Hot tub chemical calculator
One test reading becomes one dose when you use a hot tub chemical calculator. For 100 gallons (380 litres), start with 0.1 oz (2.8 g) pH down to bring pH from about 7.8 toward 7.5; high alkalinity slows the change, so run the jets and test again before adding more. Free chlorine rises by 1 ppm per 100 gallons with only 0.7 g of chlorine granules.
Choose the issue, enter your tub details and your reading, then view the dose.
- Starting pH down dose for 100 gal at pH 7.8; test again after 15 minutes
- 0.1 oz (2.8 g)
- Per 100 gal: raise alkalinity by 10 ppm
- 0.22 oz (6.2 g)
- For every 100 gal, add dichlor to raise chlorine by 1 ppm
- 0.7 g
- Run jets between chemical additions
- 15 min
Hot tub chemical levels chart: sanitizer, alkalinity level and pH level
Aim for these ideal chemical levels in your hot tub. Sanitizer refers to chlorine or bromine used to kill germs, while ppm means parts per million, the unit shown on your test strip. Look across the row to find problems caused by readings that are too low or too high.
| Chemical | Ideal range | If it is too low | If it is too high |
|---|---|---|---|
| Free chlorine | 3 to 5 ppm | Germs can grow; in hot tubs, CDC wants at least 3 ppm. | The EPA dichlor label warns against soaking above 5 ppm |
| Bromine | The AquaChek care guide lists 4 to 6 ppm | Public hot tubs have a floor of 4 ppm set by CDC; germs can grow below that. | CDC says you must stay out of the water above the manufacturer's upper limit. |
| pH | AquaChek suggests 7.4 to 7.6, though 7.2 to 7.8 is still OK | Pipes corrode more easily below 7.0, says CDC; Lay-Z-Spa notes burning eyes and itchy skin. | Chlorine kills germs less well, says CDC; scale and cloudy water are noted in the American Whirlpool manual. |
| Total alkalinity | The EPA dichlor label specifies 80 to 120 ppm | Frog notes itchy skin and sore eyes along with pH that drifts down and swings. | Frog reports scale in pipes, cloudy water and stuck pH |
| Calcium hardness | The Intex spa manual lists 150 to 250 ppm | Frog states water becomes corrosive and eats metal and grout | Frog reports scale around equipment and cloudy water |
Calculator for spa chemicals: covers chlorine, pH and alkalinity; shock has a separate page
You can use this spa chemical calculator for any spa or hot tub. Type your gallons or litres or pick a size, then correct pH, alkalinity, chlorine or bromine. Use the hot tub shock calculator for shock treatments.
Correct alkalinity first if multiple readings are wrong, then pH, then chlorine or bromine. This order comes from Lay-Z-Spa since sanitizer works poorly until pH is right and alkalinity holds pH steady.
Hot tub chemical calculator dose chart
Look at your test result, then find the column with the tub size closest to yours. The tool at the top calculates doses for other sizes. Unsure of your tub size? Calculate how many gallons your hot tub holds.
Each amount is a starting dose for the product named in the row: pH down is dry acid (sodium bisulfate) and the chlorine granules are dichlor with 56% available chlorine. Lowering alkalinity takes more of the same pH down than a pH nudge does, because alkalinity is what holds pH steady and the acid has to use that up first. Add the dose with the jets running, wait 15 minutes, then test again before adding more.
| To fix | Product | 100 gal (380 L) | 250 gal (950 L) | 400 gal (1,510 L) | Source |
|---|---|---|---|---|---|
| pH 7.8 to 7.5 | pH down (sodium bisulfate) | 0.1 oz (2.8 g) | 0.25 oz (7 g) | 0.4 oz (11 g) | Leslie's spa chart |
| pH 8.0 to 7.5 | pH down (sodium bisulfate) | 0.17 oz (5 g) | 0.42 oz (12 g) | 0.68 oz (19 g) | Leslie's spa chart |
| pH 8.2 to 7.5 | pH down (sodium bisulfate) | 0.2 oz (6 g) | 0.5 oz (14 g) | 0.8 oz (23 g) | Leslie's spa chart |
| pH 7.2 to 7.5 | pH up (soda ash) | 0.13 oz (4 g) | 0.33 oz (9 g) | 0.52 oz (15 g) | Leslie's spa chart |
| pH 6.8 to 7.5 | pH up (soda ash) | 0.37 oz (10 g) | 0.93 oz (26 g) | 1.5 oz (42 g) | Leslie's spa chart |
| pH 6.5 to 7.5 | pH up (soda ash) | 0.5 oz (14 g) | 1.3 oz (35 g) | 2 oz (57 g) | Leslie's spa chart |
| Alkalinity up 10 ppm | Alkalinity up (sodium bicarbonate) | 0.22 oz (6 g) | 0.55 oz (16 g) | 0.88 oz (25 g) | CPO dosage tables |
| Alkalinity down 10 ppm | pH down (sodium bisulfate) | 0.34 oz (10 g) | 0.85 oz (24 g) | 1.4 oz (39 g) | CPO dosage tables |
| Free chlorine up 1 ppm | Chlorine granules (sodium dichlor) | 0.02 oz (0.7 g) | 0.06 oz (1.7 g) | 0.1 oz (2.7 g) | EPA dichlor label, 56% available chlorine |
| Bromine up 1 ppm | Bromine granules | 0.03 oz (0.9 g) | 0.07 oz (2.1 g) | 0.12 oz (3 g) | AquaChek care guide |
We get the pH rows by dividing the Leslie's 300 gallon column by 3. Target values are bromine 5 ppm, chlorine 4 ppm, alkalinity 100 ppm and pH 7.5 for these doses. Leslie's uses kitchen spoons for measuring: a tablespoon holds about 0.5 oz, while a teaspoon holds about 0.17 oz.
How much chlorine to add to hot tub water
Aim for 3 to 5 ppm of free chlorine, with CDC marking 3 ppm as the minimum. Our dose targets 4 ppm.
Imagine a 350 gallon (1,320 litre) tub that reads 1 ppm. Dichlor granules for this tub come to 0.25 oz (7 g). That tub requires 0.33 oz (9.5 g) starting from 0 ppm. So how much chlorine to add to hot tub water relies on two factors: your gallon count and how many ppm the reading sits below 4.
The EPA dichlor label instructs you to wait 5 minutes after broadcasting it over the water, then test. You can find more detail on the chlorine level page.
How much bromine to add to hot tub water
Target 4 to 6 ppm for bromine, with CDC setting 4 ppm as the public hot tub floor. For every 100 gallons, use 0.03 oz (0.9 g) of granules per ppm. To reach 5 ppm, a 350 gallon tub at 1 ppm requires 0.42 oz (12 g).
After a refill, deciding how much bromine to add to hot tub water takes extra effort. Leslie's first builds a bank of bromide from sodium bromide, at 10 to 15 ppm. Next, it shocks the water and drops tablets into a floater.
The order to add hot tub chemicals
- Start by testing every pad. Before adding anything, read all pads using test strips.
- Adjust alkalinity. This step comes first in the Lay-Z-Spa sequence because it stabilizes pH.
- pH adjustment comes next. Your target is 7.4 to 7.6. For more help, refer to the guides on how to lower pH or raise alkalinity.
- Chlorine or bromine goes in last. Sanitizer performs poorly until pH and alkalinity are correct.
- Give each chemical its own turn. Leslie's lets the jets run 15 minutes before it adds another. At least 2 hours is what Lay-Z-Spa asks for in its tubs.
- Add more only after you test. Small amounts are added by Leslie's, with retesting after about an hour of circulation.
Safety with chemicals
Pour each chemical directly into the tub water while the jets run. A small amount of water can trigger a reaction, warns the EPA dichlor label. This reaction may release heat or toxic gas. That is why you must never pre-mix it in a cup.
For each product, use a clean, dry scoop and nothing else. Limit dry acid to 1 oz (28 g) per 500 gallons (1,890 litres) in a single dose, says the AquaChek guide. Keep containers separate and do not stack them.
Put on safety glasses. Rinse for 15 to 20 minutes with water if any chemical gets in your eyes, and phone poison control.