makes refrigeration units, other than where you use them there's nothing tying them together in why a manufacturer would be good at making them all really, makes sense to specialise in refrigeration, or dish washing, or gas products or whatever IMO.

The H series F&P dishwasher models have a built-in water softener. [0]

According to the user guide, a water softener is recommended if your municipal water hardness is >250ppm which would be quite hard for municipal tap water.

In my region of central Florida the city’s water quality report shows a hardness well below 250ppm, Which you can divide by 17.1 to determine how many grains of salt to take that with.

[0] https://www.fisherpaykel.com/us/dishwashing/all-dishwashers/