You cannot filter hard water in the ordinary sense, because hardness is not a particle. The USGS describes it as dissolved calcium and magnesium, which passes straight through sediment and carbon filters the way salt passes through a coffee filter. Removing it takes ion exchange or a membrane, and those are different machines with different jobs.
What hardness is, and how much you have
Hardness is dissolved calcium and magnesium, reported either as milligrams per liter of calcium carbonate or as grains per gallon. The USGS classification puts soft water at 0 to 60 mg/L as calcium carbonate, moderately hard at 61 to 120, hard at 121 to 180, and very hard above 180. Get your own number before buying anything: a test strip, a titration kit or, on a municipal supply, the annual water quality report from your utility. Every decision that follows depends on that figure and on how much water your household uses each day.
Why sediment and carbon filters do nothing here
A sediment cartridge strains out particles by size. A carbon block adsorbs chlorine and organic compounds onto a vast internal surface. Neither mechanism has any grip on a dissolved calcium ion, which is far smaller than the pore structure and chemically uninterested in carbon. This is why people install a whole-house filter, notice the chlorine smell is gone, and are baffled that the kettle still furs and the glasses still spot. The filter is working. It was never the tool for this job.
Ion exchange: the actual answer for a whole house
A softener passes water through a bed of resin beads charged with sodium or potassium and trades those ions for the calcium and magnesium. The hardness leaves the water and sits on the resin until a brine wash strips it to drain. This is the only common technology that treats every fixture at household flow rates, which is why it is what you fit when the complaint is scale in the water heater, crusted shower heads, spotted dishes and stiff laundry. Size it to your measured hardness and daily usage rather than to a headline capacity.
Reverse osmosis and template-assisted crystallization
Reverse osmosis does remove hardness along with most other dissolved solids, but it works at a trickle and is a drinking water solution, not a whole-house one. Salt-free conditioners using template-assisted crystallization take a different approach: the calcium is converted to microscopic crystals that are far less inclined to stick to pipe walls and heating elements, so scale is reduced while the minerals remain in the water. Your soap still will not lather, and a hardness test will still read hard, because nothing was removed. Decide which outcome you want before you shop.
Matching the fix to the symptom
Scale inside the water heater and on the elements of dishwashers and kettles argues for softening or conditioning at the main. Spotting on glassware alone can often be handled at the dishwasher with rinse aid and its internal softener. Dry skin and lifeless hair after showering point at a softener, since shower cartridges do not remove hardness in any lasting way, as our hard water shower filter guide explains. Bad taste alongside hardness is usually chlorine, and that is a carbon job you can do separately.
Questions people ask about filter hard water
Will a whole-house filter soften my water?
No. Sediment and carbon filters leave dissolved calcium and magnesium untouched, so scale and spotting continue exactly as before.
Does a pitcher filter remove hardness?
Most do not. Some contain a small ion exchange resin that reduces hardness in a pitcher's worth of water, which helps a kettle but does nothing for the plumbing.
Is a salt-free conditioner the same as a softener?
No. A softener removes hardness minerals through ion exchange. A conditioner leaves them in the water in a form less likely to form hard scale, so a hardness test still reads hard.
Can I just descale appliances instead?
You can, and on moderately hard water that is often enough. Very hard water rebuilds scale quickly, and the water heater is the one appliance you cannot easily descale yourself.