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Cold Plunge Chiller Calculator
Enter your water volume and how cold you want it. You get the BTU/hr and HP you need, adjusted for insulation, cover and outdoor heat, plus what it actually costs to run per month and year.
How it is calculated
- Heat load = gallons × 8.33 × ΔT ÷ pull-down hours (water is 8.33 lb/gal, 1 BTU/lb·°F).
- Corrections: no insulation ×1.5, no cover ×1.2, outdoor hot summer ×1.25.
- HP ≈ BTU/hr ÷ 10,000 (1 HP ≈ 9,000–12,000 BTU/hr).
Pull-down vs holding
There are two jobs a chiller does, and they need different power. Pull-down is dropping a warm fill to your target the first time, and it is what this calculator sizes, because it is the harder ask. Holding a tub that is already cold takes a fraction of that, since you are only replacing heat that leaks back in. Set a longer pull-down time and the required BTU/hr falls; if you do not mind waiting a couple of hours to reach temperature, a smaller unit is fine.
The biggest lever is not the chiller at all. An insulated tub with a cover holds cold with far less work, which is why the correction for an uninsulated, uncovered outdoor tub nearly doubles the number. Fix the insulation first, then match a unit from the chiller database.