Gel Ice Packs for Shipping Calculator
Estimate how many frozen gel packs an insulated shipper needs for a transit time, from box size, insulation, wall thickness and outside temperature.
Gel packs keep a parcel cold by soaking up the heat that leaks through the box walls, mostly as they melt. This calculator works out that heat leak from the box size, insulation type and thickness and the outside temperature, then sizes the frozen gel needed for your transit time. It estimates a coolant energy balance; it cannot establish food safety, product temperature or an approved shipping duration.
How to use this tool
- Choose units and enter the inside dimensions and wall thickness of the insulated shipper.
- Pick the insulation type, or enter its thermal conductivity.
- Enter the average outside temperature and how long the parcel will be in transit.
- Enter the freezer temperature, the weight of one pack and a safety margin, then read the pack count.
Formula
Heat leak Q = k × S × (T outside − 0 °C)
S = A ÷ t + 0.54 × (sum of inside edges) + 1.2 × t
Gel needed = Q × hours × 3,600 ÷ (specific heat × freezer degrees below 0 °C + latent heat) × (1 + margin)
S is the conduction shape factor of a hollow box (walls, edges and corners) from standard heat-transfer texts; A is the inside surface area and t the wall thickness, in metres. Default 334 J/g latent heat and 2.05 J/g K specific heat approximate pure-water ice. Replace them with supplier data for compatible 0 °C gel; insulation conductivities are illustrative.
Example
A 12 × 10 × 8 in box with 1.5 in EPS walls on an 80 °F day leaks about 10.9 W of heat. Over 36 hours that is 1.42 MJ. Packs from a 0 °F freezer absorb 370 J per gram, so the box needs 3.8 kg of gel — 10.54 lb with a 25% margin, or 8 packs of 24 oz.
Frequently asked questions
Can gel packs keep food frozen?
This model only covers coolant melting near 0 °C and cannot verify frozen storage. Other phase-change materials need their own transition temperature and measured properties.
Why does insulation thickness matter so much?
Heat flow through the walls is roughly proportional to 1 ÷ thickness. Edge and corner losses prevent exact inverse scaling. Seams, film resistance, changing temperatures and product heat loads are not modeled.
Should the product be pre-chilled?
Yes. The estimate assumes the contents already start cold. Packing warm product uses up much of the gel's capacity before the trip begins.
Sources
Last updated: 2026-10-30