Hot Tub Volume and Heating Cost Calculator
Find your tub's volume in litres for chemical dosing, and what it costs to heat a fresh fill from hose temperature to soaking temperature.
Your results
Worked example
A medium tub holding 1,200 L, filled with 10 °C water and heated to 38 °C at $0.15 per kWh:
- Temperature rise: 38 − 10 = 28 °C
- Energy: 1,200 kg × 4.186 kJ per kg°C × 28 °C = 140,650 kJ, and 140,650 ÷ 3,600 = 39.1 kWh
- Cost: 39.1 × $0.15 = $5.86 to heat the fresh fill
That is the one-time heat-up cost only. Holding temperature day to day depends on your cover, insulation, and climate.
How this calculator works
Water has a specific heat of 4.186 kJ per kilogram per degree, and one litre of water weighs one kilogram. Custom volume is length times width times water depth, in metres, times 1,000. Real heaters lose some heat to the air while warming, so treat the result as a best case.
Common questions
How long does a heat-up take?
Divide the kWh shown by your heater size in kW. A common 4 kW heater needs about 10 hours for the example above; a 5.5 kW heater needs about 7 hours, before heat losses.
Why does my tub cost more in winter?
Colder fill water needs more energy to reach temperature, and the tub loses heat faster to cold air every day after. A well-fitted insulated cover is the single biggest running-cost factor.
Estimates only. Results are provided for planning purposes and are based on the figures and assumptions shown on this page. Actual heating cost varies with heater efficiency, cover quality, and ambient conditions. Mississippi Maple accepts no liability for purchase decisions made using this calculator.