How to read a hot tub test strip

Test strips are the fastest way to check your hot tub’s water — dip, wait, compare colors. But the color chart on the bottle assumes you already know what each measurement means and what range you’re aiming for. This guide covers both, so a strip goes from “a row of colored squares” to a complete picture of your water in about fifteen seconds.

Getting an accurate reading

Bad technique is the number one source of confusing results. The strip itself is reliable if you use it right:

1. Test before you add anything — and ideally before anyone gets in. Jets on for a minute first helps mix the water.
2. Dip at elbow depth, away from jets and the skimmer, for about two seconds. Surface water isn’t representative.
3. Pull it out flat and hold it level. Don’t shake or flick the water off — that smears reagents between pads and corrupts adjacent readings.
4. Read at the time the bottle says — usually 15 seconds. The colors keep developing, so a strip read at 60 seconds will lie to you.
5. Read in natural light. Warm indoor lighting shifts the colors.

Also check the bottle’s expiration date and keep the lid tightly closed — humidity ruins the reagent pads, and a spa is a humid place to store them. A fresh bottle of quality strips is cheap insurance: the AquaChek Select 7-way strips are a solid choice that covers everything below.

What each pad measures

Sanitizer (free chlorine or bromine)

This is the most important pad on the strip. Sanitizer is what kills bacteria, and hot water breeds bacteria fast. Free chlorine is the chlorine still available to sanitize (as opposed to chlorine that’s already been used up). If your spa uses bromine, read the bromine scale instead — the same pad usually shows both scales, and bromine targets run higher than chlorine.

pH

How acidic or basic the water is. Low pH is corrosive — it eats heaters, seals, and pump parts, and stings eyes. High pH makes your sanitizer dramatically less effective and lets scale form on surfaces and inside plumbing. Spas naturally drift upward because hot, aerated water off-gasses carbon dioxide (see our high pH guide).

Total alkalinity (TA)

Alkalinity is pH’s shock absorber — it buffers the water against sudden pH swings. When TA is right, pH stays put; when TA is off, pH bounces around no matter how carefully you dose. This is why alkalinity is corrected first, before pH.

Calcium hardness

The dissolved calcium in your water. Too low and the water becomes aggressive, foams easily, and corrodes metal; too high and you get scale and cloudiness. Hardness barely changes week to week — it’s mostly set by your fill water.

Cyanuric acid (CYA)

A stabilizer that protects chlorine from sunlight. In an outdoor pool it’s essential; in a mostly-covered spa it matters far less, and it accumulates every time you add stabilized chlorine (dichlor). Once it climbs too high, it blunts your chlorine’s killing power. Bromine users can ignore this pad entirely — bromine can’t be stabilized by CYA.

Target ranges for a hot tub

Spa targets are not the same as pool targets — hotter water, smaller volume, and heavy aeration change the math. These ranges are based on industry-standard guidelines published by the Pool & Hot Tub Alliance (PHTA), tuned to the spa-friendly end of each band:

What to do with the numbers

Fix things in this order: alkalinity → pH → sanitizer. Alkalinity steadies pH, pH determines how well sanitizer works, and sanitizer keeps the water safe. Correcting them out of order means re-doing work — the classic example is chasing pH up and down for a week when the real problem was alkalinity all along.

How much of each chemical to add depends on your spa’s volume and the strength of the specific product in your cabinet, which is where most owners reach for a dosing chart and a calculator.