Guide
Hot tub chemistry for beginners
The numbers a hot tub runs on, chlorine or bromine: sanitizer, pH, alkalinity, calcium and CYA, doses in millilitres for a small tub, and water no hotter than 104°F (40°C).
Updated · 3 min read
A hot tub runs on the same chemistry as a pool, in a few hundred gallons instead of tens of thousands, so every dose is small. The numbers to hold: free chlorine 4 to 6 ppm with CYA around 30, or bromine 4 to 6 ppm; pH 7.2 to 7.8; total alkalinity 50 to 90 ppm; calcium hardness 100 to 250 ppm; and water no hotter than 104°F (40°C).
At this size, 15 mL of 10% liquid chlorine raises a 400-gallon tub by 1 ppm. Measure in millilitres or tablespoons, not cups.
The targets
| Reading | Chlorine hot tub | Bromine hot tub |
|---|---|---|
| Sanitizer | Free chlorine 4 to 6 ppm (never below 2) | Bromine 4 to 6 ppm |
| Shock level | 12 ppm free chlorine | not used |
| pH | 7.2 to 7.8, aim for 7.5 | 7.2 to 7.8, aim for 7.5 |
| Total alkalinity | 50 to 90 ppm, aim for 60 | 50 to 90 ppm, aim for 60 |
| Calcium hardness | 100 to 250 ppm, aim for 150 | 100 to 250 ppm, aim for 150 |
| CYA (stabilizer) | 20 to 40 ppm, aim for 30 | not used: CYA does not stabilize bromine |
| Temperature | 104°F (40°C) or lower | 104°F (40°C) or lower |
The chlorine targets assume CYA around 30. If CYA climbs above 50, replace water to bring it down.
How to balance a hot tub, step by step
- Test sanitizer, pH, alkalinity and temperature. A tub this small changes quickly, so test before you add anything.
- Check the temperature first. Above 104°F (40°C), turn the heater down before anyone gets in.
- Fix alkalinity, then pH. Raise alkalinity first if it is low, then bring pH toward 7.5 (acid lowers it). Add each product separately, with the pump running, and wait 30 minutes after each.
- Raise the sanitizer to the target. In a 400-gallon tub, 15 mL of 10% liquid chlorine raises free chlorine by 1 ppm. Add it separately, with the pump and jets running.
- Retest after 30 minutes. Never mix products, in the water or in a cup.
Chlorine or bromine?
- Chlorine is the same chemical you would use in a pool, at spa scale. A new tub builds its CYA with dichlor: every 10 ppm of chlorine from dichlor brings about 9 ppm of CYA with it. Once CYA reaches 20 to 40, switch to liquid chlorine or bleach so CYA stops climbing. In a 400-gallon tub, about 2.7 g of dichlor raises chlorine by 1 ppm.
- Bromine holds 4 to 6 ppm and needs no CYA at all, because CYA doesn't stabilize bromine. Top it up with the bromine product you use, following its label, and retest.
Both work. Pick one and stay with it; don't mix sanitizer systems in the same water.
Metric notes
- In a 1,500-litre tub, about 15 mL of 10% liquid chlorine raises free chlorine by 1 ppm, and about 12 mL of 12.5% does the same.
- In Australia, hot tubs are usually called spas; the chemistry and the targets are the same.
- Local product names: "pool acid" lowers pH, "buffer" raises total alkalinity, and "stabiliser" is CYA (built in a spa with dichlor, not granular stabiliser).
Questions people ask
How much chlorine do I add to a 400-gallon hot tub?
15 mL of 10% liquid chlorine raises free chlorine by 1 ppm, so going from 2 to 5 ppm takes 45 mL (about 1½ fl oz). Add it separately, with the jets running, and retest after 30 minutes.
Do I need stabilizer (CYA) in a bromine hot tub?
No. CYA protects chlorine, not bromine, so a bromine tub carries none. A chlorine tub aims for 20 to 40 ppm, built up with dichlor.
Can I use my pool chemicals in the hot tub?
The products are the same, but the amounts are not. A dose that barely moves a pool can overshoot a tub many times over, so work the amount out for your tub's volume and measure it.
How hot is too hot?
104°F (40°C) is the limit. Above it, turn the heater down before anyone gets in.
For a plan built from your own readings, pool or hot tub, use the water check. There is more on spas on the hot tubs page.