UPDATE: The principles and formulas for dimensional weight (DIM weight) rules below are a great resource for ecommerce retailers. For updated examples of DIM weight formulas, including the latest DIM weight calculations and how we help you cut your shipping costs in half, visit our Dimensional Weight Calculator.

FedEx and UPS changed their dimensional weight rules starting 2015. UPS dimensional weight rules from 2015 remain in place. Fedex recently announced new dimensional weight rules for 2017. For an up-to-date overview of how DIM rules apply, please visit our most recent post: New FedEx & UPS Dimensional Weight Rules.

Have you seen those USPS commercials for their Flat Rate Boxes; you know, the ones where people are so amazed at how easy shipping is?

Well, if you’re an online merchant, you know that your stuff doesn’t always fit neatly into a flat rate box, which means that shipping isn’t always easy. Sometimes, it’s flat out complicated, especially when dimensional weight comes into play.

As an eCommerce fulfillment center, we’re often asked how to calculate dimensional weight. We’ll get into that shortly, but let’s start with a quick overview of what it actually is.

What is Dimensional Weight?

Dimensional weight, also known as “dim” weight, is a tactic used by shipping and freight companies to basically ensure they don’t lose money on lightweight packages. Dim weight has long been used for air freight, and about 5 years ago, the big shipping carriers like FedEx and UPS also started applying dimensional weight to packages shipped via ground service.

Dimensional weight works by calculating the cubic size of a package by multiplying its length, width and height. Once the dimensional weight is calculated, it is then compared to the actual weight of the package in some circumstances, and the larger of the two is used to determine the package’s actual billable weight.

When it comes to eCommerce order fulfillment, online merchants are often concerned about when dim weight will actually be triggered, and how they can anticipate those charges. So let’s talk about how to calculate dimensional weight, and when it’s applied.

How Do You Calculate Dimensional Weight?

To determine the dim weight of a package, you must first measure the length, width and height of the package in inches using the longest point on each side, taking into account any bulges or misshaped sides.

You then multiply those dimensions to get the cubic size of the package. For example, let’s say your package is 30” x 12” x 12”. In this case, you multiply these 3 dimensions and determine that the package is 4,320 cubic inches.

When calculating dimensional weight, it’s important to keep in mind that most shipping carriers will have you round to the nearest whole number. For instance, if the width of your package is 12.50 inches or above, you’d round up to 13”. If it’s 12.49” or below, you’d round down to 12”.

To determine the dimensional weight of a package, the major carriers, UPS and FedEx, currently use the same calculation, which is the cubic size of a package divided by 166 for domestic shipments, and 139 for international shipments. (Note: in 2020 FedEx uses a DIM weight factor of 139. UPS uses a DIM weight factor based on rate type. For Daily Rates the UPS weight factor is 139, for Retail Rates the UPS weight factor is 166.) These calculations are shown in more detail below:

What is Dimensional Weight Pricing?

Dimensional weight pricing is defined as the price that is paid for shipping based on the cubic size of the package that is being shipped.

In many cases, the package size and overall dimensional weight are used as factors in determining the package’s billable weight, which is the final weight used to determine the actual pricing of shipping the package. But, depending on your carrier, dim weight isn’t always used to determine billable weight, and this is where it can get somewhat complicated.

Here’s how it works. If your package is being delivered via air service, the billable weight will always be subject to dimensional weight. If it’s being shipped via ground service, however, generally it is only subject to dim weight if the cubic size of the package is 3 cubic feet or larger (5,184 cubic inches). If the cubic size is less than 3 cubic feet, dimensional weight will not apply, and the actual weight of the package will always be used.

I know, a little confusing, right? To help clear all this up, let’s take a look at a few examples below.

box with bow

Example 1

Ship Method / Destination: Ground / Domestic
Actual Weight: 21 pounds
Length: 30 inches
Width: 12 inches
Height: 12 inches
Cubic Size Calculation: 30 x 12 x 12 = 4,320 cubic inches
Dimensional Weight Calculation: 4,320/166 = 26 pounds

In this first example, because it is a domestic shipment, we multiply the total cubic size in inches by 166. In doing so, we find out that the dimensional weight is 26 pounds, which is greater than the actual weight.

However, because the package is being shipped via ground service and is less than 5,184 cubic inches, the dimensional weight is not applied. So in this case, the actual weight of 21 lbs. becomes the billable weight.

Example 2

Ship Method / Destination: Ground / Domestic
Actual Weight: 32 pounds
Length: 30 inches
Width: 15 inches
Height: 15 inches
Cubic Size Calculation: 30 x 15 x 15 = 6,750 cubic inches
Dimensional Weight Calculation: 6,750/166 = 41 pounds

In example 2, this is also a domestic shipment, so the cubic size in inches is divided by 166 to determine the dimensional weight, which is 41 lbs.

As in example 1, the dimensional weight here exceeds the actual weight, but because the cubic size of the package is larger than 5,184 cubic inches, the dimensional weight of 41 pounds will be used as the billable weight.

Example 3

Ship Method / Destination: Air / International
Actual Weight: 18 pounds
Length: 24 inches
Width: 12 inches
Height: 12 inches
Cubic Size Calculation: 24 x 12 x 12 = 3,456 cubic inches
Dimensional Weight Calculation: 3,456/139 = 25 pounds

In this last example, the package is being shipped internationally, so the cubic size is divided by 139 instead of 166, which makes the dimensional weight 25 lbs. This exceeds the actual weight of 18 lbs., and because it’s an air shipment and the 3 cubic foot minimum doesn’t apply, in this case the dim weight will again be used as the billable weight.

Dimensional weight can get a little confusing, and what makes it even worse is that, just when you think you have it figured it out, the carriers go ahead and change the rules (which means I’ll probably have to update this blog post before too long). And the problem with not correctly factoring in dimensional weight, is that you will likely get hit with back charges from the shipping providers.

While those USPS commercials make parcel shipping seem easy, in many cases it’s just not that simple, and that’s where an order fulfillment partner like eFulfillment Service can save you a lot of time and money.

Sick of dealing with the headaches of shipping? Let’s talk, we’d love to help.