Technical guide

Your pool's water parameters: what they mean and why they matter

pH, chlorine, alkalinity, calcium hardness... These are words you've probably heard from your technician. Here we explain what each one is, what range it should be in, and what happens if it's neglected.

Overhead view of turquoise, crystal-clear pool water

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Pool water isn't maintained by "eyeballing" it. Behind clear water that feels pleasant to the touch is a balance of several chemical parameters that, when properly controlled, make swimming safe, comfortable and inexpensive to maintain in the long run.

pH

7.2-7.6 · scale 0-14

0 (acidic)7 (neutral)14 (basic)

The parameter that most affects the others, with unbalanced pH, chlorine barely disinfects. Decreto 99/2024 legally allows up to 8.0, but above 7.6 chlorine already starts losing effectiveness.

Free chlorine

0.5-2.0 ppm · scale 0-5 ppm

02.55 ppm

The disinfectant that eliminates bacteria and microorganisms, without it, the water is no longer safe for swimming.

Combined chlorine

max. 0.6 ppm · scale 0-2 ppm

012 ppm

This is chlorine that's already "spent," combined with contaminants, the higher it is, the more it smells of chlorine and the less it actually disinfects.

Total alkalinity

80-150 ppm · scale 0-200 ppm

0100200 ppm

Acts as a pH "buffer," preventing it from rising or falling sharply from one day to the next.

Cyanuric acid

30-50 ppm · scale 0-100 ppm

050100 ppm

Protects chlorine from breaking down in the sun, without it, chlorine gets used up within hours outdoors. The legal limit is 75 ppm; above that, chlorine stops working properly even if the amount is correct.

The key parameters, at a glance

Ideal

Free chlorine

Ideal

Alkalinity

Ideal

Calcium hardness

Ideal

Sample readings from a well-maintained pool, each needle rests in its ideal green zone. See the table below for each parameter's exact range.

Parameter	Recommended	Legal limit RD 742/2013 · Madrid region	What it's for
pH	7.2-7.6	7.2-8.0	Determines whether the water is acidic or basic; affects chlorine's effectiveness and comfort for skin and eyes.
Free chlorine	0.5-2.0 ppm	0.5-2.0 mg/l	The disinfectant that removes bacteria and microorganisms from the water.
Combined chlorine	max. 0.6 ppm	max. 0.6 mg/l	Chloramines that are already "spent", the higher it is, the worse it smells and the less it disinfects.
Total alkalinity	80-150 ppm	Not regulated	Acts as a pH "buffer," preventing it from rising or falling sharply.
Calcium hardness	150-300 ppm	Not regulated	Regulates dissolved calcium; too low corrodes fittings, too high causes scale buildup.
Cyanuric acid (stabilizer)	30-50 ppm	max. 75 mg/l	Protects chlorine from breaking down in the sun, so it lasts longer in the water.
Salt (pools with a chlorinator)	3-6 g/l	Not regulated	Raw material that the salt chlorinator converts into chlorine through electrolysis.
Temperature	24-28 °C	24-30 °C heated pools	Swimming comfort; the higher the temperature, the faster chlorine is used up.
Turbidity	Crystal-clear water, bottom clearly visible	max. 5 NTU	Measures how cloudy the water is; in pools subject to checks, Decreto 99/2024 accepts measuring it with kits meeting UNE-EN ISO 7027-1.
Redox (ORP)	Above 650 mV	250-900 mV (only if the disinfectant isn't chlorine or bromine)	Shows the water's real disinfecting power; measured by the probes of automatic dosing systems.

The "Recommended" column is the range for good maintenance; the "Legal limit" column shows the values required by RD 742/2013 (Annex I), which Madrid's Decreto 99/2024 keeps for pools for collective use (residential communities, hotels, gyms, schools…). For a private pool at a single-family home they aren't mandatory, but they're the best reference. Alkalinity, calcium hardness and salt are not regulated: they're good practice for protecting the water and the installation in the long run, not a sanitary requirement. Pools for collective use (residential communities, hotels, schools) are also subject to mandatory checks that we verify on every visit.

What happens if these parameters aren't controlled?

Cloudy or green water

Uncontrolled pH or chlorine is the gateway for algae and bad-looking water.

Skin and eye irritation

pH out of range is almost always the real cause of stinging eyes, not excess chlorine.

Equipment corrosion

Aggressive water wears down pumps, metal ladders and the pool shell itself before its time.

Higher chemical costs

Correcting badly unbalanced water requires far more product than keeping it stable from the start.

Bad smell

The characteristic "pool smell" usually indicates chloramines, not excess chlorine: it's a sign of poor maintenance.

Regulatory non-compliance

In community pools, not keeping documented records can lead to penalties during an inspection.

The real benefits of professional maintenance

Measuring the water with test strips is easy; the hard part is interpreting those numbers and knowing what product to add, in what amount and in what order, without overcorrecting. A technician who visits your pool regularly:

Saves money in the long run: avoids excessive chemical use and breakdowns caused by aggressive or scale-forming water.
Guarantees safe water: with sanitary parameters always within range.
Extends the life of your installation: pumps, filters and lining deteriorate much faster with uncontrolled water.
Gives you peace of mind: you don't have to learn chemistry or remember anything, someone does it for you.
Complies with regulations: essential in residential communities, hotels and sports centers.

And what if you also have a salt chlorinator?

If your pool already has (or you want to install) a salt chlorinator, controlling these parameters is just as important: the equipment takes care of the chlorine, but pH, alkalinity and salt level still need to be checked regularly. We cover it all in our guide on salt chlorination. If pH is the one that gets thrown off balance most often for you, we have a specific guide on how to raise or lower it step by step.

Let us take care of your pool's water

Chlorine, pH and alkalinity analysis on every visit, with exact dosing. No surprises.

Request a full water analysis

Frequently asked questions about pool water

During swimming season, it's best to check it at least once a week; with heavier use or more bathers, it's worth doing it more often. That's exactly what our regular maintenance visits include.

Yes, for basic checks between visits. But interpreting the results correctly and dosing the right product in the right amount is where an experienced technician makes the difference.

With high pH, chlorine loses much of its disinfecting power. With low pH, the water becomes aggressive and corrodes equipment and linings. Either way, it needs to be corrected as soon as possible.

With the pool closed, the frequency drops a lot, but it's worth a quick check now and then to avoid surprises (green water) when spring arrives. We cover this in our season opening and closing service.

Your pool's water parameters: what they mean and why they matter

pH

Free chlorine

Combined chlorine

Total alkalinity

Cyanuric acid

The key parameters, at a glance

What happens if these parameters aren't controlled?

Cloudy or green water

Skin and eye irritation

Equipment corrosion

Higher chemical costs

Bad smell

Regulatory non-compliance

The real benefits of professional maintenance

And what if you also have a salt chlorinator?

Let us take care of your pool's water

Frequently asked questions about pool water

Keep reading

Salt chlorination: benefits and installation

How to test chlorine and pH

Our maintenance services

m³ and m² calculator