How to Read a Pool Test Strip: What Each Number Means and What to Do

Test strips are a field tool. Fast, cheap, good enough to flag a problem before you open the test kit or load the truck. They're not a replacement for a proper drop test or a Taylor K-2006, but for a weekly route check they tell you what you need to know in 15 seconds.

Here's what each pad reads and how to act on it.

Dip it right first

Dip the strip in elbow-deep water, not the surface. Pull straight out, don't shake it. Hold level for the seconds your bottle says (usually 15). Read in shade, not glare. Strips that have been sitting in a hot truck bed all summer give garbage readings — keep the bottle sealed and out of the sun.

Free Chlorine (FC) — target 1–3 ppm

This is your sanitizer. The number that actually keeps the pool from turning green.

0 ppm: Pool is unprotected. Algae is coming or already there. Shock it.
1–3 ppm: Where you want to live.
5+ ppm: High. Usually fine, but tell the customer not to swim until it drops if it's way up there. Often means you shocked last visit and the sun hasn't burned it off yet.

What to do if it's low: add liquid chlorine or cal-hypo based on pool volume. A 15,000-gallon pool and a 30,000-gallon pool don't get the same dose. Test, dose, retest.

Total Chlorine (TC) and Combined Chlorine

Total chlorine minus free chlorine equals combined chlorine — the used-up chlorine bonded with ammonia, sweat, sunscreen. If combined is above 0.5 ppm, you've got chloramines. That's the smell people call "too much chlorine" when it's actually not enough. Shock the pool.

pH — target 7.4–7.6

Below 7.2: Acidic. Eats plaster, corrodes metal, burns eyes. Add soda ash or aerate.
7.4–7.6: Sweet spot. Chlorine works best here.
Above 7.8: Basic. Chlorine gets lazy, scale forms, water goes cloudy. Add muriatic acid based on volume and current alkalinity.

Don't chase pH without checking alkalinity first. They move together.

Total Alkalinity (TA) — target 80–120 ppm

Alkalinity is pH's shock absorber. Low TA and your pH bounces all over the place after every rain. High TA and pH wants to climb no matter what you do.

Low (under 80): Add sodium bicarbonate (baking soda).
High (over 120): Add muriatic acid. Slowly. Over multiple visits if it's way up.

Cyanuric Acid (CYA) — target 30–50 ppm (outdoor)

Sunscreen for chlorine. Without it, UV burns free chlorine off in hours. Too much and chlorine stops working — "chlorine lock."

0–20: Add stabilizer.
30–50: Good.
80+: Drain and dilute. There's no chemical that removes CYA.

If FC keeps reading fine but the pool still looks tired, check CYA. High CYA hides a sanitizer problem.

Hardness (Calcium Hardness) — target 200–400 ppm

Low: Water pulls calcium out of plaster and grout. Add calcium chloride.
High: Scale on tile, cloudy water, scale in the heater. Partial drain and refill.

Hardness moves slow. It's not a weekly problem, it's a quarterly one.

Filter pressure isn't on the strip — but check it anyway

While the strip is developing, look at the gauge. If it's 8–10 psi above clean baseline:

DE or sand filter: backwash.
Cartridge filter: pull the cart and hose it. You don't backwash a cartridge.

Logging the reading

Writing chemistry on a clipboard and copying it later is how numbers get lost between the truck and the invoice. PoolPilot has a test-strip scan — snap a photo of the developed strip and it logs the reading to that stop on that date. It doesn't replace your kit, it doesn't tell you the pool is balanced, it doesn't dose for you. It logs what you saw so you have a history per pool when chemistry drifts three weeks in a row.

Same with the customer text. The message goes out from the stop: pool clean, what you added, swim-ready time. Flat and concrete. The kind of update that ends the "did you come today?" call.

PoolPilot is built around the solo operator and the truck — the route, the stop, the photo, the text, the invoice. $30/month, 14-day free trial. That's it.

The short version

Reading	Target	If low	If high
Free Chlorine	1–3 ppm	Add chlorine	Wait or partial drain
pH	7.4–7.6	Soda ash	Muriatic acid
Total Alkalinity	80–120 ppm	Baking soda	Muriatic acid
CYA	30–50 ppm	Stabilizer	Drain and dilute
Calcium Hardness	200–400 ppm	Calcium chloride	Partial drain

Test first. Dose by volume. Retest next visit. That's the job.