About Hot Tub Calculator
Hot Tub Calculator turns a test strip reading, a tub size or an electricity price into one plain answer you can act on at the tub.
Who makes it?
A small publisher in Denver, Colorado called Oakridge Analytics LLC runs Hot Tub Calculator. We do not sell parts, chemicals or hot tubs, and no manufacturer pays us for a product mention.
Email hello@hottubcalculator.com if a number seems wrong for your tub. Every message gets read, and we fix the page once the source confirms the error.
How are chemical doses worked out?
Each dose begins with the gallons you type or pick. Each chlorine dose starts with the parts per million you must add, your water's litres, and the available chlorine share a sodium dichlor label shows. For pH, alkalinity and shock, published spa dosage charts and product labels give a rate per 100 gallons.
Spa manufacturers' care guides and the CDC provide the ideal ranges. We round doses so a kitchen spoon or scale can measure them. If your product label differs, always follow it.
Where the numbers come from
| Number | Value | Source |
|---|---|---|
| Free chlorine in a hot tub | 3 to 5 ppm | CDC, AquaChek care guide |
| pH | Target 7.4 to 7.6 for best results, though 7.2 to 7.8 is fine. | AquaChek care guide |
| Total alkalinity | 80 to 120 ppm | EPA dichlor label |
| Chlorine in dichlor granules | 56% available chlorine | EPA dichlor label |
| pH up and pH down doses | Per 100 gallons, by reading | Leslie's spa chart |
| Alkalinity doses | Per 10 ppm and 100 gallons | CPO dosage tables |
| Water weight | 8.34 lb per US gallon | USGS |
| Energy in a kilowatt-hour | 3,412 BTU | US EIA |
How are running cost and heat-up time worked out?
We use water weight and heater output to work out heat-up time. The weight of water runs about 8.34 pounds per gallon. For heat, a kilowatt-hour contains 3,412 British thermal units, the amount that raises a pound of water one degree Fahrenheit. Wind, cold air, and a cover increase the actual time.
Running cost comes from multiplying your typed price per kWh by typical monthly energy use for a hard or inflatable tub. These energy figures come from state energy test data and manufacturer running cost pages.
How are weight, wiring and water changes worked out?
To get full weight, add the water weight, the dry weight from manufacturers' spec sheets, and an average adult weight per person from the CDC. To find the load per square foot, divide that total by your tub's footprint. Most home decks are built for 40 pounds per square foot, and that's the number we compare against.
Installation guides from spa manufacturers are where the wiring answers come from. Your electrician can use them as a starting point, but they never replace a licensed one or your local code.
Spa manufacturers publish the rule for water change timing: divide gallons by 3, then divide by daily users to get days between drains.
What it does not do
You cannot use this tool to test water, inspect decks, or approve wiring. A reading that seems off means you should test once more, using fresh strips, or carry a water sample to a pool and spa store.
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