How many gel packs do I need in my shipping container? How long will my gel packs last at a given temperature given the package size? Needless to say, we get this question a lot so we decided to build a calculator based on the principles of heat transfer. While you may have heard the old industry rule of thumb of 1:2 in Winter, 1:1 in Summer (ratio of gel pack weight to perishables weight), we believe this leaves something to be desired in terms of specificity. 

This calculator is not intended to substitute for field testing or environmental chamber testing, its intended to be used as a heuristic or starting place. This calculator is calibrated with a goal temperature of 0C/32F.

We’d welcome your questions. Our technical sales team and packaging engineers are standing by to help. Email us at cs@minusworks.com or call us at 516-331-1638.

About this Calculator

This calculator is based on a popular thermal engineering equation used to determine the R value of various materials used for insulation.

We are going to transform this equation to solve for amount of gel pack needed by taking the variable “melt rate” which is units of mass divided by Time and splitting it up so we can get units of mass (or amount of gel pack required) on the left side of our equation. Time, which was the divisor in the Melt Rate parameter, then moves to the top of the right side of the equation. After the transformation, our equation will look like this:

 Parameters:

Surface Area: The surface area will be the internal surface area of your package. For example, a box with a 10 inch height, 10 inch length, 10 inch width will have an internal surface area of 600 square inches. Enter in your container dimensions (L x W x H) in the fields below. Make sure the values are in inches.

∆T: This is the temperature difference between our starting internal temperature, which we are assuming is 0 degrees C and the average ambient outside temperature. In the Ambient Temperature field, enter what you believe the average temperature to be over the course of the time period. Make sure the value is in degres Celsius.

Time Period: This is the amount of time your package will be subjected to the ambient temperature. For example, if you are overnighting your package, you may want to choose 24 hours as a Time Period. If you are delivering your package via active refrigeration but are unsure how long it many sit in a lobby or on the doorstep, you may choose 8 hours as a Time Period. Make sure the value is in hours.

Latent Heat: We are using the latent heat of fusion for the Minus Works Plant-based Refrigerant Gel. No need to enter a value but keep in mind, if you are using lower quality/performance gel packs this number will be lower, meaning you will need to use more of those gel packs than this calculator indicates. If you’d like to learn more about the concept of Latent Heat, check out our blog post on the subject.

R-Value: this is the thermal resistance (effectiveness) of the insulation you are using in your packaging. The higher the better. See list of R-Values for common insulation materials below.  If you don’t see your insulation or something close to it listed here, feel free to reach out to us.

Source for R-Values:  Performance Comparison of Thermal Insulated Packaging Boxes, Bags and Refrigerants for Single-parcel Shipments. (2008) Michigan State University, Cal Poly State University.