Sizing and running a hot tub
The hot tub tools turn the same physics used for pools into spa-sized answers. Heat-up time uses 8.34 BTU per gallon per degree Fahrenheit and the conversion 1 watt = 3.412 BTU per hour, so a heater’s wattage gives you a heating rate. Fill time is volume divided by flow rate, and a garden hose delivers roughly 9 to 10 gallons per minute. The running cost and water-cost calculators use editable assumptions such as your electricity and municipal water prices.
Chemistry and water changes
The hot tub chemical calculator scales published spa dosing rates, for example about 1 teaspoon of bromine granules per 500 gallons for a level of roughly 3 to 5 ppm, and pH and alkalinity rates scaled from standard pool dosing. The water change interval calculator uses a commonly cited rule of thumb of gallons divided by users times 3 days, capped at roughly 90 to 120 days. Rules of thumb vary by manufacturer, so check your tub’s manual.
Weight and electrical planning
The pad weight and breaker and wire size calculators are planning aids only. A hot tub full of water and people is very heavy, since water alone weighs 8.34 pounds per gallon, and electrical work near water is dangerous. Ask the manufacturer and a licensed electrician before you build or wire anything. The disclaimer explains the limits.