Hot Tub Chemical Calculator
Calculate sanitizer and pH doses for a hot tub based on its water volume.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
Compare the current sanitizer, pH and total-alkalinity readings with targets you choose. The calculator reports reading differences only; it cannot determine a chemical dose because products have different active ingredients and strengths. Follow the spa manual and exact product label, retest, and never mix chemicals.
Worked example
calculated with the default values below
Current sanitizer reading 1 ppm · Target sanitizer reading 4 ppm · Current pH 7.2 · Target pH 7.4 · Current total alkalinity 60 ppm · Target total alkalinity 100 ppm Sanitizer change to target 3.00 ppm · pH change to target 0.20 pH · pH status pH is below target · Alkalinity change to target 40 ppm · Water-care note Reading differences only—not chemical doses. Follow the exact product label and spa manual, retest the water, and never mix products. CDC advises against stabilized chlorine (including dichlor) in hot tubs.
General background references
Disclaimer
These calculators provide planning estimates only. Building codes, wiring regulations and safety requirements differ by country and change over time, and nothing here replaces a licensed professional or a permit. For structural, electrical or gas work, follow local code and have the installation inspected.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
ΔS = S_target − S_current ; ΔpH = pH_target − pH_current ; ΔA = A_target − A_current
- S
- sanitizer reading (ppm)
- pH
- measured pH
- A
- total alkalinity reading (ppm)
These are reading differences only. No product dose is calculated because chemical formulations and strengths vary.
Frequently asked questions
Does this calculator tell me how much chemical to add?
No. It calculates the difference between your current and target sanitizer, pH and alkalinity readings; it does not calculate a product dose. Dosing depends on the exact product formulation and test results. CDC guidance for hot tubs is at least 3 ppm free chlorine or 4–8 ppm bromine and pH 7.0–7.8; CDC advises against cyanuric-acid stabilized chlorine products such as dichlor in hot tubs. Follow your spa manual and product label, retest the water, and never mix products.