How the calculation works

Chlorine tests read in parts per million (ppm), and 1 ppm is 1 milligram per litre. The calculator uses the same maths as our pool chlorine calculator, scaled down to a hot tub:

Chlorine needed (grams of available chlorine) = (target ppm − current ppm) × water volume in litres ÷ 1,000. A US gallon is 3.785 litres.
Dichlor granules: grams = chlorine needed ÷ (available chlorine % ÷ 100). Use the "available chlorine" figure on the label (55% to 57% for dichlor dihydrate and at least 62% for anhydrous dichlor, according to the FAO/WHO specification), not the active ingredient percentage.
Liquid chlorine and bleach: the label strength is turned into grams of available chlorine per litre. US labels give sodium hypochlorite as a percentage by weight (EPA), so the calculator multiplies by 0.952 (the available chlorine in a gram of sodium hypochlorite) and by the density: about 1.10 for household bleach (Clorox's safety data sheet), rising to 1.20 at 12.5% (Formosa Plastics) and 1.242 at 15% (Rowell Chemical). If your label gives grams per litre or "trade percent", switch the setting; 125 g/L is 12.5 trade percent.

Granules are given by weight, not in teaspoons, because we could not find a published density for dichlor granules to convert reliably. If you can't weigh a few grams accurately, follow the dosing directions on the product label instead.

Worked example

A 400 US gallon hot tub reads 1 ppm and you want 4 ppm. 400 gallons is 1,514 litres, so the 3 ppm rise needs 3 × 1,514 ÷ 1,000 = 4.54 g (0.16 oz) of available chlorine.

6% household bleach: 72.3 mL (2.44 fl oz).
Pool liquid chlorine labelled 12.5% sodium hypochlorite: 31.8 mL (1.08 fl oz).
Dichlor at 56%: 4.54 ÷ 0.56 = 8.1 g (0.29 oz). It also adds 2.73 ppm of cyanuric acid.

In metric, a 1,500 litre hot tub raised from 1 to 4 ppm needs 4.5 g of available chlorine: 71.6 mL of 6% bleach or 8 g of 56% dichlor.

Hot tub targets

What	Level	Who says so
Free chlorine	At least 3 ppm; 3 to 5 ppm recommended	CDC; MAHC minimum for public spas, with 3–5 ppm in its annex
Free chlorine, upper	Speak to the owner above 10 ppm	CDC advice to hot tub users
Bromine	4 to 8 ppm	CDC; MAHC for public spas
pH	7.0 to 7.8	CDC; MAHC
Stabilizer (cyanuric acid)	None	CDC recommends not using it in hot tubs
Water temperature	No hotter than 104°F (40°C)	CDC; MAHC
Testing	Chlorine or bromine, and pH, at least twice a day	CDC

The MAHC figures are legal requirements for public spas where a state or city adopts the code. For a home hot tub they are a useful benchmark, alongside CDC's advice for home owners.

Liquid chlorine or dichlor?

CDC recommends not using cyanuric acid, or chlorine products that contain it, in hot tubs. Dichlor is one of those products: in water, each dichlor molecule splits into cyanuric acid and two units of chlorine (WHO), which works out at about 0.91 ppm of cyanuric acid for every 1 ppm of chlorine (molar masses from PubChem). The Model Aquatic Health Code bans cyanuric acid and stabilized chlorine in new or re-equipped public spas, and requires all public spas to stop using them within four years of a state or city adopting the code.

The Oregon Health Authority explains why: chlorine breaks down faster in hot water, and cyanuric acid makes it slower to kill germs on top of that. A hot tub's small volume also means a few people can use up chlorine quickly, and cyanuric acid builds up faster. It only comes down when you drain some water and refill. If you do use dichlor, the calculator shows how much cyanuric acid each dose adds.

Liquid chlorine and household bleach add no cyanuric acid. They do weaken in storage: EPA asks sodium hypochlorite labels to warn that the product degrades with age, sunlight and heat, so use a test kit and add more if needed.

Bromine

CDC suggests 4 to 8 ppm of bromine in a hot tub, and the MAHC requires public spas to keep at least 4 ppm and never more than 8 ppm. The calculator does not work out bromine doses: bromine is sold in different forms, such as the BCDMH and DBDMH compounds the MAHC names, and each product label gives its own dosing directions. Follow the label, and test as often as you would for chlorine.

Hot tub safety

Heat: keep the water at or below 104°F (40°C). CDC says children under 5 should not use hot tubs, and anyone who is pregnant should talk to a healthcare provider before using one.
Never mix chemicals. Don't mix chlorine with acid, which can release toxic gas, and don't mix different chemicals, including different types of chlorine, with each other or with anything else (CDC). The MAHC annex says to keep hydantoin bromine compounds, dichlor and other stabilized chlorine, and cal-hypo away from other chemicals.
Add chemical to water, never water to chemical, and only pre-dissolve a product if its label says so (CDC).
Protect yourself: read the label, wear goggles and gloves, work in fresh air and open one container at a time (CDC).
Keep people out of the water while you add chemicals (New Jersey Department of Health), and test again before anyone gets in.
Add a big dose in stages, retesting between additions. Sodium hypochlorite labels say to use a test kit and increase the dose as necessary, because the product weakens with age (EPA).
Store products closed in their original containers, cool, dry and locked away from children and pets, with acid and chlorine kept apart (CDC).

Common questions

How much bleach do I add to a hot tub?: Raising a 400-gallon hot tub by 1 ppm takes 1.51 g of available chlorine: about 24.1 mL (0.81 fl oz) of 6% bleach, or 17.5 mL of 8.25% bleach. For a 1,500-litre tub it is 1.5 g, or about 23.9 mL of 6% bleach. Multiply by the number of ppm you want to add, or let the calculator do it.
Why does CDC say not to use dichlor in a hot tub?: Dichlor contains cyanuric acid (stabilizer), which slows down how fast chlorine kills germs. CDC recommends not using cyanuric acid or chlorine products that contain it in hot tubs, and the MAHC is phasing it out of public spas. Every 1 ppm of chlorine from dichlor adds about 0.91 ppm of cyanuric acid, so the 3 ppm dose in the worked example adds 2.7 ppm, and it only goes down when you drain and refill (Oregon Health Authority).
What bromine level should a hot tub have?: CDC gives 4 to 8 ppm. The MAHC requires public spas to keep at least 4 ppm and never more than 8 ppm. This calculator does not work out bromine doses, because bromine products differ; follow the dose on your product label.
How hot can a hot tub be?: No hotter than 104°F (40°C), according to CDC and the MAHC. CDC also says children under 5 should not use hot tubs, and anyone who is pregnant should talk to a healthcare provider first.
How often should I test hot tub water?: CDC recommends testing chlorine (or bromine) and pH at least twice a day, and more often when the hot tub is busy. Keep pH between 7.0 and 7.8.

Sources

Last checked and updated: October 31, 2026.