Hot Tub Chemical Calculator
Hot tub minima are higher than a pool's. The dose fields start with an example: 400 gallons, from 1 ppm to the 3.0 ppm minimum, at 12.5 percent liquid. 400 gallons is not a standard tub size.
Result: add about
of 12.5% liquid chlorine (trade %), about 24 mL. Raises free chlorine by 2.0 ppm in 400 gallons.
Chlorine needed: 3.0 g = 400 gal × 3.785 L × 2.0 mg/L ÷ 1,000.
Follow the product label.
Chemical safety: read before you add anything
- Never mix pool chemicals together, in or out of the water. Add each one separately.
- Never mix chlorine with acid. Never mix cal-hypo (calcium hypochlorite) with trichlor.
- Add acid to water. Never pour water into acid.
- Split large doses into smaller portions. A numeric per-dose limit is not verified.
- This result is an estimate. Follow the product label.
How Much Sanitizer Does Your Hot Tub Need?
The code minimum for free chlorine in a hot tub is 3.0 ppm. CDC's hot tub page says to tell swimmers when chlorine is above 10 ppm. Bromine on that page is 4 to 8 ppm. pH is 7.0 to 7.8. Water temperature tops out at 104 F.
CDC does not recommend cyanuric acid in a hot tub. Calcium hardness when water is above 90 F is 100 to 200 ppm in the MAHC annex.
Product amount uses the same chlorine formula as the pool page: gallons × 3.785 × ppm increase ÷ 1,000 ÷ strength
The example dose for 400 gallons, a 2.0 ppm rise, and 12.5 percent liquid is 0.8 fl oz. Source: CDC, 2023 Model Aquatic Health Code (MAHC), §5.7.3.1.1.2.3. No CDC endorsement implied; available free at cdc.gov.
Liquid chlorine labels use trade % or weight %. The calculator defaults to trade %. A bromine product dose in ounces is not computed, because a strength table for bromine products is not in the value file. The 4 to 8 ppm range is the cited target, not a scoop count.
Hot Tub Readings From CDC
| Reading | Cited figure |
|---|---|
| Free chlorine minimum | 3.0 ppm |
| Tell swimmers above | 10 ppm |
| Bromine | 4 to 8 ppm |
| pH | 7.0 to 7.8 |
| Cyanuric acid | Not recommended |
| Water temperature maximum | 104 F |
| Calcium hardness above 90 F | 100 to 200 ppm |
Source: CDC. Use of CDC information does not imply endorsement by CDC; available free at cdc.gov.
Common Mistakes With the Hot Tub Chemical Calculator
- Using the pool's 1 ppm minimum. The hot tub minimum cited here is 3.0 ppm.
- Adding cyanuric acid because the pool uses it. CDC does not recommend it in a hot tub.
- Heating past 104 F. That is the maximum on the CDC hot tub page.
Pool Chlorine Uses the Same Dose Formula
Hot Tub Chemical Calculator FAQ
What free chlorine minimum does the MAHC set for a hot tub?
3.0 ppm. Source: CDC, 2023 Model Aquatic Health Code (MAHC), §5.7.3.1.1.2.3. No CDC endorsement implied; available free at cdc.gov. Follow the product label.
When does CDC say to tell swimmers the chlorine is high?
Above 10 ppm. Source: CDC. Use of CDC information does not imply endorsement by CDC; available free at cdc.gov. Follow the product label.
What is the highest water temperature CDC lists for a hot tub?
104 F. Source: CDC. Use of CDC information does not imply endorsement by CDC; available free at cdc.gov.
Does CDC want cyanuric acid in a hot tub?
No. CDC does not recommend cyanuric acid in a hot tub. Follow the product label.