How to tell if you have hard water
Before you buy anything, confirm the problem. Two of these four checks cost nothing and one of them gives you a number you can act on.
The quickest reliable check is your utility's annual water quality report, which publishes hardness for your supply. A soap-shake test in a bottle gives you a free yes-or-no in two minutes. A test strip gives you grains per gallon. Anything above about 7 gpg is hard, and above 10 gpg is very hard.
1. The soap test — free, two minutes, yes or no
- Fill a clear bottle about a third with cold water straight from the tap.
- Add a few drops of plain liquid soap — not detergent, not anything with added softeners.
- Cap it and shake hard for a few seconds.
- Soft water gives a thick head of suds and clear water beneath. Hard water gives thin, quickly-collapsing bubbles and cloudy, milky water.
The cloudiness is the actual chemistry happening in front of you: calcium and magnesium reacting with the soap's fatty acids to form insoluble scum. That is the same reaction that is consuming your shampoo in the shower and leaving a film on your skin.
2. Your utility's report — free, four minutes, definitive for municipal supply
Every community water system in the United States must publish a Consumer Confidence Report annually. Hardness is usually reported in milligrams per liter as calcium carbonate, or sometimes in grains per gallon. The same document tells you which disinfectant your utility uses, which is the other thing you need before buying a filter — so this is two answers for one search. How to find and read yours.
3. A test strip — a few dollars, and it gives you a number
Hardness test strips are inexpensive and adequate for this purpose. Dip, wait, compare against the color chart. They are less precise than a lab titration and precise enough to tell 5 gpg from 15 gpg, which is the distinction that changes your decision. The advantage over the utility report is that it measures the water arriving at your fixture, after whatever your building's plumbing has done to it.
4. A lab test — the definitive version
If you are on a private well, this is not optional and hardness is the least of it. Wells are not covered by the utility reporting requirement, nobody is testing yours but you, and the things worth knowing about — coliform bacteria, nitrate, arsenic, iron — are not things a strip will tell you. The EPA's private well guidance sets out what to test for and how often. Well water and shower filters.
Reading the numbers
The conversion is roughly 17.1 mg/L per grain per gallon. Utilities report in either unit, so check which one you are reading.
The signs that are not hardness
Several complaints get filed under "hard water" that are something else, and the distinction changes what you should buy:
- A chemical or pool smell is chlorine, not hardness. That is the thing a shower filter actually fixes.
- Rotten-egg smell is hydrogen sulfide, common on wells, and a KDF filter helps at the fixture. Well water.
- Orange or brown staining is iron. Also a well problem, also not hardness, and usually needs treatment upstream of the bathroom.
- Cloudy water that clears from the bottom up is dissolved air, and it is harmless.
- Blue-green staining is copper, usually from acidic water attacking the pipes — a plumbing issue, not a filter one.
Questions people actually ask
+ How do I know if my water is hard?
Shake plain soap in a bottle of tap water: thin bubbles and cloudy water mean hard. For a number, check your utility's annual report or use a hardness test strip.
+ What is considered hard water?
Above roughly 7 grains per gallon, or about 120 mg/L as calcium carbonate. Above 10 gpg is classified very hard.
+ Does my water report say how hard my water is?
Usually yes, in mg/L as calcium carbonate or in grains per gallon. It also names your disinfectant, which is the other thing you need before buying a filter.
+ Is a rotten egg smell hard water?
No, that is hydrogen sulfide, most often on private wells. It is a different problem with a different solution.
+ Can I test hard water without buying anything?
Yes — the soap-shake test costs nothing and takes two minutes, and your utility report is free.
Sources
Everything on this page traces to one of these. Where a source is brand-owned we say so, because a filter maker explaining filter chemistry has an interest and you are entitled to weigh it.