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, more calculators for volume, filled weight, 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. Every page on the site is listed, with a one-line summary, on the sitemap.
Who writes this
HotTubCalc is written and maintained by Sam Calder. Sam keeps the calculators honest and the guides readable: every dose starts from published water-chemistry references, every spec starts from the manufacturer's sheet, and anything that is an estimate says so on the page.
The size and capacity database is compiled from manufacturer spec sheets, product pages, and archived brochures, and each model entry records where its numbers came from. Estimates like heat-up times and running costs show their assumptions right next to the figure.
Spotted a number that looks wrong? Email contact@hottubcalc.com and it gets checked against the source. Anything that does not hold up is corrected.
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 against its source. 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 2026-08-08. The checks run every time the site builds — see how we check the numbers. Substantive fixes are dated on the corrections log.
What we will not do
- Tell you something that contradicts your product label. The label always wins.
- Sell you chemicals. The gear and accessories pages carry tagged Amazon affiliate links to product categories, which fund the site at no extra cost to you, and nothing more. The calculators and guides carry none.
- Pretend a calculator replaces testing. Test your water; the tools size the response.
Contact
Corrections or a suggestion: contact@hottubcalc.com.