HotTubCalc: Spa Gallons Tool

Plan your next spa fill with clear numbers. Enter the tub's shape and size to get water volume, weight, fill time and the energy needed to heat it.

• Rectangular, round and oval tubs, with presets for compact, family, round and oval spas
• Choose open tub, inflatable or molded seats so seats and footwells are not counted as water
• Feet or metres, gallons or litres, °F or °C
• Volume, surface area and total water weight when full
• Fill time with a slow, standard or fast garden hose
• Heating energy in kWh, heat-up time and cost from your start and target temperature, heater power and electricity price
• Standby running cost per day, month and year from the California Title 20 spa benchmarks or your data-sheet figure, with air temperature and cover-off time
• Water chemistry: free chlorine, pH and total alkalinity checked against the CDC Model Aquatic Health Code spa ranges, with chlorine and baking-soda amounts
• Water change interval from the number of users per day
• Support load: total weight and pressure on a deck or pad, compared with the IRC deck design load and your platform rating
• Alert when the target temperature is above 104 °F (40 °C)
• Save tub setups to compare later and share a summary

Official sources:
• CDC Model Aquatic Health Code (MAHC), 2024 edition: https://www.cdc.gov/model-aquatic-health-code/media/pdfs/2024/11/5th-Ed-MAHC-Code-508.pdf
• CDC, About the MAHC Current Edition: https://www.cdc.gov/model-aquatic-health-code/php/our-work/index.html
Not affiliated with or endorsed by any government agency.