Hot Tub Chemical Calculator
Enter your hot tub test strip readings and get how many spoonfuls of chlorine, bromine, pH up or down and baking soda to add, in order.
A hot tub holds so little water that a spoonful too much throws it off, and the order matters: alkalinity steadies pH, and pH decides how well the chlorine or bromine works. This reads your test strip and gives level spoonfuls for your tub's size, only for what's off.
Your hot tub
Pick its size and what keeps it clean.
What keeps it clean
Your test strip
The three readings from a test strip or drop kit.
Chlorine
Free chlorine. 3 to 5 ppm is the goal.
pH
7.2 to 7.8 is the goal.
Alkalinity
Total alkalinity. 80 to 120 ppm is the goal.
Your water
One thing to add
Then retest in 20 minutes
Add it with the jets running and the cover off. These are level spoonfuls; product strengths vary, so check the label and retest before adding more.
💡About this calculator▼
A hot tub holds a few hundred gallons, so the amounts are small, often a teaspoon or two, and easy to overdo. The order matters too: alkalinity keeps pH steady, and pH decides how well the chlorine or bromine works.
This turns your test strip into spoonfuls. Pick the tub's size by seats, or enter its gallons, choose chlorine or bromine, and type in the three readings. You get a numbered list of only what's off, in the order to add it, with the sanitizer last.
Level spoonfuls, checked against the labels. The amounts come from the registered product labels and the chemistry behind them, converted to level teaspoons and tablespoons by how much each product weighs per spoonful.
Alkalinity first. If total alkalinity reads below 80 ppm, it's raised to 100 with baking soda, at the label rate of 1.5 pounds per 10,000 gallons for every 10 ppm. In a 300-gallon tub, going from 60 to 100 takes about 82 grams, roughly 6 tablespoons. Baking soda nudges pH up a little, so the pH step starts from where it leaves the water.
Alkalinity too high works differently. Above 120 ppm there is no product that only lowers alkalinity; pH down lowers it along with pH. So the calculator gives one round of pH down, enough to reach 100 or to bring pH to 7.0, whichever comes first, since CDC warns that pipes corrode below 7.0. Running the jets with the cover off for 4 to 6 hours lets pH climb back without raising alkalinity, and then the next round goes in. With alkalinity at 300 and pH 8.2 in a 300-gallon tub, a round is about 8½ tablespoons.
pH next. The goal is 7.2 to 7.8. Above 7.8, dry acid (pH down) brings it to 7.5. How much it takes depends on the alkalinity, which resists the change, so the calculator works it out from both readings. Below 7.2 is handled more carefully, because soda ash (pH up) raises alkalinity too: it gives only enough to reach 7.2, and never so much that alkalinity passes 120. If that isn't enough, running the jets with the cover off finishes the job, since it lets carbon dioxide escape and lifts pH without touching alkalinity.
Sanitizer last. Chlorine below 3 ppm is raised to 4 with chlorine granules, which are 56 percent available chlorine. In a 300-gallon tub, going from 1 to 4 ppm takes about 6 grams, 1¼ level teaspoons. Bromine below 3 gets the label dose of 1 teaspoon per 200 gallons, repeated every 20 minutes until the test reads 3 to 6.
Too much. The chlorine label prohibits getting in above 5 ppm, and the bromine label above 8, so those readings say to wait with the cover off instead.
Spoonfuls. Grams become level spoonfuls by each product's weight: about 4.6 grams per teaspoon for chlorine granules and baking soda, 6.6 for dry acid and 5.1 for soda ash. Under 3 teaspoons it rounds to the nearest quarter teaspoon, above that to the nearest half tablespoon.
📐How it's calculated▼
Baking soda (alkalinity below 80): grams = (100 − reading) ÷ 10 × 1.5 lb × 453.6 × gallons ÷ 10,000.
pH: alkalinity and pH give bicarbonate and carbonic acid (pH = 6.35 + log of their ratio). Dry acid to reach 7.5 = the bicarbonate that must turn to carbonic acid, with r = 10^(7.5 − 6.35). Soda ash to reach 7.2 = (r × carbonic acid − bicarbonate) ÷ (r + 2), with r = 10^(7.2 − 6.35), but no more than keeps alkalinity at 120 or less.
Chlorine (below 3 ppm): grams = (4 − reading) × gallons × 3.785 ÷ 1000 ÷ 0.56. Bromine (below 3 ppm): 1 teaspoon per 200 gallons per dose.
Teaspoon weights: chlorine granules 4.6 g, baking soda 4.6, dry acid 6.6, soda ash 5.1.
300 gallons, chlorine 1, pH 8.0, alkalinity 100: 2¼ tsp of pH down first, then 1¼ tsp of chlorine granules. 300 gallons, alkalinity 60, pH 7.5, chlorine 4: 6 tbsp of baking soda only.
📎Sources:U.S. EPA — Spa 56 chlorinating granules label (available chlorine, dose, soak limit, pH and alkalinity ranges),U.S. EPA — SpaGuard brominating concentrate label (dose and limits),CDC — Home Pool and Hot Tub Water Treatment and Testing
🔍Finding your inputs▼
Size: Pick by seats: 2–3 seats holds about 200 gallons, 4–5 seats about 300, and 6 or more about 400. Makers' own figures run from about 200 gallons for a 3-seat tub to over 500 for a large 7-seat one, so if you know yours, choose Enter gallons and type it in. It's in the owner's manual or on the maker's website.
What keeps it clean: Chlorine or bromine, whichever your tub uses. The chlorine doses are for chlorine granules; the bromine doses are for bromine granules, used to bring the level up even if you normally run tablets in a floater.
Chlorine or bromine: The reading from your test strip or drop kit, in ppm. For chlorine, use free chlorine; for bromine, total bromine.
pH: The goal is 7.2 to 7.8.
Alkalinity: Total alkalinity, in ppm. The goal is 80 to 120.
⚠️Special situations▼
My alkalinity is very high, and pH is already low
Then pH down would push pH too low, and pH up would raise the alkalinity further. Run the jets with the cover off for 4 to 6 hours first; that raises pH without adding alkalinity. Then retest and lower the alkalinity in rounds of pH down, as the calculator shows.
I just filled the tub with fresh water
Test the fresh water first and fix alkalinity and pH before adding sanitizer, as the calculator orders it. With bromine, the first few doses build up a reserve in the water, so the reading climbs slowly; keep adding the label dose every 20 minutes until it reads 3 to 6.
My chlorine reads high after a shock
The chlorine label prohibits getting in above 5 ppm (8 for bromine). Leave the cover off with the jets or blower running and test again later; it falls on its own as it is used up. There is nothing to add.
I use liquid bleach instead of granules
The chlorine doses here are for chlorine granules, which are 56 percent available chlorine. Bleach is much weaker by volume and comes in different strengths, so these spoonfuls do not apply to it; follow the bleach label or a dosing chart made for it.
❓Common questions▼
How much chlorine do I add to my hot tub?
In a typical 300-gallon tub, about 1¼ level teaspoons of chlorine granules raise the level from 1 to 4 ppm; a 200-gallon tub takes about 1 teaspoon and a 500-gallon tub about 2¼. Aim for 3 to 5 ppm. Add it with the jets running and the cover off, and test again after 20 minutes.
What should hot tub chemical levels be?
Chlorine 3 to 5 ppm or bromine 3 to 6, pH 7.2 to 7.8, and total alkalinity 80 to 120 ppm. Chlorine should never be lower than 3. Don't get in when chlorine is above 5 or bromine above 8; those are the limits on the product labels.
What order do I add hot tub chemicals?
Alkalinity first, then pH, then the chlorine or bromine. Alkalinity holds pH steady, and chlorine and bromine work best once pH is in range. Add one product at a time with the jets running, and wait about 20 minutes before the next.
How do I lower the pH in my hot tub?
Add dry acid, sold as pH down or pH decreaser. In a 300-gallon tub with alkalinity around 100, about 2¼ level teaspoons bring pH from 8.0 to 7.5. The jets push gas out of the water, so pH tends to drift back up; retest before adding more.
How do I lower alkalinity in my hot tub?
With pH down, in rounds. Each round lowers alkalinity and pH together; then run the jets with the cover off for 4 to 6 hours so the pH climbs back, retest, and add another round until alkalinity reads 120 or less. In a 300-gallon tub at 300 ppm and pH 8.2, a round is about 8½ level tablespoons.
Is chlorine or bromine better for a hot tub?
Both keep the water clean, and the calculator covers either. One thing to know: most chlorine granules, and some bromine granules, are made with dichlor, which adds a stabilizer called cyanuric acid. The CDC recommends against it in hot tubs, and the chlorine label caps it at 100 ppm, which is one reason to drain and refill now and then.
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