Hot tub chemical dosing calculator
Tell it your water volume, what the test says now, and where you want it. You get the exact amount to add, in ounces and approximate tablespoons, for pH, alkalinity, chlorine, bromine, and shock. Everything runs in your browser, so it works at the tub with no signal.
Enter your tub volume and a reading to size the dose.
Add in small steps with the pump running, wait, then retest before adding more. Never mix chemicals together, and always follow your product label, which wins over any calculator.
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How to use it
- Find your water volume once. If you do not know it, the volume calculator gets you close, or use your tub's spec.
- Test the water with strips or a kit. Balance total alkalinity first, then pH, then sanitizer; alkalinity is the buffer that holds pH steady.
- Pick the chemical, enter the current reading and your target, and add the amount shown in small steps.
- Run the pump to circulate, wait, and retest before adding any more. It is easy to add; it is slow to take back out.
Quick reference: common doses
Computed from the same chemistry the calculator uses, for three typical spa sizes. Amounts are ounces by weight.
| To do this | Chemical | 200 gal | 350 gal | 500 gal |
|---|---|---|---|---|
| Raise total alkalinity +10 ppm | sodium bicarbonate | 0.48 oz | 0.84 oz | 1.2 oz |
| Raise free chlorine +3 ppm | dichlor granules | 0.14 oz | 0.25 oz | 0.36 oz |
| Raise bromine +3 ppm | bromine granules | 0.13 oz | 0.23 oz | 0.33 oz |
| Lower pH 7.8 to 7.5 | sodium bisulfate | 1.5 oz | 2.6 oz | 3.8 oz |
| Chlorine shock (+10 ppm) | dichlor | 0.48 oz | 0.83 oz | 1.2 oz |
| Non-chlorine shock (routine) | potassium monopersulfate | 0.80 oz | 1.4 oz | 2.0 oz |
Last verified 2026-06-13. pH and alkalinity move together, and pH dosing depends on your alkalinity, so treat the pH row as a starting step and retest. Sources are listed under the method note below.
Where these numbers come from
The dosing math is computed from published chemistry, not copied from one brand's chart, and every constant is checked by an assertion when the site is built so a wrong number cannot ship. Total alkalinity uses the standard rate of about 1.5 lb of sodium bicarbonate per 10,000 gallons for a 10 ppm rise. Chlorine and bromine are dosed by their available-halogen content (dichlor at about 56 percent available chlorine, bromine granules at about 60 percent available bromine). pH adjusters are buffer-dependent, so those figures are representative starting steps, not exact.
- Sodium bicarbonate raises total alkalinity about 10 ppm per 1.5 lb in 10,000 gallons (standard pool/spa dosage tables; Taylor Technologies; Trouble Free Pool PoolMath).
- Dichlor (sodium dichloro-s-triazinetrione dihydrate) is about 56 percent available chlorine; dosing is by available-halogen content (1 mg/L product per 1 mg/L residual, divided by the available fraction).
- Bromine granules / BCDMH are about 60 percent available bromine; spa bromine is carried at 4 to 6 ppm.
- Balanced-water ranges follow common spa water-care guidance (PHTA/APSP ranges and typical spa product labels): pH 7.2 to 7.8, total alkalinity 80 to 120 ppm, free chlorine 3 to 5 ppm, bromine 4 to 6 ppm.
- pH adjuster rates are representative spa-label increments and are buffer-dependent estimates; always add in small steps and retest.
Common questions
Should I balance alkalinity or pH first?
Alkalinity first. Total alkalinity is the buffer that keeps pH from bouncing around, so get it into the 80 to 120 ppm band, then adjust pH. If you chase pH while alkalinity is off, the pH will not hold.
Why does the pH dose feel approximate?
Because pH is logarithmic and held in place by alkalinity, the exact amount of acid or soda ash depends on your water. The calculator gives a sensible starting step at typical spa alkalinity. Add it, circulate, wait, and retest, then repeat if needed.
Chlorine or bromine for a hot tub?
Both work. Bromine is stable at spa temperatures and is gentler on skin and eyes, which is why many hot tub owners prefer it; it is usually carried a little higher (4 to 6 ppm) than chlorine (3 to 5 ppm). Chlorine, usually dichlor granules in a spa, is cheaper and acts faster. The calculator sizes either one.
My reading is already past the target. What does zero mean?
It means nothing to add for that chemical right now. If a level is too high, you generally wait for it to fall, dilute with a partial water change, or in the case of high chlorine use a neutralizer; you do not add more of the same product.
Does it work offline?
Yes. The calculator is just math in your browser, and the site installs as an app, so once you have opened it the calculators keep working at the tub with no signal.