Coolers and ice, by the numbers

A cooler makes nothing cold. It holds melting ice, and melting ice absorbs 334 kilojoules for every kilogram that turns to water, all of it at 0 degrees C. Everything else, wall thickness, lid seals, shade, only changes the rate at which heat finds that ice. Put numbers on the leak and the ice quantity stops being a guess.

The only job a cooler has#

Ice at 0 degrees C and water at 0 degrees C are the same temperature. The energy separating them, the latent heat of fusion, is 334 kilojoules per kilogram, absorbed with no change in temperature. That is why a box sits near 0 degrees C for days and then warms fast once the last ice goes: the buffer is the phase change, not the cold.

The target is set by food safety. The FDA Food Code puts cold holding at 5 degrees C (41 degrees F) or below, and USDA guidance calls 4 to 60 degrees C (40 to 140 degrees F) the danger zone. A box with ice in it meets that easily. A box with the last ice gone does not, and the transition is fast.

Where the heat gets in, and what wall thickness buys#

Steady conduction through a wall is conductivity divided by thickness, times area, times temperature difference. Take a 50 litre cooler with 3 centimetre walls. Its outer surface is around 1.2 square metres and its inner surface around 0.8, and heat crosses something between the two, so the effective area is about 1.0 square metre. Using that, rather than the outer figure most published estimates quote, is why our numbers come out lower than the usual scare figures. Closed cell foam runs about 0.026 watts per metre kelvin, so the coefficient is 0.026 divided by 0.03, or 0.87 watts per square metre per kelvin. With the inside at 5 degrees C and a shaded 25 degrees C outside, the difference is 20 kelvin:

• Leak = 0.87 x 1.0 x 20 = 17 watts
• Over 24 hours: 17.3 x 86,400 seconds = 1,497 kilojoules
• Ice melted: 1,497 divided by 334 = 4.5 kilograms per day

Double the wall to 6 centimetres and the coefficient halves to 0.43, the leak falls to about 9 watts, and the melt drops to roughly 2.2 kilograms a day. The real saving is slightly less than half, because thicker walls enlarge the box and add surface area. This is why heavy rotomoulded boxes hold ice for days where a thin-walled picnic box does not last a weekend.

Ice does two jobs, and you pay for both#

Most "how much ice" arguments go wrong because two separate jobs get added together.

Job one: the pull-down. Warm contents come to temperature once. Take 12 kilograms of mixed food and drinks at an average specific heat around 3.8 kilojoules per kilogram per kelvin, from 20 degrees C to 5 degrees C: 12 x 3.8 x 15 = 684 kilojoules, or 684 divided by 334 = 2.05 kilograms of ice. The box counts too: 4 kilograms of plastic and foam at an effective 2.0 kilojoules per kilogram per kelvin over 15 kelvin is 120 kilojoules, another 0.36 kilograms. Total: about 2.4 kilograms.

Job two: the leak. The 4.5 kilograms a day calculated above, continuing for as long as the trip does.

A three day trip in deep shade with 12 kilograms of room-temperature contents therefore needs 2.4 + (3 x 4.5) = 16 kilograms of ice. Against 12 kilograms of contents that is 1.3 to 1. Run the same trip in full sun and the daily term rises to 7.9 kilograms, the total to 26, and the ratio to 2.2 to 1. The familiar 2 to 1 rule is really a sunny weather rule that got repeated without its conditions.

The useful consequence is that job one is optional. Load food and drinks at fridge temperature and pre-chill the empty box overnight, and the pull-down term collapses to near zero: 2.4 kilograms recovered for nothing, close to a fifth of a two day ice budget.

What sun and a parked car cost, in kilograms of ice#

Placement changes the temperature difference, and the leak scales linearly with it. Same box, same 1.0 square metre effective area, same 3 centimetre walls, interior held at 5 degrees C.

Read that as a shopping list. Moving the box from full sun into deep shade saves 3.4 kilograms of ice a day, or 10 kilograms over three days, and it costs nothing but a tarp or a tree. No wall thickness available at normal prices buys as much, because thickness halves the coefficient at best while placement can more than double the temperature difference.

Two extensions: get the box off hot ground, because dark tarmac is a heat source touching your largest panel, and cover the lid with a light coloured or reflective sheet in sun.

Block or cubes, and how much of each#

Block and cubed ice store the same 334 kilojoules per kilogram. They differ only in surface area, which sets how fast that energy is drawn down.

Ice has a density of about 917 kilograms per cubic metre, so 5 kilograms occupies 5.45 litres. As a single cube that is 17.6 centimetres on a side, with a surface area of 6 x 0.176 x 0.176 = 0.186 square metres. The same 5 kilograms as 3 centimetre cubes is about 202 cubes, each with 0.0054 square metres of surface, totalling 1.09 square metres. That is 5.9 times the area for the same stored capacity. Block ice melts far slower and cubed ice chills contents far faster, and those are the same fact.

So mix them against the two jobs: cubes for job one, since they fill gaps and touch every surface, budgeted at the 2.4 kilograms the pull-down needs, and block for job two, giving up capacity slowly from the bottom.

Sealed containers of frozen water behave like block ice, use space you were giving to drinking water anyway, and thaw into a drink rather than slurry. Treat that water before freezing, since freezing does not disinfect: see water treatment.

Packing order, draining, and the lid#

1. Pre-chill the box overnight, with sacrificial ice or in a cold space.
2. Block ice on the bottom. Cold water sinks, so the slow store belongs low.
3. Meat and anything that must stay coldest directly on the ice, in sealed containers.
4. Dense pre-chilled items next, packed tight: air is a poor thermal store, so a full cooler outlasts a half-empty one.
5. Cubed ice poured into the gaps, not layered on top.
6. Items needed often on top, where the reaching happens, and a folded towel in any remaining air space.

On draining. Draining melt water is usually a mistake. That water sits at 0 degrees C and does two useful things: it holds cold, at 4.186 kilojoules per kilogram per kelvin, so 10 litres absorbs 42 kilojoules per degree it warms, and it fills gaps that would otherwise hold air. The one good reason to drain is packaging: if cardboard is turning to pulp, drain and repack in sealed containers.

On opening the lid. The usual advice needs more precision than it gets. The 25 litres of air above the contents in a half-full 50 litre box weighs about 30 grams, and warming it by 20 kelvin takes 0.6 kilojoules, under 2 grams of ice. One quick opening costs a handful of grams.

So the ice budget is not what opening costs you. What it costs is the temperature of the top layer: each opening warms the exposed surface of the contents, and in a frequently opened box that layer never fully re-equilibrates with the ice below. You end up with ice in the bottom and food on top spending hours above 5 degrees C. That is a food safety problem rather than an ice problem, and it argues for two boxes rather than for lid technique.

A lid that no longer seals is different. A failed gasket replaces slow conduction through foam with continuous air exchange, and continuous exchange genuinely multiplies losses. Check the seal before every trip, with the rest of your car camping checklist.

Two coolers, and where a 12 volt fridge wins#

Split by opening frequency, not by food type. The drinks cooler is opened 20 to 40 times a day, runs on cheap cubed ice, and holds nothing that is a food safety risk. The food cooler is opened at meal times only, on block ice, packed tight with pre-chilled contents.

The arithmetic justifying the second box: 21 kilograms of ice occupies 23 litres, leaving 27 litres of a 50 litre box for 12 kilograms of food and drink that packs into 15 to 20 litres. It fits with no margin, which is why single-cooler trips run out of ice a day early. Our cooler ice calculator runs this model for your box and temperatures, and the camp food calculator sizes the contents.

Where a fridge takes over. Model the same box as a fridge with 5 centimetre walls holding 4 degrees C against a 30 degree day. The coefficient is 0.03 divided by 0.05, or 0.6, so the load is 0.6 x 1.2 x 26 = about 19 watts. At a coefficient of performance near 1.3 that is roughly 14 watts electrical, about 345 watt hours a day, or 29 amp hours at 12 volts.

As a cooler at the same 26 kelvin difference the box leaks 31 watts, melting 8.1 kilograms a day, and it cannot hold much over 30 kilograms of ice alongside useful contents, so the ceiling is under four days. Past that point the fridge stops competing on convenience and starts deciding whether the trip is possible, at the cost of battery capacity: see 12 volt power and fridges.

Measure your own cooler over one weekend#

1. Pre-chill the empty box overnight, then load a known, weighed mass of ice and nothing else.
2. Put it where you would put it on a trip and leave the lid shut for 24 hours.
3. Log the outside air temperature with a cheap min-max thermometer beside the box.
4. At 24 hours, drain and weigh the melt water. That mass is your ice loss.
5. Convert to watts: mass in kilograms x 334, divided by 86.4. So 5 kilograms melted is 5 x 334 divided by 86.4 = 19 watts.
6. Divide by the average temperature difference for your box's real coefficient in watts per kelvin, then use it to predict any trip.

A box much worse than the model has a leak worth finding: a failed gasket, a drain plug that no longer seats, or a lid built thinner than the body.

Common mistakes#

Leaving the box in the car during the shopping stop. The parked car row is the worst number here. Two hours before the trip starts can cost more than a night at camp.

Draining melt water on principle. It is stored cold and it fills air gaps. Drain only when packaging is failing.

Loading warm food and drinks. A real 2.4 kilograms of ice, spent in the first few hours, avoidable with an overnight fridge stay.

Running one cooler for everything. Drinks traffic warms the food layer, and the volume leaves no room for enough ice.

Assuming ice in the bottom means the food is cold. Put a thermometer in with the food and look at it. The rest of the kitchen is in cookware and camp kitchen kit.

Frequently asked questions#

How much ice do I need for a three day camping trip?#

For about 12 kilograms of contents in a 50 litre box in deep shade at 25 degrees C, the model gives 2.4 kilograms for the pull-down plus 4.5 kilograms a day, so roughly 16 kilograms, a ratio of 1.3 to 1. In full sun the daily figure rises to 7.9 kilograms and the ratio to 2.2 to 1, which is the number everyone quotes without saying it assumes sunshine.

Should I drain the water from my cooler?#

Usually not. Melt water sits at 0 degrees C, holds cold at 4.186 kilojoules per kilogram per kelvin, and fills gaps that would otherwise hold air. Draining swaps cold water for warm air and shortens ice life. The exception is packaging: if cardboard is disintegrating, drain and repack in sealed containers.

Is block ice really better than cubes?#

For duration, yes. Five kilograms as one block has about 0.19 square metres of surface; the same mass as 3 centimetre cubes has about 1.09 square metres, nearly six times more. Both store 334 kilojoules per kilogram, but cubes give it up far faster. Use cubes to chill and fill gaps, block ice to carry the days.

Does pre-chilling a cooler actually make a difference?#

Yes, measurably. A 4 kilogram plastic and foam box cooled by 15 degrees C absorbs about 120 kilojoules, and 12 kilograms of contents over the same range absorbs 684 kilojoules. That is 2.4 kilograms of ice spent before the trip has done anything, and pre-chilling recovers essentially all of it.

How much does keeping a cooler in the shade save?#

In the model above, a 50 litre box with 3 centimetre walls melts 4.5 kilograms of ice a day in deep shade and about 7.9 kilograms in full sun: 3.4 kilograms a day, or 10 kilograms over three days. Getting the box off hot ground works on the same term.

At what point is a 12 volt fridge better than a cooler?#

Roughly beyond three or four days without access to more ice. A 50 litre cooler cannot carry much over 30 kilograms of ice alongside useful contents, and in heat it loses around 8 kilograms a day. A similar fridge with thicker walls draws in the region of 300 to 400 watt hours a day, which needs battery capacity but does not run out.