Hot tub chemical calculator
A 6 seat tub holds
330gal 1,250LFirst thing out of band: Free chlorine at 0.5 ppm, which takes 1.0 fl oz of HASA CHLOR. Seats and steps take the rest: 32% of the shell is water.
Your tub and your readings
The tub's gallons, the drawing, every reading and every dose on this page are worked again as you type.
What to add
| Fixes | Product | Add |
|---|---|---|
| Free chlorine +3.0 ppm |
HASA CHLOR sodium hypochlorite |
1.0 fl oz 30 mL |
| Free chlorine +3.0 ppm |
Multi-Chlor sodium hypochlorite |
1.0 fl oz 30 mL |
| Free chlorine +3.0 ppm |
Cal Hypo 68% Granular Pool Shock calcium hypochlorite |
0.2 oz 6 g |
| pH −0.5 pH |
HASA Dry Acid sodium bisulfate |
0.7 oz 19 g |
| pH −0.5 pH |
HASA Muriatic Acid hydrochloric acid |
no quantity — see the label |
Add one thing at a time, and let the pump turn the water over before the next. The only published wait we have found is for acid: test again 30 minutes after each dose. For anything else we put no number on it.
The bands for a tub
| Reading | Range |
|---|---|
| pH COMAR 10.17.01.45 A(1) |
7.2–7.8 pH |
| Free chlorine Florida 64E-9.004 (1)(d)2 |
2–5 ppm |
| Total alkalinity COMAR 10.17.01.45 A(2) |
60–180 ppm |
| Calcium hardness COMAR 10.17.01.45 A(3) |
150–400 ppm |
| Stabilizer Florida 64E-9.004 (1)(d)4 |
up to 40 ppm |
| Water temperature COMAR 10.17.01.45 C(2) |
up to 104 °F |
A hot tub holds a fraction of a pool and heats the water, so pool quantities are wrong here. The volume here is worked out from five published shell sizes and water capacities of one maker's moulded acrylic tubs; if yours is inflatable, use the capacity printed on its box. Never mix chemicals, and never add anything while anyone is in the water.
Fix a reading
Updated October 2026
Pool and spa rules are adopted and amended state by state, and most of the ranges here are written for public pools. Confirm every figure against the code your jurisdiction has adopted and against your own product label.
Sources and dates
Ranges are quoted from the code named beside them, product strengths from the manufacturer's own product page, each retrieved on the date shown.
- COMAR 10.17.01.45 A(1) · https://www.law.cornell.edu/regulations/maryland/COMAR-10-17-01-45 · 2026-09-17
- Florida 64E-9.004 (1)(d)2 · http://flrules.elaws.us/fac/64e-9.004 · 2026-09-17
- COMAR 10.17.01.45 A(2) · https://www.law.cornell.edu/regulations/maryland/COMAR-10-17-01-45 · 2026-09-17
- COMAR 10.17.01.45 A(3) · https://www.law.cornell.edu/regulations/maryland/COMAR-10-17-01-45 · 2026-09-17
- Florida 64E-9.004 (1)(d)4 · http://flrules.elaws.us/fac/64e-9.004 · 2026-09-17
- COMAR 10.17.01.45 C(2) · https://www.law.cornell.edu/regulations/maryland/COMAR-10-17-01-45 · 2026-09-17
- HASA CHLOR · https://hasa.com/product/hasachlor · 2026-09-17
- Multi-Chlor · https://hasa.com/products/ · 2026-09-17
- Cal Hypo 68% Granular Pool Shock · https://pyrockchemical.com/products/pool-shock-calcium-hypochlorite-65-for-sale · 2026-10-01
- HASA Dry Acid · https://hasa.com/product/dry-acid · 2026-09-17
- HASA Muriatic Acid · https://hasa.com/product/muriatic-acid · 2026-09-17
- HASA · https://hasa.com/blog/how-to-lower-the-ph-in-pool-water-safely-hasa · 2026-10-01
This site publishes calculated water volumes and chemical quantities for home pool and spa owners. It is not a health, safety or professional service, and it does not replace the instructions printed on the product you are holding. Not affiliated with, endorsed by or sponsored by HASA, Pyrock Chemical, Pentair, Hayward, Sonoma Hot Tubs, the Centers for Disease Control and Prevention, or any state health department. Product names are the trademarks of their owners and appear here only to identify the strength of what is in the container.