Hard water cannot be filtered, in the sense people usually mean. Filtration works by straining particles or adsorbing molecules onto a surface, and hardness is dissolved calcium and magnesium that passes through both mechanisms untouched. That is why a household that buys a filter for limescale sees no change, and why the useful answer starts by naming what actually works.
Why filters do nothing here
A sediment cartridge stops particles above its rating, and dissolved minerals are not particles. Activated carbon adsorbs organic molecules and chlorine onto its internal surface, and calcium and magnesium ions offer it nothing to hold. Ultraviolet changes no chemistry at all. So a filter that is genuinely working, removing chlorine taste and sediment exactly as specified, will leave a hardness test reading exactly what it read before, and the kettle will fur at the same rate.
Removal: ion exchange, or a membrane
A softener holds resin that swaps calcium and magnesium for sodium as water passes, treating the whole house so every tap, shower and appliance benefits, and it is the only common domestic technology that removes hardness from the entire supply. At a single tap, reverse osmosis rejects hardness along with most dissolved material, and nanofiltration rejects it specifically. Both are genuine removal. The USGS classifies water as hard at 121 to 180 mg/L as calcium carbonate and very hard above 180, which is where these become worth fitting.
Management: scale control
Template assisted crystallization and sequestering devices leave the minerals in the water and reduce their tendency to deposit on hot surfaces. They protect the water heater and the pipework, need no salt, no drain and no power, and they change nothing about lather, laundry or a hardness test. That is the right trade for a household whose concern is equipment and whose site cannot take a softener, and the wrong one for a household that wants the softened water experience.
Living with it
In the moderately hard band, plenty of households treat nothing and simply descale the kettle occasionally and squeegee the shower screen. Mild acid dissolves deposits, so vinegar or a commercial descaler with contact time is the method, avoiding acid on natural stone which it etches. That is a rational choice rather than a compromise. If the reason for searching was a shower experience, our hard water shower filter guide covers what a point of use device can and cannot change.
Questions people ask about how to filter hard water
Is there a filter that removes hard water?
No. Hardness is dissolved, so it passes through sediment and carbon filters untouched. Removal needs ion exchange or a membrane.
Will a whole house filter help with limescale?
No. It will remove sediment and chlorine as specified and leave a hardness test reading exactly the same.
What are the actual options?
A softener for the whole house, reverse osmosis or nanofiltration at a tap, or scale control that manages deposition without removing anything.
Is doing nothing reasonable?
In the moderately hard band, often yes: occasional descaling with mild acid and drying surfaces keeps it manageable.