How long does a cold pack stay cold?

The most common question about cold packs: how long will my shipment stay cold? The honest answer is that the pack itself is only half the story. The box around it determines the rest.

Practical guide values

A 400-gram gel pack in a thick-walled EPS box at a constant outside temperature of 25 degrees performs as follows: after 12 hours the contents are still well below freezing, after 24 hours still below 0 degrees, and it stays below zero for up to around 30 hours. After that the box slowly warms up but stays within the safe cooling range below 8 degrees for up to 48 hours.

In a thin-walled box that is roughly halved. In a cardboard box without insulation you'll be above 8 degrees within half a day, no matter how many packs you add.

Four factors that determine it

Insulation value of the packaging. This is by far the biggest factor. A thick-walled EPS box doubles the cooling duration compared to a thin-walled one.

Outside temperature. The guide values above apply at 25 degrees. In a delivery van parked in the sun that can rise to 50 degrees, halving your cooling duration.

Number of packs. Rule of thumb: one 400-gram pack per 5 liters of box volume for 24 hours. Double that for 48 hours or frozen transport.

Fill level. A full box stays cold longer than a half-full one. Air is the space in which heat spreads; fill any remaining space.

Calculating for your transit time

Work backwards from the longest conceivable transit time, not the average. A shipment that normally arrives in 24 hours can be in transit for 36 hours in case of a delay. Calculate with those 36 hours, not 24.

What not to do

Stacking packs in the freezer, so the core doesn't freeze through. Placing the packs at the bottom of the box, because cold sinks. And compensating for a thin-walled box with more packs, because that only works to a limited extent: the heat comes in through the wall, not from a shortage of cooling.