How to Test Pool Water

How do you test pool water?

You test pool water one of three ways: paper test strips, a liquid reagent (drop) kit, or a digital tester, plus the free water test most pool stores offer. Strips are the fastest. Dip one in the pool, hold it level for the time on the bottle, and match the pads to the color chart. A liquid kit takes more work but is far more accurate, because you count drops of reagent until the sample changes color. A digital tester or photometer reads the numbers for you and removes the guesswork of matching colors by eye.

For everyday checks, strips are cheap and good enough. When something looks off, or when you are clearing algae and need a precise reading at high chlorine, reach for a liquid kit. The standard for serious home testing is a FAS-DPD drop kit, which measures free and combined chlorine accurately even at high levels where strips and simpler tests max out. If you are choosing one, see the guide to the best pool test kits.

What should you test for?

Test free chlorine, pH, and total alkalinity every time, and check cyanuric acid, calcium hardness, and salt less often. Free chlorine is your active sanitizer and the reading that moves the most. pH belongs at 7.4 to 7.6 for swimmer comfort and chlorine strength. Total alkalinity, which buffers pH so it does not swing, belongs at 80 to 120 ppm.

The slower-moving readings still matter. Cyanuric acid (stabilizer) belongs at 30 to 50 ppm on a chlorine pool and 60 to 80 ppm on a salt pool, and it sets your real chlorine target: the more CYA you run, the more free chlorine you need for the same sanitizing power. Calcium hardness belongs at 200 to 400 ppm (150 to 250 ppm on a vinyl pool). Salt, on a saltwater pool, belongs at 2,700 to 3,400 ppm. Combined chlorine, the used-up chlorine that stings eyes, should read 0.5 ppm or less. The full target chart is in ideal pool chemistry levels.

Which is more accurate, test strips or a drop kit?

A liquid drop kit is more accurate than test strips, and a FAS-DPD kit is the most accurate home method for chlorine. Strips are convenient but coarse: they read in wide bands, fade if the bottle gets damp, and depend on your eye matching colors under changing light. A drop kit measures by counting reagent drops until the color flips, which is a number, not a shade, so two people get the same result.

To use strips well, dip a fresh strip elbow deep for a second, pull it out without shaking off the water, hold it level, and match the pads at the time the label says (often 15 seconds), in daylight rather than under a colored deck light. Replace the bottle every season. To use a liquid kit, fill the sample tube to the line and add reagents. Chlorine tests come in two chemistries: OTO turns yellow and reads total chlorine only, while DPD turns pink and can separate free from combined. Follow the kit's instructions exactly, because the drop count is the measurement.

Cyanuric acid uses a different style of test entirely. You mix pool water with a cloudy reagent and pour it into a tube with a black dot at the bottom until the dot just disappears, then read the level off the tube. It is normal for this one to feel imprecise. Get it close, and retest if your chlorine starts behaving oddly.

Where and when should you take the sample?

Take the sample about 18 inches deep (elbow deep), away from the returns, the skimmer, and any spot where you recently added chemicals, with the pump running or just after it has run. Water that sits still stratifies, so a sample from a dead corner reads differently from the main body of the pool. Push the container down upside down, then flip it at depth to fill, which avoids scooping the surface film.

Test soon after you collect the sample, because chlorine and pH start to drift once the water is out of the pool and warming in your hand. For the chlorine reading specifically, morning is the most useful time, before the sun has burned off the day's free chlorine. A noon reading looks lower than what you actually started with, and a morning reading also tells you whether chlorine held overnight, which is the real test of whether your sanitizer is keeping up.

How often should you test pool water?

Test free chlorine and pH every day or two in swim season, and check total alkalinity, calcium hardness, and cyanuric acid about once a week or when something seems off. A busy pool, a heat wave, or heavy rain all call for an extra test, since each one moves your chemistry. In the off-season with the pool covered and cold, monthly is usually enough.

Beyond the calendar, always test right before you add any chemical and again after it has circulated for a few hours. Dosing against yesterday's number, or against a reading you never took, is how pools drift out of balance. Consistency helps too: test at about the same time and the same spot each time so the changes you see reflect your water and not your routine.

How do you fix a reading that is off?

Fix one thing at a time, in order: total alkalinity first because it steadies pH, then pH, then free chlorine. Correcting them out of order means chasing your tail, since alkalinity and pH pull on each other and CYA sets the chlorine target. If a strong chlorine smell or a combined chlorine reading above 0.5 ppm shows up, that is a signal to shock the pool, not to add a little more of the same.

Once you know which reading is off, put your numbers into the matching calculator so the dose fits your gallons rather than a guess: the chlorine calculator, the pH calculator, or the alkalinity calculator. Add chemicals in stages, run the pump, and retest before adding more. One calculated dose is a starting point, not a final answer, and the retest is what keeps the water right.

Frequently asked questions