A 400 gallon hot tub at 1 ppm free chlorine needs 3 ppm more to reach a 4 ppm target, one ppm above the CDC spa minimum of 3 ppm: 0.29 oz of 56% dichlor granules, which is about 1.6 teaspoons (8 g). That dose also adds 2.7 ppm of cyanuric acid. Choose your tub size from 200 to 600 gallons or type your own, pick chlorine or bromine, and add your pH and alkalinity readings for the balancing doses.
How the hot tub dose is calculated
Product = (target − current) × gallons ÷ 10,000 × product per ppm per 10,000 gal
A hot tub is a small volume, so the sums are the pool sums scaled down. Dichlor 56% needs 2.38 oz per ppm in 10,000 gallons; in 400 gallons that is 2.38 × 0.04 = 0.095 oz per ppm, and 3 ppm is 0.29 oz. Solid figures in teaspoons and tablespoons are approximate because granules pack differently: weigh with a kitchen scale when you can.
| Spa size | Dichlor 56% for 1 to 4 ppm | 12.5% liquid chlorine for 1 to 4 ppm |
|---|---|---|
| 200 gal | 0.14 oz | 0.61 fl oz |
| 300 gal | 0.21 oz | 0.92 fl oz |
| 400 gal | 0.29 oz | 1.23 fl oz |
| 500 gal | 0.36 oz | 1.54 fl oz |
| 600 gal | 0.43 oz | 1.84 fl oz |
Free chlorine in a spa: which number
The CDC Model Aquatic Health Code sets 3 ppm as the minimum in spas, and the CircuPool manual gives 3 to 5 ppm. Swim University recommends 1 to 3 ppm with 3 as the target. We default to a 4 ppm target, above the CDC minimum, and warn if you go under 3 ppm. Swim University's weekly dose, 0.5 oz for 500 gallons, and double for shocking, is the same order of size as the doses above.
Dichlor adds cyanuric acid with every dose. The CDC code says no new stabilizer in spas, and CYA does nothing for bromine, so a spa on dichlor needs a partial drain when the test shows a high CYA. The liquid chlorine and cal-hypo options avoid that at the price of a more careful measure.
Bromine
PHTA gives an ideal bromine range of 4 to 6 ppm in spas, and the CDC code sets 4 ppm as the minimum and 8 ppm as the maximum. Spa Depot's guide keeps it at 3 to 5 ppm. Bromine tablets and granules differ by brand, and we could not verify one product's dose that works for all, so the bromine tab asks for the numbers from your label: "X of product per Y gallons raises bromine Z ppm". The tool scales that to your tub. A chlorine test kit reads bromine low, so multiply its result by 2.25, according to PHTA. Test kits cannot tell free from combined bromine, and PHTA notes that bromamines still sanitize but organic waste builds up, so shock with a non-chlorine oxidiser on the label schedule. Spa Depot's guide, as one example, uses 1.5 to 2 oz of non-chlorine shock per 500 gallons weekly and after use.
pH, alkalinity and hardness for a spa
Spa targets in the meters come from the same ranges as pools with the spa bands where a source gives one: pH 7.2 to 7.8, total alkalinity 80 to 120 ppm, calcium hardness 150 to 450 ppm (CircuPool manual). Correct alkalinity before pH, as PHTA advises. With pH 7.9 and TA 70 ppm the tool suggests 2.7 oz of baking soda to bring TA to 100 ppm, then 0.4 oz of dry acid to bring pH to 7.5, using the same buffer model as the pool pH calculator. Pour small parts, wait, retest.
Temperature
The CDC code caps spa water at 104 °F (40 °C). Enter your water temperature and the tool flags anything above it. Hot water also uses chlorine faster.
Metric
Switch to metric and a 400 gallon tub is 1,514 L; the dichlor dose shows in grams.
What to do next
If the tub is cloudy or smells of chlorine, the ideas in the pool shock calculator apply at spa scale. Enter all your readings in the pool chemical calculator, see how alkalinity works in the pool alkalinity calculator, handle acid with the muriatic acid pool calculator and the pool volume calculator if you need to measure your tub. More tools sit in the pool chemistry hub.