Total Hardness
What nearly every test strip actually prints. Calcium plus magnesium, where the published pool bands are written for the calcium alone.
- Safe range
- 80 – 400 mg/L (The safe band is the union of the aquatic health code's two figures — one for masonry surfaces, a lower one for vinyl and fiberglass — because a single row cannot know which surface it is looking at. A vinyl pool is entitled to narrow its own target; the ideal band shown is the masonry figure.)
- Ideal range
- 200 – 400 mg/L
- Measured in
- mg/L
What it is
Hardness is dissolved mineral, and there are two ways to count it. Total hardness counts calcium and magnesium together, which is what almost every consumer strip reports, because that is what the chemistry on a strip pad responds to. Calcium hardness counts only the calcium, which is the half pool chemistry actually cares about — it is calcium that scales a heater and calcium that plaster gives up when the water is short of it.
Why it matters
Water that is short of calcium is aggressive: it takes what it needs out of plaster, grout and the metal in a heater. Water with too much of it drops the surplus as scale, on the tile line first and inside the equipment where you cannot see it. Neither is a health question, which is why this parameter is about what the pool costs to own rather than whether it is safe to swim in.
How to measure it
Read the label on your kit before you act on the number. If it says Total Hardness you have calcium plus magnesium; if it says Calcium Hardness you have the number the bands were written for, and the two are not interchangeable when you are deciding whether to add anything.
Drop-count titration kit
- How trustworthy
- Dependable
- Cost
- Low cost
- Effort
- A few minutes, and it rewards being unhurried
Add reagent one drop at a time until the sample changes colour, and count the drops. The count is the answer, so there is no colour to judge by eye and no lighting to get wrong.
The EDTA drop count, which is the standard hardness test and the one most pool kits use: the sample turns red, and you count drops until it goes blue. Kits sold for pools usually include a reagent that masks magnesium, which turns the same test into a calcium hardness test — that is the version to buy.
Watch out: Count the drop where the colour stops changing, not the one where it starts. Overshooting by two or three drops is what makes a hardness test read high and sends somebody draining a pool that was fine.
Where to find it: Pool shops and online, usually as part of a multi-test kit. Aquarium shops sell the same chemistry in smaller bottles.
Test strips
- How trustworthy
- Ballpark
- Cost
- Low cost
- Effort
- Under a minute, wherever you drew the sample
A plastic strip with reagent pads on it. Dip it, hold it level, and match each pad against the chart on the bottle at the time the label tells you to. Most strips read several parameters at once.
Almost universally printed Total Hardness, and the reason this guide exists. Good for noticing that the number has moved; not good for deciding to add calcium, because it is counting magnesium you do not care about.
Where to find it: Pool shops, hardware stores, supermarkets and online, sold both per parameter and as multi-packs. Buy strips made for your kind of water: pool strips and drinking water strips read very different scales for the same chemistry.
Free test at a pool shop
- How trustworthy
- Dependable
- Cost
- Free
- Effort
- A trip, and a sample carried carefully
Take a sample in and they will run it, usually on a photometer, usually free, and print you a full panel. The catch is that the printout ends in a list of things to buy.
A pool shop panel usually reports calcium hardness specifically, which makes it the easy way to check whether your strip's total is hiding a calcium problem.
Where to find it: Most pool and spa retailers. Ask which parameters they cover before driving over.
Taking the sample
For several of these the sample is where the error is, and no instrument recovers from a bad one.
- • Test the fill water as well as the pool, once. Hardness only arrives one way, and knowing what comes out of the hose explains most of what the pool does over a season.
- • Test before topping up rather than after, or you are measuring the hose.
Further reading: CDC: the Model Aquatic Health Code, the guidance US pool codes are written from
If it is too low
The water is hungry for calcium and will take it from whatever is nearest. In a plaster or concrete pool that means etching and a rough surface over time; in a vinyl or fiberglass one there is nothing for it to attack and the number matters much less.
- • Add calcium chloride, and add it in stages with the pump running rather than all at once.
- • Check the surface before spending anything. A vinyl or fiberglass pool with soft water is not in trouble the way a plaster one is.
- • Look at your fill water. If it is naturally soft, this will keep happening, and knowing the number going in is what makes it predictable.
If it is too high
Scale. Cloudy water that no amount of chlorine clears, a rough deposit at the waterline, and — most expensively — a coating inside the heater and on a salt cell's plates, both of which lose efficiency long before anyone sees anything.
- • Partial drain and refill is the only real way down, and the arithmetic is worth doing first: replacing a third of the water moves the reading by roughly a third of the gap to your fill water.
- • A sequestrant holds calcium in solution as a holding action. It stops scale forming for a while; it does not remove anything.
- • Keep pH and alkalinity at the lower end of their ranges meanwhile. High hardness only scales when the rest of the balance lets it.
Common questions
Is total hardness the same as calcium hardness?
No. Total hardness is calcium and magnesium together; calcium hardness is the calcium alone. The published pool ranges are calcium figures, and the magnesium in your water inflates a total hardness reading above it — which is why a strip can read comfortably in range while the calcium is genuinely low.
My strip and my drop kit disagree. Which do I believe?
The drop kit, if it is a calcium hardness test, and the gap between them is roughly the magnesium. A strip reading higher than a calcium test is the expected result rather than a fault.
Does hardness matter in a vinyl pool?
Much less. There is no plaster for soft water to attack, which is why the code's figure for vinyl and fiberglass runs lower. The upper end still matters: scale forms on a heater and a salt cell whatever the pool is lined with.
WiseWaterGauge helps you record and understand your own readings. It is not a substitute for a certified laboratory test, and nothing here is medical or regulatory advice. If a health parameter is out of range, confirm it with an accredited lab and speak to your water provider.