Why is my pool chlorine low?

Why free chlorine matters

Free chlorine is the chlorine that is available to kill germs and algae. For pools, keep it between 1 and 3 ppm. Hot tubs need more because the water is warm and used by several people in a small volume - keep it between 3 and 5 ppm.

Below these levels bacteria can survive, algae can take hold within a day or two in warm weather, and the water may start to look dull or smell stale.

Common causes

• Sunshine - UV light destroys unstabilised chlorine within hours, which is why outdoor pools need stabiliser.
• Lots of bathers, sweat, suncream and pets.
• Warm weather and warm water.
• Heavy rain, leaves and debris using chlorine up.
• An empty chlorine floater or feeder, or dosing too infrequently.
• Algae starting to grow, which consumes chlorine fast.

How to raise chlorine, step by step

Check pH first. Chlorine works best at pH 7.2-7.6 - adding chlorine to high-pH water wastes much of it.

• Correct pH if needed and wait at least an hour before adding chlorine.
• With the pump running, add chlorine granules according to the app's plan or the label. As a guide, stabilised granules need about 1.8 g per 1,000 litres to raise free chlorine by 1 ppm.
• Pre-dissolve granules in a bucket of pool water (chemical to water) and pour in front of a return, or follow the label if it says to broadcast them. For hot tubs, add with the jets running and the cover off.
• Never mix chlorine with pH products or any other chemical - dangerous gases can form. Use a clean, dry scoop and a separate bucket.
• Add in the evening if you can, so the sun does not burn it off straight away.
• Retest after a few hours (20-30 minutes in a hot tub). Swim once free chlorine is back in range.

If chlorine is zero or the water is cloudy

If free chlorine reads zero, or the water is cloudy or turning green, a normal dose will not be enough - shock the water instead. See our guides to cloudy water and green pools.

Preventing low chlorine

Test two or three times a week (hot tubs before each use), keep stabiliser at 30-50 ppm in outdoor pools, top up floaters regularly, and use a cover when the pool is not in use. Ask bathers to shower before swimming.

When to call a professional

If chlorine disappears overnight despite a healthy dose and good pH, you may have hidden algae, high stabiliser or a problem with your filter. A professional water test can find the cause.

Frequently asked questions