Spa Calc

Hot tub chemical calculator

Enter your hot tub test and get every dose in order, plus whether your readings are in the soak range right now. It works with chlorine or bromine, in acrylic or inflatable tubs.

All-in-one hot tub calculator

gal

Not sure? Find your tub’s volume.

ppm
ppm
ppm
ppm
ppm

Dichlor adds it. Blank if you have not tested.

°F
%

Not safe: do not get in yet.

Two things to fix. Start with ph.

  1. pH 7.9 → 7.5

    Lower it

    Add

    0.42oz

    of Dry acid / pH minus (sodium bisulfate 93%)

    12 gabout 2⅛ tsp by volume

    • Side effects: TA −3 ppm.
    • Run the jets with the lid open and wait at least 15 minutes (20 after pH decreaser) before anyone gets in.
    • Never add acid and chlorine together or in the same spot: mixing them releases toxic chlorine gas. Let the first one circulate before adding the other (the wait is shown on each step).

    Then wait 15 minutes with the jets running.

  2. Free chlorine 1 ppm → 4 ppm

    Raise it

    Add

    0.29oz

    of Dichlor granules 56%

    8.1 gabout 1⅞ tsp by volume

    • Side effects: CYA +2.7 ppm, TA −1.1 ppm.
    • Also raises cyanuric acid (stabilizer). It only comes down by replacing water.
    • Run the jets with the lid open and wait at least 15 minutes (20 after pH decreaser) before anyone gets in.
    • Free chlorine is below the minimum for your stabilizer level.
    • Use dichlor for the first week or two until stabilizer reaches about 30 ppm.

In range

  • CYA 20 ppm
  • TA 70 ppm
  • Use dichlor for the first week or two until stabilizer reaches about 30 ppm.

Check the product label. Strengths vary by brand, so enter the % printed on yours. Add one chemical at a time, into the water, and never mix chemicals.

  • CDC recommends not using stabilizer (cyanuric acid) in hot tubs, because it slows chlorine down. The dichlor-then-bleach method keeps it to about 30 to 40 ppm; bleach only keeps it at zero.

Save this test and get a reminder before your next soak.

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The order

  1. Alkalinity first, then pH. Hot tub pH drifts up fast with jets and heat.
  2. Sanitizer last: dichlor while stabilizer is low, then bleach once CYA reaches about 30 ppm; or MPS and bromine tablets for bromine tubs.
  3. Wait 15 minutes with the jets running and the cover open between steps.

Before you soak

Test free chlorine (or bromine) and pH. The tub is ready when free chlorine is 3 to 5 ppm and at least 7.5% of your CYA (bromine 4 to 6) and pH is 7.2 to 7.8. Above 5 ppm chlorine, wait and retest; never soak above 10 ppm chlorine or 8 ppm bromine, or with pH below 7.0 or above 7.8. Hot tub limits based on CDC guidance and EPA product labels.

Questions

What order do I add hot tub chemicals?

Alkalinity, then pH, then sanitizer. Let each circulate for about 15 minutes.

How often should I test?

Before every soak, or at least two or three times a week.

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