You moved to a new city and your hair changed. It feels stiffer, the shampoo barely lathers, and a conditioner you've used for years has stopped doing much. Then a week at your parents' place, or one hotel shower in another city, and your hair is suddenly soft again.
The obvious suspect is the water, and it's a reasonable one. Hard water carries dissolved calcium and magnesium, and those minerals do end up on your hair. How much harm they do once they're there is a far less settled question than the internet suggests.
What the evidence shows, and where it splits
The best single summary is a 2025 systematic review of nine human studies. Calcium and magnesium deposits were clearly higher on hair washed in hard water, and tensile strength (how much force a strand takes to pull apart) dropped slightly. The reviewers rated the certainty of that strength loss as moderate, and the certainty of everything else as very low. Their conclusion was that hard water probably increases breakage, and that better real-world studies are needed.[1]
The individual results explain the caution. One soaked shed hair from 15 women in hard water for 10 minutes every other day for 30 days, and found no difference in strength or elasticity compared with distilled water.[2] Another, using hair from 70 men, found hard water did reduce strength.[3] A third, with 20 volunteers, found more magnesium on hard-water hair but no significant difference in calcium, and no visible surface damage under an electron microscope.[4]
Much of this work treats cut or shed hair in a lab, away from any scalp or normal wash routine. It tells you what minerals can do to a fiber. It can't tell you what your shower does to your head.
Not every effect is a bad one, either. In controlled lab work, minerals stiffened both virgin and bleached hair, and they improved style retention in virgin hair while slightly reducing it in bleached hair. Combing force actually went down, which the researchers put down to that same stiffening.[6] The 2025 review reports combing forces going up, so even that result isn't settled.[1]
Your hair's condition may matter more than your water
The most useful finding in this field gets the least attention. When researchers tested what controls how many minerals hair picks up, the condition of the hair was the biggest factor. Water hardness itself had only a small effect: damaged hair picked up notable amounts of minerals after repeated exposure even to soft water. Water pH mattered too, with more alkaline water driving more uptake.[5] That was one lab's work, and the 2025 review did find deposits rising with hardness,[1] so softer water isn't irrelevant. It's just not the only lever, and possibly not the biggest.
The reason is chemistry. Damage leaves more negatively charged sites on the fiber, and calcium and magnesium are positively charged, so they have more places to bind. This is why the problem so often shows up after bleaching or coloring, and why a global study of over 300 women in nine countries found hardness minerals significantly higher on the hair of those who colored it.[7]
That study also found calcium, magnesium and copper rising from root to tip, and that women who rated their hair health as poor had higher metal levels.[7] That's a correlation, and it can't show which came first. It does fit the pattern though: the oldest, most worn part of the strand holds the most.
Minerals can also pair up with what's already on your hair. Calcium combines with fatty acids from natural sources such as sebum (the scalp's own oil) to form deposits between the cuticle layers, the overlapping scales on the strand's surface. These can grow large enough to make the structure bulge, changing how hair looks and behaves.[8] If you've ever noticed hair that feels coated despite being freshly washed, that's a plausible explanation, though nobody has directly tied those deposits to the sensation.
Hard water also seems to interfere with washing itself. In human skin studies, more surfactant (the cleansing agent in shampoo) stayed behind after washing in harder water.[9] That was skin rather than scalp or hair, so it's a reasonable inference and nothing more.
What removes minerals
Ordinary surfactants lift oil and dirt. Minerals bound to hair are a different job, and that's where chelators come in: ingredients that grip metal ions and hold them so they can rinse away.
Citric acid has the strongest hair-specific data we found. In a lab study on chemically treated hair, it reduced calcium by 21% to 42% and raised the number of repeated stretch cycles hair survived by 56% to 124%.[10] The authors attribute that to several things at once, including chelation and the treatment's pH, so it isn't a pure chelation effect, and the result depended on how damaged the hair already was. It also wasn't a retail shampoo.
The evidence for chelators in shampoo is thinner. One study showed that a shampoo containing the chelator EDDS reduced how much copper built up on hair.[11] Copper matters because it worsens sun damage, and Roots' own ingredient research found chelators in general are better supported for managing calcium and magnesium in the wash than for protecting hair from oxidative damage.
Here's the gap: we found no controlled study of a retail chelating shampoo on people's hair showing a lasting difference in how it feels or breaks. The chemistry is sound and it's a sensible thing to try, but anyone promising a transformation is ahead of the data.
On a label, look for names like disodium EDTA, tetrasodium EDTA, sodium phytate or citric acid. Don't dismiss them for sitting near the bottom of the list, since EDTA does its job at around 0.05% to 0.2%. Our research also flags two cautions. In conditioners and leave-ins, chelators are mostly there to keep the formula stable, so the shampoo is where they count. And EDTA can help other ingredients through the skin, which is worth bearing in mind if your scalp is sensitive and the formula is heavily fragranced. The ingredient list guide explains how to read the rest.
“Water hardness turned out to matter less than expected. Damaged hair picked up notable amounts of minerals even from soft water.”
What to do about it
Start by finding out whether this applies to you. Your water supplier usually publishes your local hardness, often online by postal code. If your water is soft, look elsewhere.
If it's hard, try a chelating or clarifying wash every few weeks and judge it on your own hair. Clarifying too often has its own costs, and the clarifying and buildup guide covers how to pace it and how to tell mineral buildup from product buildup.
Protect the hair that absorbs the most. Since damage drives uptake,[5] anything that limits bleaching and heat also limits how much your hair takes on. Keep up your conditioner too. One imaging study reported that coconut oil applied beforehand, or conditioner, prevented calcium pickup,[12] which is promising but far from proven.
Shower filters and softeners are where the evidence runs out. We found no controlled study testing them on hair, and in trials on people with eczema, water softeners didn't significantly improve objective severity,[9] which is a different question but a useful reminder that softer water isn't an automatic fix. Before spending on one, rule out the cheaper explanations: an old routine, buildup, or ends that are simply worn. The guide to hair concerns helps you separate them. If your hair feels dry and your roots are oily at the same time, the oily roots and dry ends guide may explain more than your water does.
Water is one variable
You can't choose what comes out of your tap, but it's rarely the whole explanation. Hair that reacts badly to hard water is usually hair that's already porous or processed, and that's a question about your hair as much as about your water.
The Roots quiz looks at damage, porosity and scalp together and matches products at the ingredient level, so you can see which cleansers fit the hair you actually have. If you haven't taken it, it's a good way to find out how much of this is the water.