Insulated Shipping Box Hold Time Calculator
Estimate how many hours an insulated shipper stays cold with a given amount of gel packs or ice, from box size, insulation and outside temperature.
An insulated shipper stays cold for as long as its coolant can absorb the heat seeping through the walls. This calculator estimates that time from the box's inside size, insulation type and thickness, the outside temperature and the gel packs or ice you put in. Use it to compare modeled heat loads; shipping duration and product temperature require tests with the actual payload and route profile.
How to use this tool
- Choose units and enter the inside dimensions and wall thickness of the box.
- Pick the insulation material, or enter its thermal conductivity.
- Enter the average outside temperature the parcel will see.
- Enter how many packs you use, their weight and freezer temperature, and a safety margin.
Formula
Heat leak Q = k × S × (T outside − 0 °C)
Cooling capacity E = coolant grams × (specific heat × freezer degrees below 0 °C + latent heat) J
Hold time = E ÷ Q ÷ 3,600 hours
S is the conduction shape factor of a hollow box: inside area ÷ wall thickness, plus 0.54 × the total length of the inside edges, plus 0.15 × thickness for each of the 8 corners. The adjusted duration divides by (1 + allowance). This arbitrary allowance does not validate seam losses, temperature excursions or product safety. The coolant defaults approximate pure ice; use measured material properties.
Example
A 12 × 10 × 8 in EPS box with 1.5 in walls on an 80 °F day leaks about 10.9 W. Six 24 oz gel packs from a 0 °F freezer weigh 9 lb and can absorb about 1,513 kJ, enough for 38.4 hours in theory, or 30.7 hours after a 25% margin under the stated idealized model. These durations do not establish a suitable delivery service.
Frequently asked questions
Is more ice or thicker insulation better?
Both help, but insulation often wins: thicker walls reduce planar conduction, although edge and corner losses prevent exact inverse scaling. Test the configuration to establish real performance.
Does it work for dry ice?
No. Dry ice sublimes at −78.5 °C and absorbs about 571 kJ per kg, so it needs a different calculation. Use the dry ice calculator for frozen shipments.
Why use average rather than peak temperature?
Hold time depends on the total heat over the trip, so the average matters most. For summer routes through hot regions, enter a higher average or raise the safety margin.
Sources
Last updated: 2026-10-30