How it works
Ice cools water mostly by melting: every kilogram of ice absorbs 334 kJ as it turns to water at 0 °C, then a little more as the meltwater warms to your target. Cooling 150 litres of water by 5 °C takes about 3,100 kJ, which is roughly 8 kg of ice. The colder your tap water, the less ice you need, which is why winter ice baths are cheap and summer ones are not.
Getting the most from your ice
- Measure the tap water with a thermometer after running it for a minute; it varies a lot with the season.
- Run less water first: the melted ice adds volume. The calculator tells you how much.
- Add the ice last, just before you get in, and stir.
- Big blocks last longer than cubes because they melt more slowly; cubes cool faster.
- Plunging every day? A chiller usually works out cheaper than bags of ice within weeks: compare them in the cold plunge running cost calculator.
Safety first: start with short sessions, never plunge alone or after alcohol, and ask a doctor first if you have a heart condition. See cold plunge vs ice bath.