How to read pool test strips
Dip the strip for the time on your bottle, hold it flat, and match each pad to the color chart in the shade, before the reading time runs out.
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
On this page (8 sections)
Read it in shade rather than direct sun. A strip reads in steps (for example, free chlorine 0, 0.5, 1, 3, 5, 10 ppm), so treat the result as a range and use a drop kit to confirm borderline readings.
Step by step: dip, wait, read
- Check the bottle. Look at the expiration date and make sure the cap was closed tightly. Strips that picked up moisture give wrong colors before you even dip them.
- Take the sample from the right spot. About elbow deep, away from return jets, the skimmer, and any chlorine floater, with the pump running and at least 30 minutes after you last added chemicals.
- Dip with dry hands. Take out one strip, close the cap right away, and dip for exactly as long as your bottle says. Brands differ, so don't rely on another brand's timing.
- Hold it flat, pads up. Don't shake it hard or hold it upright; otherwise, color from one pad can run into the next.
- Read inside the time window. Some brands read all pads at once; others read chlorine first and the rest after a wait. Colors keep changing after the window, so a strip read late gives a wrong result.
- Use good light. Open shade or indirect daylight is best. Direct sun, yellow porch lights, and phone flashlights all shift colors.
- Write it down or enter it. Note each value (or "between 1 and 3") with the date. A log is how you see trends, such as how fast your chlorine drops.
Strips do expire. AquaChek, for example, says strips have a definite shelf life, should not be used past the date, and should be stored at room temperature in low humidity with the cap on tight. If a bottle sat in a hot shed all summer, buy a new one.
Each pad explained
Most pool strips have four to seven pads. The steps below are typical color blocks on common bottles, not any one brand. Your chart may use different steps, so always compare with your own bottle.
| Pad | Typical steps on the chart | Aim for | If it's off |
|---|---|---|---|
| Free chlorine (FC) | 0 · 0.5 · 1 · 3 · 5 · 10 ppm | 2–4 ppm without stabilizer; about 3–5 ppm at CYA 40 | Add chlorine; the minimum depends on your CYA |
| Total chlorine (TC) | same steps as FC | Equal to FC, or less than 0.5 ppm above it | A gap means combined chlorine: see FC vs TC |
| pH | 6.2 · 6.8 · 7.2 · 7.5 · 7.8 · 8.4 | 7.4–7.6 | Acid to lower it, soda ash to raise it |
| Total alkalinity (TA) | 0 · 40 · 80 · 120 · 180 · 240 ppm | 80–120 ppm | Baking soda to raise, acid plus aeration to lower |
| Total hardness (reads close to CH) | 0 · 100 · 250 · 500 · 1000 ppm | 200–400 ppm for plaster; 150–250 ppm for vinyl or above-ground | Calcium chloride to raise; only fresh water lowers it |
| Stabilizer (CYA) | 0 · 30 · 50 · 100 · 150 · 300 ppm | 30–50 ppm (60–80 with a salt system) | Stabilizer to raise; only draining lowers it |
Targets are our calculator defaults. The full list with sources is the pool chemistry chart. Hardness pads on strips usually read "total hardness," which includes magnesium, so they can read a little higher than a calcium-only drop test.
Two pads need extra care. The pH pad is unreliable when chlorine is very high: strong chlorine bleaches the indicator, so after a shock, wait until FC is back under 10 ppm before trusting pH (PHTA). CYA pads are the least precise on most strips because the steps are far apart. A reading of "50" can mean anything between the neighboring blocks. For more on each value, see the pH levels and alkalinity levels pages.
Reading between colors
Most of the time your pad will land between two blocks. Don't invent a decimal. Pick the range, and when you decide what to do, lean toward the smaller correction:
- Chlorine between two blocks: read it as the lower one. Topping up chlorine by an extra ppm is low risk. Letting it run out is how algae start.
- pH between 7.5 and 7.8: treat it as the lower value when you calculate acid, add that smaller dose, and retest after 30 minutes. It's easier to add a second small dose than to undo too much acid.
- TA, CH, or CYA between blocks: these change slowly. Don't dose on a borderline strip; confirm with a drop test or a second strip from another bottle first.
- Colors that match nothing on the chart: usually an expired or damp bottle, a late reading, or chlorine so high it bleached the pad. Retest with a fresh strip before you add anything.
Why strips and camera apps disagree
If you photograph the same strip twice and get two different answers, the strip may be fine. The camera is the problem. App Store reviews of scanner apps describe exactly this. One Clorox Pool review (August 2, 2026) says the numbers are "all over the place" for the same strip. An HTH Test to Swim review (July 10, 2026) says you can "test the same strip seconds apart and it will give you different results."
The physics explains it. A strip reads in a few steps, so a camera cannot be more precise than the chart on the bottle, even if the app prints a decimal. Phone cameras also adjust white balance and exposure on their own, wet pads reflect light, and the color keeps changing after the reading time. Brand scanner apps usually work only with that brand's strips, so if you switch strips, the app no longer applies. The honest output from any strip, read by eye or by camera, is a step or a range.
Strips vs drop kits (Taylor K-2006, FAS-DPD)
The CDC's advice for home pools is direct: use a DPD test kit for chlorine and pH; strips can be used, but they "are less accurate than the DPD test kit." Drop kits cost more up front and take a few minutes, but they give finer numbers where it matters.
| Test strips | FAS-DPD drop kit (for example Taylor K-2006) | |
|---|---|---|
| How chlorine is read | Color match against a chart | Count drops until pink turns clear (titration) |
| Chlorine resolution | Chart steps, for example 1, 3 and 5 ppm | 0.2 or 0.5 ppm per drop, depending on sample size (Taylor) |
| At high chlorine (shock) | Pads bleach and read wrong | Still readable: no color matching at the endpoint (Taylor) |
| Free vs combined chlorine | Some strips have a TC pad; small gaps are hard to see | Measures free and combined chlorine separately |
| Best use | Quick daily check of FC and pH | Weekly full test, shocking, and any time a strip is borderline |
A good routine for most owners: strips for quick daily checks, a FAS-DPD kit once a week and whenever you shock or treat algae. If your strip and your drop kit disagree, trust the drop kit and check the strip bottle's date.
Enter your strip result
Pick the block that matches each pad. The calculator uses the value of the block you pick and turns it into a plan with amounts for your products. If a pad sits between two blocks, pick the one the rules above suggest. For drop-kit numbers, use the full pool chemical calculator, which accepts exact values.
In the Pooldex app, entering a strip result takes five taps: choose the block for each pad. It works with any brand. A camera helper that compares your strip with the chart on your own bottle is planned for a later version. It will show the step it sees, not a made-up decimal, and you can always correct it.
Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling.
FAQ
How to read pool test strip results?
Dip the strip for the time on the bottle, hold it flat, and match each pad to the bottle's chart in the shade, before the reading time runs out. Write down each pad as the closest block, or as "between two blocks" when it falls in between, and dose from the conservative end of that range.
Are pool test strips or drops better?
Drops are more accurate, especially for chlorine: a FAS-DPD kit reads in 0.2 or 0.5 ppm steps and still works at shock levels. Strips are faster and fine for quick daily checks. Many owners use both.
Sources
- CDC, Home pool and hot tub water treatment and testing: cdc.gov
- CDC, Model Aquatic Health Code 2024, Annex (HOCl share by pH): cdc.gov (PDF)
- Taylor Technologies, How FAS-DPD tests for free and combined chlorine: taylortechnologies.com
- Taylor Technologies, FAS-DPD drop test, 1 drop = 0.2 or 0.5 ppm: taylortechnologies.com
- AquaChek, Water testing 101 (storage and expiration): aquachek.com
- PHTA fact sheet, Sodium hypochlorite (high chlorine and test interference): phta.org
- App Store reviews: Clorox Pool (id957130735, review of 2026-08-02) and HTH Test to Swim (id643360256, review of 2026-07-10), read 2026-09-29 via the public reviews feed.