Calculators / Hot tubs, saunas and plunges

What size cold plunge chiller do you need?

Put in the tub and how cold you want it. You’ll get how long each chiller takes, and what it costs to keep it cold.

Updated November 2026 · How we check our numbers

These are example numbers. Put in yours.

The tub

Water

gal

A 100-gallon stock tank, or the tub’s listed capacity.

Starting water temperature

°F

Hose water is often 55 to 70°F.

Target temperature

°F

Running cost

How fast it warms with the chiller off (optional)

°F/hr

Penguin’s test. Leave blank if you haven’t measured it.

Where you live

Pull-down, best case

3.3 hours

100 gal from 65°F to 45°F is 16,680 BTU; at 5,000 BTU/hr that’s about 3.3 hours.

Heat to remove: 16,680 BTU
Runs to hold: 2 h/day
Holding cost: $4.98/mo27 kWh at 18.19¢

Estimate for planning only, not professional advice. Check the manufacturer’s instructions, your local code and a licensed professional before you buy, build or install. Use at your own risk. Terms.

heat = 100 gal × 8.34 lb × 20°F = 16,680 BTU

time = 16,680 ÷ 5,000 BTU/hr = 3.3 h

heat gain = 100 × 8.34 × 0.5°F/hr = 417 BTU/hr

Every chiller on this tub

100 gal

Penguin 1/2 HP5,000 BTU/hr
3.3 h to chill
Penguin Cold Therapy7,500 BTU/hr
2.2 h to chill
Active Aqua 1/2 HP4,020 BTU/hr
4.1 h to chill
Active Aqua 1 HP10,050 BTU/hr
1.7 h to chill

Ratings from Penguin and Active Aqua. Best-case pull-down, before heat gain.

A best case

The pull-down time ignores heat the tub picks up from the air and the ground while it chills, so real pull-down takes longer, more so outdoors in summer. Penguin: “When in doubt, size up.”

How we work it out

BTU = gallons × 8.34 × °F drop · hours = BTU ÷ chiller BTU/hr

A BTU warms a pound of water 1°F, and a gallon weighs 8.34 lb (USGS). Penguin Chillers uses the same formula and points out it ignores heat leaking in while you chill, so treat the time as a best case: 100 gallons from 65 to 45°F is 16,680 BTU, about 3⅓ hours on a 5,000 BTU/hr chiller.

Keeping it cold depends on how fast your tub warms, which only you can measure. Penguin's test: chill it, switch the chiller off, and time the rise. Gallons × 8.34 × degrees an hour is the heat the chiller has to pull back out, which tells us how many hours a day it runs.

Where the numbers come from

Cold plunge

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