Select an incorrect box size and you’re either paying for a bunch of empty space or you’re watching your product bounce around in your box while in transit. Learning how to measure a box correctly takes two minutes, and it could save your business hundreds of dollars on every shipment it sends out. Here’s exactly how to do it.
Why Box Measurements Actually Matter
Shipping carriers don’t just charge you based on the weight of your package. They also consider how much room it takes up on a truck or plane. If you get your measurements wrong, you may be paying for a larger box class than you actually need, or even worse, your carrier may flag the shipment and charge you more later.
Doing this correctly the first time makes your shipping costs predictable and keeps your customers happy.
Inside Dimensions vs. Outside Dimensions
This is confusing for many people, so let’s clarify it right away.
When You’re Buying a Box
Box suppliers and distributors base dimensions on inside measurements of the box. It’s the space we can use, where the product really sits. When you are ordering boxes for a product that measures 10 x 8 x 6 inches, you want to ensure that the interior of the box that you choose comfortably accommodates those dimensions, with a little extra room for padding.
When You’re Shipping a Box
The box is measured on the outside by carriers such as UPS, FedEx, and USPS. After your product is packed and the box is taped up, your shipping cost and the box classification will be based on the outside dimensions. This is the number that will be used to create your shipping label.
Confusing these two is one of the most common packing errors small businesses make, so keep them straight in your mind: inside dimensions when purchasing, outside dimensions when shipping.
How to Measure a Box, Step by Step
You will need a tape measure or ruler and two minutes. Measure in inches only unless your provider requests centimeters.
Step 1: Measure the Length
The length is the longest side of the box, and is usually the side with the longer flap on top. To measure the length of the box, you must first lay the box flat, and then measure from one edge to the other.
Step 2: Measure the Width
Turn the box 90 degrees from the length measurement. The width is the shorter side, often denoted by the shorter flap.
Step 3: Measure the Height
With the box closed and the flaps flat, measure straight from the bottom to the top. That is your height.
A Quick Note on Accuracy
Round up to the nearest inch and don’t cut off fractions to make the box look smaller. Carriers generally allow some variance (about 1/8 of an inch), but purposely rounding down can cause a dimension mismatch and a surprise rebill.
What Is Dimensional Weight (and Why Should You Care)?
Once you have your three measurements, you’re then ready for the part that actually influences your purse strings: dimensional weight, usually abbreviated “DIM weight.” Sub Carriers levy charges based on the greater of the two, which is either the actual weight of your parcel or its dimensional weight. A big, lightweight box (imagine a lampshade or a pillow) can cost more to ship than a tiny, dense one — simply because of how much room it takes up.
How to Calculate Dimensional Weight
- Find the cubic size. Multiply length x width x height (in inches).
- Divide by the carrier’s dim factor. Common divisors:
- UPS and FedEx: divide by 139
- USPS and DHL: divide by 166 (for packages under 1,728 cubic inches)
This is your dimensional weight in pounds. Compare this number to the actual weight on your scale, and you will be charged for the higher of the two.
A Simple Example
For example, a box that is 12 inches by 10 inches by 8 inches.
- Cubic size: 12 x 10 x 8 = 960 cubic inches
- Dimensional weight (UPS/FedEx): 960 ÷ 139 ≈ 6.9 lbs
[If] your real product is 3lb, you’ll still get charged for the 6.9 lb dimensional weight since that is the larger number.
Tips for Choosing the Right Box Size
- Match the box to the product. Oversized boxes waste money and require more filler material.
- Leave just enough room for padding. An extra inch or two on each side is usually plenty for bubble wrap or packing paper.
- Check with your carrier before shipping oddly shaped items. Irregular packages sometimes have separate pricing rules.
- Weigh and measure every time. Even boxes that look identical can vary slightly, and that variance adds up across high shipping volumes.
Getting It Right Pays Off
After all, a few extra seconds with a tape measure can be the difference between a shipping bill that makes sense and one that has you scratching your head. Once length, width, height, and dimensional weight measurement are second nature, you’ll be able to spend less time resolving invoices and more time on the running of your business.
