About HotTubCalc
HotTubCalc is a free, independent set of calculators and guides for keeping hot tub water balanced. No account, no app to buy, and no chemical line we are trying to sell you.
What this is
A dosing calculator that turns your gallons and a test reading into an exact amount to add, three more calculators for volume, running cost, and heat-up time, and a set of plain-English guides for the problems every tub runs into. Everything runs in your browser and works offline once loaded, so it is usable at the tub with no signal.
How the numbers are made
The dosing math is computed from published chemistry, not copied from one brand's chart, and every constant is checked by an assertion when the site is built, so a value that drifts away from its source fails the build instead of shipping. Total alkalinity uses the standard rate of about 1.5 lb of sodium bicarbonate per 10,000 gallons for a 10 ppm rise. Sanitizers are dosed by available-halogen content. Balanced-water ranges follow common spa water-care guidance. pH adjusters are buffer dependent, so those figures are honest starting steps rather than exact amounts, and every dosing output says to add in small steps and retest.
Dosing data last verified 2026-06-13.
What we will not do
- Tell you something that contradicts your product label. The label always wins.
- Sell you chemicals. Shopping links are tagged Amazon affiliate links to product categories, which fund the site at no extra cost to you, and nothing more.
- Pretend a calculator replaces testing. Test your water; the tools size the response.
Contact
Corrections or a suggestion: contact@hottubcalc.com.