How much ice do you need for an ice bath?
The sensible-heat and latent-heat arithmetic for any tub size and any starting temperature, then the week each chiller in our catalog pays for itself against the ice bill it replaces.
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Short answer
Three worked examples
Rendered on the server from the same function the calculator uses, at three plunges a week and $3.50 for a 10 lb bag.
| Tub and temperatures | Ice | Bags | Per plunge | Per month |
|---|---|---|---|---|
| 100 gal, 68°F tap, 50°F target | 109 lb | 11 bags | $38.22 | $497 |
| 100 gal, 68°F tap, 40°F target | 181 lb | 19 bags | $63.37 | $824 |
| 130 gal, 78°F summer tap, 45°F target | 269 lb | 27 bags | $94.00 | $1,222 |
How the math works
This is the one page on this site where the headline number is exact rather than quoted. Two standard constants do all the work, and neither of them needs a bench.
Lf = 334 kJ/kg latent heat of fusion of ice at 0 °C
Q = mwater · c · (Ttap − Ttarget)
q = Lf + c · Ttarget
mice = Q ÷ q ÷ (1 − loss)
A US gallon of water is 3.785 litres and a litre of water is about a kilogram, so the mass is the volume. Temperature differences convert from °F to kelvin by five ninths. Ice does two jobs: it melts, which is the big term, and then the melt-water warms from 0 °C up to the bath temperature, which is the small one. That second term is why a 40 °F target costs proportionally less ice per degree than a 55 °F target — colder melt-water has less warming left to do.
The loss allowance is the only assumption in the calculation, and it is an input so you can see what it costs. Fifteen percent stands in for heat picked up through the tub walls and the open surface while you fill, stir and wait. Set it to zero and the tool shows the ideal-mixing floor, which is the least ice that could ever work.
A number on our own site that does not survive this
Our cold plunge running-cost guide puts 100 gallons to the mid-40s at 40 to 60 lb of ice, which is the figure that circulates everywhere. The arithmetic above does not reproduce it: the same job comes out near 150 lb. We think the widely repeated range is low, the guide is queued for correction, and we would rather say so here than quietly change a number behind you.
When a chiller pays for itself
A chiller has to earn back the premium over the ice-only tub you would otherwise buy, not its whole sticker price, plus the dedicated 120V 20A GFCI circuit our electrical guide puts at $250 to $600. That is the premium column below. Electricity is deliberately left out — we have no verified hold-mode figure, so real payback is a little slower than these weeks suggest.
| Chilled tub | Price | Compared against | Premium + circuit | Pays for itself |
|---|---|---|---|---|
| Redwood Outdoors Alaskan All-In-One Cold Plunge | $6,999 | Redwood Outdoors Alaskan Cold Plunge Tub at $2,700 | $4,549–$4,899 | week 40–43 (0.8–0.8 yr) |
| Ice Barrel Ice Barrel 500 + Chiller Bundle | $5,750 | Ice Barrel Ice Barrel 300 at $1,150 | $4,850–$5,200 | week 43–46 (0.8–0.9 yr) |
| Plunge All-In Gen 2 | $9,990 | Ice Barrel Ice Barrel 300 at $1,150 | $9,090–$9,440 | week 80–83 (1.5–1.6 yr) |
Prices are manufacturer and retailer listings from our catalog, dated below. The Plunge All-In has no ice-only sibling in our catalog, so its premium is measured against the cheapest ice-capable tub we list — a harsher comparison than a like-for-like one would be, and the table says so rather than hiding it.
What the output points at
Prices verified September 2026 from manufacturer and retailer pages. This category discounts constantly, so check before you buy.
Redwood Outdoors Alaskan All-In-One Cold Plunge
Buyers who want a thermowood tub that chills and heats, on a standard outlet
A 104-gallon thermowood tub with a Balboa 1.5 HP chiller-heater and UV sanitation built in. It is not the cheap way to a chilled plunge; it is the one that looks like garden furniture and also heats to 104°F.
- Capacity
- 104 gallons
- Exterior
- 57" L x 37" W x 44" H
- Dry weight
- 360 lb
- Chiller
- Balboa CLIM8ZONE Deluxe, 1.5 HP, 37°F to 104°F
- Sanitation
- UV, one filter
- Electrical
- 120V, 12A max, GFCI
- Material
- Canadian thermowood, moulded acrylic liner
- Warranty
- 1 year, tub and chiller
What we liked
- Chills to 37°F and heats to 104°F on one Balboa unit, manufacturer-stated
- Runs on a standard 120V outlet, 12A max, with UV sanitation built in
- Thermowood shell, insulated lid with child lock, built-in steps
What we did not
- 1-year warranty on the tub and the chiller is short for $6,999
- 104 gallons — smaller than the 130-gallon ice-only Alaskan it is named after
- No ozone, and Redwood does not cover damage from use below 32°F
Ice Barrel 500 + Chiller Bundle
Tall users, and buyers looking into HSA/FSA payment
The horizontal answer to Ice Barrel's own vertical design. Fits users up to 6'9", made in the USA, and carries a limited lifetime warranty no competitor here matches.
- Dimensions
- 57.6" L x 30.7" W x 42.1" H
- Empty weight
- 106 lb
- Filled weight
- 900 lb
- Fits
- Users to 6'9"
- Warranty
- Limited lifetime
- Made in
- USA
What we liked
- Limited lifetime warranty — unmatched in this roundup
- Fits users up to 6'9" lying back, not crouching
- Ice Barrel states HSA/FSA eligibility — qualifying usually needs a Letter of Medical Necessity
What we did not
- 900 lb filled — floor loading matters indoors
- Bundle price is close to premium all-in-one territory
- Horizontal footprint is large for a garage bay
Plunge All-In Gen 2
Buyers who want one appliance, no assembly, and the lowest temperatures available
The category benchmark. Cools to 37°F, filters the whole tub every 15 minutes with onboard ozone, and Gen 2 does it on 50% less power than Gen 1.
- Min temp
- 37°F
- Filtration
- Full volume every 15 min + ozone
- Electrical
- Standard 120V outlet
- App
- iOS / Android
- Financing
- From ~$65/mo
What we liked
- 37°F floor — colder than nearly every competitor
- Full water volume filtered every 15 minutes with ozone sanitation
- 50% less energy than Gen 1 and 31% faster cooling
What we did not
- $9,990 before promotions is the most expensive plunge here
- Integrated chiller means a single point of failure
- App-dependent for scheduling and remote temperature control
What we have not measured
The arithmetic on this page is standard. The figures going into it come from the people selling the equipment: The loss allowance and the chiller pull-down times behind the payback table; the underlying thermodynamic constants need no bench.
- The physics is exact and needs no bench: 4.18 kJ per kg·K for liquid water and 334 kJ per kg for the latent heat of fusion are standard values, and the mixing calculation is closed-form.
- The 15 percent loss allowance is not. It stands in for heat gain through the walls and the surface while you fill, stir and wait, and it is an assumption we have not verified with a probe. Dial it to zero and the tool shows the ideal-mixing floor.
- Chiller prices and the pull-down hours behind them are manufacturer and retailer figures. We have not logged a pull-down, which is why the payback table prices the chiller against the ice it replaces rather than against a measured running cost.
- The chiller premium in the payback table excludes its electricity, because we have no verified hold-mode figure. Real payback is therefore slightly slower than the table shows.
- Our own cold plunge running-cost guide puts a 100-gallon session at 40 to 60 lb of ice, a figure that circulates widely. The arithmetic on this page does not reproduce it and we now think the guide's range is low; the guide is queued for correction rather than quietly edited behind you.
Our testing method says the same about every figure on the site, and lists which bench runs are still owed.
Questions buyers actually ask
How much ice do you need for an ice bath?
For 100 gallons going from 68°F tap water to 50°F, about 109 lb — eleven 10-pound bags — including a 15 percent allowance for heat gain while you fill and stir. Colder targets rise steeply: the same tub to 40°F needs roughly 181 lb. The two levers that matter are how cold your tap water already is and how much water is actually in the tub.
Is a chiller cheaper than ice?
It depends entirely on how often you plunge, which is why this page prices the chiller against your own ice bill rather than quoting a rule of thumb. At three plunges a week the ice bill is large enough that the chiller premium over an equivalent ice-only tub, plus its $250 to $600 dedicated GFCI circuit, is repaid inside the first year in most configurations. At one plunge a week it is not.
Why does the calculator ask for tap water temperature?
Because it sets more than half the answer. The ice you need is proportional to the temperature drop you are asking for, so 68°F tap water to 50°F is a third of the job that 78°F tap water to 40°F is. Summer tap water in a hot climate is a materially different problem from winter tap water in a cold one, and no fixed bag count covers both.
Does melted ice overflow the tub?
Yes, if you fill to the brim first. Ice displaces roughly nine tenths of its own volume while floating and then melts into the water, so plan to fill the tub to about the level you want after the ice and your own body are in it. The calculator sizes the ice for the water volume you enter, not the tub's rated capacity.
The guides this tool belongs to
A calculator answers one question with a number. These answer the ones around it.
Prices captured September 2026 from manufacturer and retailer pages and subject to frequent promotion. Nothing on this page is medical, electrical or financial advice — sauna and plunge circuits are licensed work in every US jurisdiction.