Hot tub chemical calculator
Enter your tub's gallons and your test results. You get each dose, in the order to add it, for the products you own.
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
On this page (7 sections)
Example: a 400-gallon hot tub at 1 ppm free chlorine (FC) needs about 8.1 g (about 1½ tsp) of 56% dichlor granules to reach 4 ppm, then about 9 g of 93.2% dry acid to bring pH from 7.8 to 7.5.
What the calculator needs
In order of importance:
- Volume. Our calculator accepts 50–1,500 gallons. Use the number from your manual or the maker's spec sheet. If you don't have it, time a fill with a bucket and a hose; guessing from seat count can be off by a third or more.
- Sanitizer. Chlorine or bromine. For chlorine, enter FC and, if your kit reads it, total chlorine (TC): the difference is combined chlorine (CC), the used-up chlorine that causes the "chlorine smell."
- pH and total alkalinity (TA). The acid or base dose depends on both. A strip that reads TA in steps of 40 ppm is good enough to start.
- Calcium hardness (CH) and temperature are optional. They are used for the water balance check.
- Products you own. Pick the strength printed on your label. The dose of 6% bleach is about twice the dose of 12.5% liquid chlorine.
The plan follows a fixed order: sanitizer first if it is below the minimum, then alkalinity, then pH, then calcium. Wait about 30 minutes with the jets running between products. Doses that would move pH more than 0.4 at once are split into smaller steps so you can retest in between. Results are estimates from our published formulas. Always follow the product label.
Hot tub targets (CDC and Taylor): FC 3–4, bromine 4–6, pH 7.4–7.6
- Free chlorine: at least 3 ppm (CDC minimum). The calculator aims for 3–4 ppm, because 4 ppm is the soaking limit on many spa chlorine labels. If your label allows a different number, enter it under "Swim limit on your chlorine label" and the calculator uses yours.
- Bromine: at least 4 ppm; target 4–6 ppm (CDC model code, Annex). A bromine test reads 2.25 times what the same chlorine level would.
- pH: CDC accepts 7.0–7.8; test kits and industry charts use 7.2–7.8. We aim for 7.4–7.6.
- Total alkalinity: 80–120 ppm (Taylor); 60–180 ppm is still acceptable.
- Calcium hardness: 150–250 ppm (Taylor); the industry standard range for spas is 100–800 ppm.
- Stabilizer (CYA): none.
When one number is off, the fix pages go further: pH too high, pH too low, and cloudy or green water. The full table is also on the hot tub chemical chart.
Why small tubs need small, precise doses
The same 1 ppm change takes about 37 times less product in a 400-gallon tub than in a 15,000-gallon pool. A "capful" that is right for a pool is a large overdose in a spa.
| Change | Product | 400 gal hot tub | 15,000 gal pool |
|---|---|---|---|
| FC +1 ppm | Dichlor 56% | 2.7 g (0.1 oz) | 101 g (3.6 oz) |
| FC +1 ppm | Household bleach 6% | 24 mL (0.8 fl oz, about 5 tsp) | 914 mL (31 fl oz) |
| TA +10 ppm | Baking soda | 25 g (0.9 oz) | 953 g (2.1 lb) |
| TA −10 ppm | Dry acid 93.2% | 38 g (1.3 oz) | 1.46 kg (3.2 lb) |
| CH +10 ppm | Calcium chloride 77% | 22 g (0.75 oz) | 818 g (1.8 lb) |
Two practical consequences. First, weigh dry products on a kitchen scale that reads in grams: a level teaspoon of granules weighs different amounts for different products, so we don't convert grams to spoons. Liquids are easy to measure, so liquid doses also show teaspoons (1 tsp = 1/6 fl oz). Second, get the volume right. A 20% error in gallons is a 20% error in every dose, and in a small tub that is the difference between 4 ppm and going over your label's limit.
Some pool apps won't accept a volume under 1,000 gallons. Ours starts at 50, so a 177-gallon inflatable spa gets its own numbers instead of a pool's.
Why no stabilizer (CYA) in hot tubs
Stabilizer protects chlorine from sunlight in outdoor pools. A hot tub is covered most of the time, so there is little to protect against, and CYA ties up a large share of the chlorine you add. The CDC recommends not using cyanuric acid, or chlorine products that contain it, in hot tubs.
That matters because the most common hot tub chlorine, dichlor granules, is stabilized: every 1 ppm of chlorine from dichlor adds about 0.9 ppm CYA (PHTA). Add 3 ppm a day for a month and the tub gains roughly 80 ppm of CYA. If you use dichlor, drain and refill on schedule, or switch to unstabilized liquid chlorine or plain bleach once the tub is running. Only water replacement removes CYA. If you enter a CYA reading, the calculator warns you.
Temperature limit: 104 °F
The U.S. Consumer Product Safety Commission says hot tub water should never exceed 104 °F and calls 100 °F safe for a healthy adult. Heat also affects chemistry: sanitizer is used up faster, pH tends to rise as the jets drive off carbon dioxide, and calcium is more likely to scale at high temperature. The calculator flags any reading above 104 °F.
Safety
- Never mix chlorine and acid, or two different chlorine products. Add each product separately to the water, with the jets on and the cover open.
- Add chemicals to water, never water to chemicals. Wear gloves and eye protection.
- No soaking until chlorine is back at or below the swim limit on your product label (4 ppm on many labels), or bromine is in its target range.
- If someone swallows spa chemicals or gets them in their eyes, call Poison Control at 1-800-222-1222.
- Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling.
FAQ
How much chlorine do I add to a hot tub?
It depends on the product and your tub size. In a 400-gallon tub, each 1 ppm takes about 2.7 g of 56% dichlor, 15 mL of 10% liquid chlorine, or 24 mL of 6% bleach. The hot tub chlorine chart covers 200–700 gallons.
Sources
- CDC, Home pool and hot tub water treatment and testing (3 ppm chlorine in hot tubs, pH 7.0–7.8, no CYA in hot tubs): cdc.gov
- CDC, Model Aquatic Health Code 2024, Annex (spa chlorine and bromine ranges, bromine factor): cdc.gov (PDF)
- U.S. Consumer Product Safety Commission, hot tub temperatures: cpsc.gov
- PHTA fact sheet, Dichlor (CYA added per ppm chlorine): phta.org
- Taylor Technologies, K-1005 instruction manual (ideal ranges, treatment tables): PDF