Water Testing

Chlorine Too High to Read? How to Test It

A test that goes blank after shocking is not telling you zero. Here is how to get a real number.

Updated November 2026

Short answer

Dilute the sample. Mix one part pool water with one part distilled water, run the test, and double the result. If it is still off the chart, use one part pool water to four parts distilled water and multiply by five.

Very high chlorine can bleach the test dye, so the sample turns pale or clear and looks like zero. A flash of color that fades is the giveaway.

How to test pool water

Once you have a number, the safe to swim checker tells you if the pool is ready.

Why does the test show nothing when chlorine is high?

Most kits and strips top out at 5 or 10 ppm. Above that, each type fails in its own way:

How each pool chlorine test behaves when chlorine is very high
Test type	Reads up to	What you see when it is too high
DPD drops or tablets (pink)	About 5 to 10 ppm	Flashes pink, then fades to pale or clear
OTO drops (yellow)	About 5 ppm	Dark yellow, orange or brown
Test strips	About 10 ppm	Darkest pad color, or a bleached white pad
FAS-DPD drop count kit	About 20 ppm	Still works. It just takes more drops. Above about 20 ppm, dilute the sample

Ranges vary by brand. Check the color card or instructions in your kit.

How do I do a dilution test?

Rinse the test vial with distilled water. Tap water often contains chlorine and skews the result.
Fill the vial halfway with pool water, taken from elbow depth, away from the return jets.
Top up to the fill line with distilled water, cap and mix.
Test as usual and multiply the reading by 2.
Still off the chart? Use one part pool water and four parts distilled water, and multiply by 5.

A diluted reading is an estimate. Any error is multiplied along with the result, so treat 12 ppm as about 12. Only the chlorine reading is valid in a diluted sample. Test pH on undiluted water.

How do I bring high chlorine down?

Wait. Sunlight and normal demand lower chlorine by a few ppm a day. Leave the cover off.
Stop adding. Turn off the chlorinator or salt system and remove tablet floaters until the level is back in range.
Dilute. Draining part of the water and refilling works when you are in a hurry.
Neutralizer. Sodium thiosulfate removes chlorine fast, but it is easy to overshoot and end up at zero. Use a partial dose and retest.

High chlorine also throws off pH tests. Above about 10 ppm, most pH tests read falsely high. Do not add acid based on a pH reading taken while chlorine is very high.

Frequently Asked Questions

My chlorine test is clear right after I shocked. Is it really zero?

Almost certainly not. Chlorine well above the top of the scale bleaches the dye. Run a dilution test to see the real level.

Can I use tap water to dilute the sample?

Distilled water is better. Most tap water contains chlorine or chloramine, which adds to the reading.

How high is too high to swim?

Wait until free chlorine is 4 ppm or below, the re-entry level printed on chlorine product labels. See how long after adding chemicals you can swim.

Is there a test kit that reads high chlorine without diluting?

Yes. A FAS-DPD drop kit counts drops instead of matching a color, so it reads up to about 20 ppm without diluting. Above that, dilute the sample the same way. It is the standard choice when shocking for algae.

Sources

Get the rest of your readings right

Strips, drops and digital testers, and how to take a sample that gives a true number.

How to Test Pool Water Safe to Swim Checker