Free chlorine is the one that matters
Free chlorine (FC) is the chlorine in your water that is still available to kill bacteria and algae. It is the single most important number in pool care, and it is what you dose against. The right level depends on your cyanuric acid, which is why there is no one safe FC number; see the ideal levels for the relationship.
When people talk about keeping a pool sanitary, they mean keeping free chlorine in range for the CYA. Use the chlorine calculator to bring it to target.
Combined chlorine is the used-up part
When chlorine does its job and reacts with sweat, sunscreen, and other contaminants, it becomes combined chlorine, also called chloramines. It is still measured as chlorine but it is mostly spent and a weak sanitizer. Total chlorine (TC) is simply free chlorine plus this combined chlorine.
So if your free chlorine is 4 and your total chlorine is 4, you have no combined chlorine and the water is healthy. If free is 4 but total is 5, you have 1 ppm of combined chlorine, which is too much.
Combined chlorine is what you smell
The strong chlorine smell at a pool is not too much chlorine. It is combined chlorine, the chloramines. A well-run pool with plenty of free chlorine and almost no combined chlorine barely smells at all. A pool that reeks of chlorine usually does not have enough.
Keep combined chlorine at or below 0.5 ppm. If it climbs above that, the fix is to shock the pool: raise free chlorine to the shock level for your CYA and hold it until the combined chlorine burns off. The shock calculator sets that level.
Free chlorine (FC) wins on
- +The active sanitizer; the number you manage.
- +Tells you the pool is actually protected.
- +Drives every chlorine dosing decision.
Total chlorine (TC) wins on
- +Reveals combined chlorine when compared to free.
- +A quick health check: total should equal free.
- +Flags when a shock is needed.
The verdict
Manage free chlorine; it is the active sanitizer and the number every dose targets. Use total chlorine only to spot combined chlorine, which is total minus free and should stay at or below 0.5 ppm. If combined chlorine rises, or the pool smells strongly of chlorine, shock it to the level for your CYA and the smell clears. Run the numbers with the chlorine calculator.
Related: Pool chlorine calculator, Pool shock calculator, Ideal pool chemistry levels.
Frequently asked questions
What is the difference between free and total chlorine?
Free chlorine is the chlorine still available to sanitize. Total chlorine is free chlorine plus combined chlorine, the part that has already reacted with contaminants and is mostly used up. The difference between total and free is your combined chlorine, which should be 0.5 ppm or less.
Which chlorine reading should I use?
Free chlorine. It is the active sanitizer and the number you dose against. Look at total chlorine only to check for combined chlorine: if total is higher than free, the gap is combined chlorine that has built up and may need shocking.
Why does my pool smell strongly of chlorine?
Counterintuitively, a strong chlorine smell usually means not enough chlorine, not too much. The smell is combined chlorine, or chloramines, which builds up when free chlorine runs low. Shocking the pool to the right level for your CYA burns it off and clears the smell.
What should combined chlorine be?
At or below 0.5 ppm. Combined chlorine is total chlorine minus free chlorine. When it rises above 0.5, the water can smell and irritate eyes and skin, and it is a sign to shock the pool until it drops back down.