TubTender / Hot tub chemical calculator
Hot tub chemical calculator
Enter your tub size and test strip readings to get how much of each chemical to add, in the right order: alkalinity, then pH, then calcium, then sanitizer, with 20 minutes of jets between each. It runs the same chemistry engine as the TubTender app, works for bromine and chlorine tubs, and is free with no account.
How much chemical to add to a hot tub
The right amount depends on three things: how many gallons your tub holds, how far each reading is from its target, and how strong your product is. Two bottles both called pH decreaser can differ in strength, and a pH dose also depends on your alkalinity, because alkalinity buffers pH. That is why a single scoop per 100 gallons chart is often wrong.
This calculator models the carbonate buffer at your water temperature, so acid and base doses account for alkalinity, and it doses bromine tubs through the bromide reserve, where each ppm of chlorine from shock turns into about 2.25 ppm of bromine.
Add chemicals in this order
- Alkalinity first. It steadies pH, so fixing it first stops pH from bouncing back.
- pH next. High pH makes chlorine and bromine much weaker; low pH stings eyes and wears on the heater.
- Calcium. Low calcium can etch surfaces and add to foaming; high calcium scales, and only fresh water lowers it.
- Sanitizer last, so it is fresh when you get in.
Run the jets with the cover open while you add each one, and wait about 20 minutes before the next. Always follow the label on your product.
Hot tub chemical levels
| Reading | Aim for | Safe to soak |
|---|---|---|
| Free chlorine | About 4 ppm (more with stabilizer) | 3 to 10 ppm |
| Bromine | About 5 ppm | 4 to 8 ppm |
| pH | About 7.5 | 7.2 to 7.8 |
| Total alkalinity | 80 to 120 ppm, about 90 | Out of range makes pH drift |
| Calcium hardness | 150 to 400 ppm, about 200 | Out of range is not a soak risk |
| Stabilizer (chlorine tubs) | Under 50 ppm | Above that, drain part of the water |
| Temperature | Your preference | No hotter than 104°F |
The soak limits follow CDC hot tub guidance.
Questions
How much chlorine do I add to my hot tub?
It depends on your tub size, your reading and your product's strength. For example, raising a 400 gallon tub from 1 to 4 ppm takes about 0.3 oz (about 1¾ tsp) of 56 percent dichlor granules. Enter your numbers in the calculator above for your exact amount, and follow the label on your product.
What order should I add hot tub chemicals in?
Alkalinity first, then pH, then calcium, then sanitizer last. Run the jets with the cover open while adding each one and wait about 20 minutes before the next.
How do I raise bromine in a hot tub?
Add a chlorine shock such as dichlor granules. It turns the used up bromide in the water back into bromine, about 2.25 ppm of bromine for every 1 ppm of chlorine it adds, which works faster than tablets alone. Keep tablets in the floater so the level holds between tests.
When is it safe to get in the hot tub after adding chemicals?
When a retest shows chlorine between 3 and 10 ppm or bromine between 4 and 8 ppm, pH between 7.2 and 7.8, and water no hotter than 104°F. After a normal dose that is usually about 20 minutes with the jets running.
Should I fix pH or alkalinity first?
Alkalinity first. Alkalinity buffers pH, so if it is low, pH bounces around, and a pH dose made before fixing it will not hold.
Is this hot tub calculator free?
Yes. It runs in your browser with no account and no download. It uses the same chemistry engine as the TubTender iPhone app.