Hot tub chlorine levels: what is right and how to fix high or low

Keep free chlorine in a hot tub at 1–3 ppm, and do not soak when it is above 5 ppm. If it is low, add spa chlorine granules (dichlor) per the label for your tub's volume. If it is high, take the cover off, run the jets, and wait. Test 2 to 3 times a week, because hot water uses up chlorine quickly.

Chlorine level chart

Free chlorine	What it means	What to do
0 ppm	No protection. Bacteria can grow within hours in warm water.	Do not soak. Add chlorine or shock.
Under 1 ppm	Too low	Add chlorine per the label.
1–3 ppm	Target range	Enjoy the tub.
3–5 ppm	Fine. Common after dosing.	OK to soak.
Over 5 ppm	Too high for soaking	Cover off, wait, retest.
10 ppm or more	Shock level	Stay out until it drops to 5 ppm or less.

The 1–3 ppm range is what most residential spa product labels state. The CDC's guidance for hot tubs is at least 3 ppm, so treat 3 ppm as the level to hit before a group soak.

How to raise chlorine

1. Test first. Check pH too. Chlorine works best at pH 7.2–7.6.
2. Turn the jets on and open the cover.
3. Measure dichlor granules per the label for your tub's volume, with a clean dry scoop.
4. Sprinkle over the water at the center of the tub.
5. Circulate 15 to 20 minutes with the cover open.
6. Retest before soaking.

Most chlorine owners add a small dose right after each soak. That deals with what bathers left behind and means the level is right next time.

How to lower chlorine

• Wait. Remove the cover and run the jets with the air on. Chlorine gasses off and gets used up. A few hours handles a mild overdose.
• Sunlight. An open cover on a sunny day burns off chlorine faster.
• Dilute. Drain some water and top up with fresh.
• Chlorine neutralizer. Sodium thiosulfate products work instantly, but they are easy to overdose, which leaves you with zero chlorine. Use per the label and only when you cannot wait.

Be careful with strips at very high levels. Chlorine above about 10 ppm can bleach the pad white, so a badly overdosed tub can read as zero. If you just shocked and the strip says zero, do not add more. Wait and retest.

Why chlorine keeps dropping to zero

• Heat and use. At 100–104°F with bathers, chlorine demand is high. A level that was 3 ppm before a soak can be 0 after it.
• Dirty filter. A filter full of oils consumes chlorine around the clock. Clean it. See how to clean a hot tub filter.
• Combined chlorine. If total chlorine is well above free chlorine, the chlorine is tied up. A shock clears it.
• Biofilm. Slime inside the pipes eats sanitizer. If chlorine vanishes overnight in an unused tub, do a line flush and change the water.
• Old water. After 3 to 4 months, built-up waste and stabilizer make chlorine less effective.

Free, combined, and total chlorine

Free chlorine is the active sanitizer. Combined chlorine (chloramines) is chlorine that has already reacted with sweat and other waste. It sanitizes poorly and is what causes the harsh smell and red eyes. Total chlorine is the two added together.

A strong "chlorine" smell from a hot tub usually means too much combined chlorine and too little free chlorine. The fix is more oxidizer, in the form of shock, not less chlorine.

Stabilizer (cyanuric acid) build-up

Dichlor is about half cyanuric acid by weight. Every dose adds stabilizer, and it never leaves except by draining. As it climbs past roughly 50 ppm, a given free chlorine reading does less work. By 100 ppm, water can turn cloudy even at 3 ppm chlorine. This is the main chemical reason to change the water every 3 to 4 months. See how often to change hot tub water.

What kind of chlorine to use

• Dichlor granules: the standard for hot tubs. Dissolves fast and is close to pH neutral.
• Trichlor tablets: do not use. Too acidic and too slow to control in small volumes.
• Calcium hypochlorite pool shock: not recommended. Adds calcium and can leave residue.
• Salt chlorine systems: generate chlorine in the water. You still test and hold 1–3 ppm.

Never mix different chlorine types, in a feeder, a scoop, or in storage. Follow the label for every product.

If chlorine bothers your skin

Check pH first. Irritation is more often from pH outside 7.2–7.8 or from combined chlorine than from free chlorine at 1–3 ppm. If it still bothers you with everything in range, consider bromine. The comparison is in chlorine vs bromine.