Hard water is not established as a cause of dandruff. What it reliably does is leave residue on the scalp and hair, which can make flaking, itching and dryness harder to control and easier to mistake for dandruff. Separating the two matters, because the fix is completely different depending on which one you have.
What hard water is doing to your scalp
The USGS defines hardness as dissolved calcium and magnesium, reported as calcium carbonate: soft up to 60 mg/L, moderately hard 61 to 120, hard 121 to 180 and very hard above that. Those minerals react with soap to form an insoluble curd that does not rinse away cleanly. On skin and hair that residue sits where it lands, which is why hard-water households describe hair that feels coated, dull and stiff even straight out of the shower.
Residue and flaking are not the same thing
Classic dandruff is an ongoing scalp condition, usually linked to yeast that lives on the skin, and it responds to medicated shampoos rather than to plumbing. Mineral residue and a dried-out scalp produce flakes too, but the flakes are typically finer, worse right after washing, and they improve when the rinse improves. If a medicated shampoo has never made any difference, water is worth suspecting.
Testing the theory cheaply
Before buying equipment, run a simple experiment. Wash and rinse with bottled or distilled water for a week, or wash your hair at a friend's house in a soft-water area, and see whether the flaking and the coated feeling change. Use less shampoo than usual, since hard water tempts people to compensate with more product and more product means more residue to leave behind. A chelating or clarifying shampoo occasionally does the same job of stripping mineral buildup.
What actually fixes the water side
A shower filter reduces chlorine and some sediment, and many people find their scalp calmer for it, but a small cartridge in a shower arm does not remove dissolved calcium and magnesium in any meaningful way. Only a proper ion exchange softener changes the hardness of the water reaching the shower. Be plain with yourself about which problem you are buying for: a softener is a good purchase for scale, laundry and appliance life, and a hopeful one if dandruff is the only complaint.
Questions people ask about can hard water cause dandruff
Will a water softener cure my dandruff?
It may reduce dryness and residue, and that can improve flaking. It will not treat true dandruff, which is a scalp condition and needs a medicated shampoo.
Do shower filters soften water?
No. They target chlorine, odor and particles. Removing dissolved calcium and magnesium takes an ion exchange softener plumbed into the supply.
How do I know if my water is hard?
Look for scale on the kettle and faucets, soap that will not lather, and spotted glassware, then confirm it with a hardness test kit or a lab number in grains per gallon.