How to test pool water: strips, drop kits and getting accurate readings
Every calculator on this site is only as good as the readings you feed it. Here's what each type of test kit does well, how often to test, and the mistakes that most often lead to wrong doses.
Test strips vs. drop test kits
| Test strips | Drop (titration) kits | |
|---|---|---|
| How they work | Dip a strip and compare pad colors with a chart. | Add reagent drops to a water sample. Many tests count drops until a color changes, which gives a number directly. |
| Speed | Seconds | A few minutes for a full set of tests |
| Precision | Rough: colors between chart steps are hard to read | Better, especially for chlorine, alkalinity and calcium |
| High chlorine | Usually can't read shock levels | Kits using the FAS-DPD chlorine test can read high levels accurately |
| Best for | Quick daily checks of chlorine and pH | Weekly full tests, dosing decisions and clearing algae |
The CDC notes that DPD test kits give the most accurate chlorine results, and that test strips are less accurate. A practical setup is strips (or a simple kit) for daily chlorine and pH checks, plus a good drop kit for anything you're about to dose for.
Which tests you need
- Free chlorine and combined chlorine. Ideally with a test that reads free and total chlorine separately, since combined chlorine = total − free.
- pH. Usually a phenol red color test.
- Total alkalinity and calcium hardness, by titration.
- Cyanuric acid (CYA), usually a turbidity (cloudiness) test.
- Salt, for saltwater pools: strips or a titration test.
The target ranges for each are in our pool chemical levels chart.
How often to test
The CDC recommends testing chlorine and pH at least twice a day for home pools in use, and more often with many swimmers. Alkalinity changes slowly, so weekly is usually enough. CYA, calcium and salt change slower still, so test monthly and after adding water.
Getting accurate readings
- Sample from elbow depth, away from return jets, skimmers and chemical feeders. Turn the bottle upside down, push it down, then turn it upright to fill.
- Test right away. Chlorine in a sample bottle left in the sun drops within minutes.
- Rinse vials with pool water before and clean water after each test. Leftover reagent skews the next result.
- Hold dropper bottles vertically so every drop is the same size.
- Read colors in daylight, against a white background, not under indoor lights.
- Replace reagents and strips by their expiry date and keep them cool and dry. Old reagents are a common cause of strange readings.
Readings that mislead
- Very high chlorine bleaches DPD tests. The CDC notes that above about 10 ppm, a DPD test can partly or fully bleach out and show a false low or zero reading. If the sample flashes pink and then clears, chlorine is high, not absent. Use a FAS-DPD test or dilute the sample with distilled water.
- High chlorine also skews pH. Phenol red pH tests can read falsely high when chlorine is very high. Wait for chlorine to drop before acting on the pH reading.
- CYA tests depend on light and temperature. Read them outdoors in daylight with your back to the sun, with the sample near room temperature. Cold samples tend to read high.
- pH tests top out. Phenol red tests read up to about 8.2. A reading at the top of the scale may mean pH is higher.
Store testing
Many pool stores test water for free. It's a useful cross-check, especially for less frequent tests. Ask for the actual numbers, not just "add these products", and compare them with your own results. Water samples should be tested soon after collection.
Frequently asked questions
Are digital or photometer testers better?
Photometers and digital readers remove the color-matching guesswork, but still depend on fresh reagents, clean vials and good technique. A well-used drop kit is very reliable.
Why do my strips and drop kit disagree?
Strips are less precise, and their reading depends on timing and lighting. When they disagree, trust a carefully done drop test, especially for chlorine.
Do I need to test salt if my generator shows it?
Yes, now and then. Cell readings can drift with age and water temperature. Confirm with a test before adding salt.
Source: CDC, Home Pool and Hot Tub Water Treatment and Testing.