Most people find out they live in a hard water area the way we did: by scrubbing the shower screen for the third time in a month. But you do not have to guess. Every water company in Britain publishes the hardness of the supply going into your postcode, and it takes about a minute to look up.
Find your number
Search for your water supplier's water quality page and enter your postcode. You will get a report covering hardness, chlorine residual, pH and a long list of tested substances. The number you want is hardness, usually given in milligrams per litre of calcium carbonate, sometimes in degrees Clark or French.
As a rough guide, anything above about 200 mg/l as calcium carbonate counts as hard. Much of the south and east of England sits well above that. Scotland, Wales, the north west and the south west are largely soft.
What hardness actually is
Hard water is water that has travelled through chalk or limestone and picked up dissolved calcium and magnesium along the way. It is completely safe to drink. Some studies have even suggested mild benefits from the mineral content.
The problems are practical rather than medical. Calcium and magnesium react with soap instead of lathering with it, which is why shampoo feels like it does nothing in Cambridge and foams up instantly in Glasgow. The same reaction leaves a residue on skin and hair, and the same dissolved minerals precipitate out as limescale on your screen, your taps and the inside of your kettle.
The other thing in your shower water
Hardness is not the only variable. UK water companies are legally required to disinfect mains water, and chlorine is how they do it. That is a genuine public health measure and nobody should want it removed from the mains.
What it means at your shower is a small residual chlorine concentration arriving with warm water in an enclosed room. Warmth drives chlorine out of solution and into the air, which is why a hot shower smells faintly of a swimming pool and a cold glass of the same water does not.
What a shower filter changes, and what it does not
This is the part worth being clear about, because the two get conflated constantly.
A shower filter reduces chlorine, sediment and trace metals. Media like KDF-55 and activated carbon convert or adsorb chlorine and catch the rust and grit picked up from ageing pipework. Most people notice the smell go within a shower or two.
A shower filter does not soften water. It does not remove dissolved calcium and magnesium. Softening requires a plumbed-in ion exchange unit with a salt reservoir, and it costs several hundred pounds installed. If a shower filter is advertised to you as a softener, that is a claim worth questioning.
So if your goal is to stop limescale on the taps, a filter is the wrong tool. If your goal is water that does not smell of chlorine and lathers more easily, it is the right one.
If you are in a hard water area
Two practical notes. First, cartridges in hard water areas load up faster, so plan on replacing nearer three months than six. Second, flow dropping off is not a fault, it is the cartridge telling you it is full.
More detail on the filter media is on our science page, and fitting instructions are on our installation guide.