By The Zagosa Team. Published .
Reviewed or updated .
Whenever you test chlorine, you get free, total, or both. Free chlorine is the working part, the bit actually sanitising. Total chlorine adds in the chloramines, the leftovers formed once chlorine has reacted with organics. The space between those two numbers is the most useful clue you have.
What each figure is telling you
Free chlorine is the live, available chlorine ready to deal with the next bug that gets in. Aim for 3 to 5 ppm in a hot tub.
Total chlorine is the lot: free plus combined (chloramines). It is always the same as, or higher than, the free reading.
Combined chlorine is the gap between them. It sanitises nothing. It is the smell, the eye sting, and the sign the water is due a shock.
The half-a-ppm rule
Keep combined chlorine at no more than 0.5 ppm. If free reads 4.0 and total reads 4.3, your combined is 0.3, and all is well.
If free reads 4.0 but total reads 5.5, combined is 1.5, and you want a non-chlorine shock that same day.
Most test strips report total chlorine only (often just labelled chlorine). To see the gap you need a two-pad strip or a DPD drop kit. Both are cheap and worth parking next to the tub.
Reading the gap like a diagnosis
Combined creeping up slowly between sessions is normal. A weekly non-chlorine shock resets it.
Combined shooting up within a single day points to heavy bather load or low free chlorine. A shock plus a small free chlorine top-up covers both.
Combined high while free sits at zero is a chlorine demand episode. See the dedicated FAQ on that, and fix the underlying cause before you simply dose more.
Common questions
Can a pool DPD kit work on a hot tub?
Yes. The chemistry is the same. Pool DPD kits often cover a wider range, which helps when you are diagnosing a high-combined episode.
And what about bromine?
Bromine splits the same way (free and total), but the gap tells you less because bromine's combined products are themselves partly active. Combined bromine up to 1 ppm above free is fine.