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Calcimator

Hot Tub Chemical Calculator

Chemical doses for hot tub from volume and test results.

Inputs

Results

pH Increaser / Soda Ash (oz)

9.6

pH Decreaser / Dry Acid (oz)

0

Chlorine Granules / Dichlor (oz)

2.4

Alkalinity Up / Baking Soda (oz)16
Total Chemicals (oz)28
Est. Monthly Chemical Cost ($)$12.64
Water per Change (gal)400
How to Use This Calculator
  1. Test your hot tub water with a test strip or meter and note the current pH, alkalinity (ppm), and free chlorine (ppm).
  2. Enter your hot tub volume in gallons (check owner's manual; typical 200-500 gal).
  3. Input current and target values for pH (target 7.2-7.6), alkalinity (target 80-120 ppm), and free chlorine (target 3-5 ppm).
  4. Review the calculated doses: pH increaser (soda ash) or decreaser (dry acid), alkalinity up (baking soda), and chlorine granules (dichlor) in ounces.
  5. Add chemicals one at a time, circulate for 15 minutes, then retest before adding more.

How the result changes with Current pH

Current pHpH Increaser / Soda Ash (oz)pH Decreaser / Dry Acid (oz)Chlorine Granules / Dichlor (oz)
5.44802.4
6.42402.4
7.602.42.4
8.6014.42.4

What each input means

Tub Volume (gallons)
Water capacity. Typical hot tubs: 200-500 gallons.
Current pH
Current pH reading from test strip or meter.
Target pH
Ideal hot tub pH: 7.2-7.6.
Current Alkalinity (ppm)
Total alkalinity in parts per million.
Target Alkalinity (ppm)
Ideal range: 80-120 ppm.
Current Free Chlorine (ppm)
Current free chlorine level.
Target Free Chlorine (ppm)
Ideal hot tub range: 3-5 ppm.

What each result means

pH Increaser / Soda Ash (oz)
Ounces of sodium carbonate to raise pH.
pH Decreaser / Dry Acid (oz)
Ounces of sodium bisulfate to lower pH.
Alkalinity Up / Baking Soda (oz)
Ounces of sodium bicarbonate to raise total alkalinity.
Chlorine Granules / Dichlor (oz)
Ounces of dichlor granules to raise free chlorine.
Total Chemicals (oz)
Combined weight of all chemicals to add.
Est. Monthly Chemical Cost ($)
Approximate ongoing monthly chemical cost.
Water per Change (gal)
Gallons needed for a full water change (every 3-4 months).

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Tub Volume (gallons) = 400, Current pH = 7, Target pH = 7.4, Current Alkalinity (ppm) = 60 = 7 input(s) provided
  2. Calculate pH Increaser / Soda Ash
    pH Increaser / Soda Ash
    9.6 = 9.6
  3. Calculate pH Decreaser / Dry Acid
    pH Decreaser / Dry Acid
    0 = 0
  4. Calculate Chlorine Granules / Dichlor
    Chlorine Granules / Dichlor = clDiff * 0.5 * (tubVolumeGal / 250)
    2.4 = 2.4
  5. Calculate Alkalinity Up / Baking Soda
    Alkalinity Up / Baking Soda
    16 = 16
  6. Calculate Total Chemicals
    Total Chemicals = round((phIncreaserOz + phDecreaserOz + alkIncreaserOz + chlorineOz) * 100) / 100
    28 = 28

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