Reduce Shipping Costs by Understanding Dimensional Weight​

Dimensional (DIM) weight represents the physical space a package occupies. Also known as volumetric weight, DIM weight is an important cost factor because many carriers calculate shipping costs based on whichever value is greater: the package's actual weight on the scale or its calculated DIM weight. The reason for this comes down to transit space.

For businesses, understanding DIM weight matters because it directly affects what you pay to ship. If a business ships lightweight products in oversized packaging, for example, it risks paying for the box's volume rather than the actual weight of the items inside. On the other hand, if businesses right-size their packaging and eliminate unnecessary void space, they can reduce their DIM weight and potentially lower shipping costs as a result.

Knowing how to calculate DIM weight and how it compares to actual package weight helps you accurately estimate shipping costs before you ship.

Step 1: Measure the length, width, and height of your package, rounding each measurement up to the next-higher centimeter or inch*. Multiple the rounded numbers for length, width, and height.

Step 2: Divide the result by the dimensional factor, or DIM factor, used by your carrier.

Length × Width × Height ÷ Dimensional Factor = Dimensional Weight

Step 3: Round the calculated result to the next full kilogram or pound*

Once you've completed this calculation, compare the result to your package's actual scale weight. The greater of these determines your shipping cost.

Let’s look at the DIM weight formula in practice. A package measuring 36 cm × 25 cm × 16 cm, shipped with a carrier using a dimensional factor of 5,000 (standard density in cubic centimeters per kg), has a DIM weight of 3 kg. If the same package has an actual scale weight of 2 kg, shipping costs would be based on the DIM weight of 3 kg, since it's the larger number.

*Shippers should refer to their carrier’s rounding rules when calculating DIM weight

The dimensional factor can vary by carrier and shipment type, so it's important to confirm the correct figure with your carrier before calculating DIM weight and estimating shipping costs.

For shipments measured in centimeters, FedEx uses a dimensional factor of 5,000 (standard density in cubic centimeters per kilogram). For shipments measured in inches, the FedEx dimensional factor is 305 (standard density in cubic inches per kilogram), or 139 (standard density in cubic inches per pound).

Rounding rules determine how individual package measurements are adjusted before a DIM weight calculation is made, and they can have a meaningful effect on the final number used to price a shipment.

In recent years, some carriers, including FedEx, have updated their rounding rules to round each package dimension (length, width, and height) up to the next whole centimeter or inch before calculating DIM weight. Once that calculation is complete, any fraction of a kilogram or pound is then rounded up to the next whole weight.

Old rounding rule vs new rounding rule:

Because rounding now happens earlier in the process and always rounds up rather than to the nearest whole number, packages with dimensions just above a whole number can see a small increase in their calculated DIM weight compared to the previous method.

The rounding changes make it worth revisiting your packaging strategy, even for products you've shipped the same way for years. Small, seemingly minor reductions in packaging size can add up to meaningful savings for businesses shipping at volume, since the effect of rounding compounds across every dimension and every shipment.

The most effective ways to minimize DIM weight are choosing packaging that closely matches your product's size, avoiding excess filler and loose wrapping materials, and auditing your packaging regularly to eliminate unnecessary bulk. Here are some practical ways to put this into practice:

• Choose packaging that matches the size and fragility of your item. The closer your packaging fits your item, the lower your calculated DIM weight is likely to be.

• Consider flat rate boxes for predictable pricing. FedEx flat-rate boxes are available for shipments up to 10 kg and 25 kg, offering set pricing regardless of DIM weight calculations. However, excess weight above these limits will be charged accordingly.

• Compare packaging types for cost efficiency. Some packaging formats are priced more competitively than others for the same shipment. For example, using FedEx Reusable Paks (available for shipments up to 2.5 kg) is often more cost-effective than sending items of the same weight in custom packaging.

• Test your packaging before you ship. Weak or inadequate packaging can change shape during transit as items shift within and push out against the sides. To avoid these issues and potential surcharges, test your packaging before you ship for durability.

• Avoid loose shrink-wrap, overlapping tape, or bulging materials. These can inflate a package's outer dimensions, increasing DIM weight and shipping costs.

• Swap oversized fillers for fitted inserts. Bubble wrap and crumpled paper are commonly used to protect items in transit, but they often add unnecessary bulk. Fitted inserts that hug the item’s shape can protect items just as effectively without adding to the package's overall dimensions.

• Audit your packaging regularly. Assess your current packaging and ensure it achieves the right balance between minimizing DIM weight and keeping items secure in transit. Additionally, consider the pros and cons of right-sizing packaging for each item or SKU you ship vs using a small set of standard packaging sizes.

Adopt these simple tips to manage DIM weight and control shipping costs:

• Know how DIM weight works

• Audit packaging regularly

• Avoid fractional measurements

• Use flat-rate options

• Right-size, don't over-pack

Ready to put this into practice? Visit the FedEx packaging page to explore shipping supplies and download the packaging guide