To calculate package size for shipping, you need three measurements: length, width, and height. From those, you can find the combined length plus girth, which is the number carriers use to determine surcharges and whether they’ll accept your package at all. You may also need to calculate dimensional weight, which carriers use to charge you based on how much space a package takes up rather than how much it actually weighs.
Measure Length, Width, and Height
Start by identifying the longest side of your package. That measurement is your length. Rotate the package 90 degrees and measure the next longest side, which is the width. Rotate once more and measure the remaining side for the height.
Round each measurement up to the nearest whole inch. Carriers like UPS are explicit about this: anything from 1.01 to 1.99 inches rounds up to 2 inches. This rounding applies before you plug numbers into any formula, so measure carefully but don’t stress over fractions of an inch.
For irregularly shaped packages (think oddly wrapped items, bags, or anything that isn’t a clean rectangle), measure as if the item were inside a rectangular box. Find the extreme points on each axis and use those as your length, width, and height. Protruding handles, straps, or bulges all count toward the outer dimensions.
Calculate Girth
Girth is the distance around the two shorter sides of your package. The formula is simple: add the width and height together, then multiply by two.
- Girth = (Width + Height) x 2
For example, if your box is 24 inches long, 10 inches wide, and 8 inches tall, the girth is (10 + 8) x 2 = 36 inches. You can also think of girth as wrapping a tape measure around the thickest cross-section of your package, perpendicular to the longest side.
Calculate Combined Length Plus Girth
This is the measurement most carriers care about for size limits and surcharges. Take your length (the longest dimension) and add the girth you just calculated.
- Combined Length + Girth = Length + Girth
Using the same example: 24 + 36 = 60 inches combined. USPS walks through a similar calculation in its guidelines. For a box that is 20 inches long with a girth of 18 inches (sides of 6 inches each), the combined length plus girth is 38 inches.
How Tubes and Odd Shapes Work
Cylindrical tubes, triangular tubes, and rolls follow the same principle. Length is still the longest dimension (end to end), and girth is the distance around the thickest part, measured perpendicular to the length.
For a triangular mailing tube that is 10 inches long with sides measuring 8, 5, 8, and 5 inches, the girth is 8 + 5 + 8 + 5 = 26 inches, and the combined length plus girth is 36 inches. For a round tube, wrap a flexible tape measure around the circumference at the widest point to get the girth directly.
Calculate Dimensional Weight
Carriers charge you the greater of your package’s actual weight or its dimensional weight (sometimes called DIM weight). Dimensional weight reflects how much space a package occupies in a truck or plane, so a large, lightweight box can cost more than its scale weight would suggest.
The formula multiplies all three dimensions, then divides by a number called the DIM factor:
- Dimensional Weight = (Length x Width x Height) / DIM Factor
Round the result up to the nearest whole pound. The DIM factor varies by carrier. FedEx and UPS use 139. DHL and USPS use 166 for packages larger than 1,728 cubic inches (one cubic foot).
Here’s a practical example from USPS: a box measuring 21 x 20 x 20 inches has a volume of 8,400 cubic inches. Divide by 166, and you get 50.6, which rounds up to a dimensional weight of 51 pounds. If the box actually weighs 15 pounds on a scale, you’d be billed based on 51 pounds. That difference can be significant, so checking dimensional weight before you ship helps you avoid surprise charges.
Size Limits and Surcharges
Knowing your combined length plus girth also tells you whether your package triggers extra fees or gets rejected entirely. UPS, for instance, applies a Large Package surcharge when any of these conditions are met for domestic shipments:
- Combined length plus girth exceeds 130 inches
- Length alone exceeds 96 inches
- Cubic size (length x width x height) exceeds 17,280 cubic inches
- Weight exceeds 110 pounds
The absolute maximum at UPS is 165 inches of combined length plus girth, 108 inches of length, and 150 pounds. Packages that exceed any of those limits are subject to an Over Maximum Limits charge or may be refused altogether. FedEx and USPS have similar thresholds, though the exact numbers and fees differ. Check your carrier’s current schedule before shipping anything close to these boundaries.
Putting It All Together
Say you’re shipping a box that measures 30 x 18 x 14 inches and weighs 12 pounds. Here’s the full calculation:
Girth = (18 + 14) x 2 = 64 inches. Combined length plus girth = 30 + 64 = 94 inches. That’s under the 130-inch large package threshold, so no surcharge on size alone.
Dimensional weight through UPS or FedEx = (30 x 18 x 14) / 139 = 7,560 / 139 = 54.4, rounded up to 55 pounds. Through USPS or DHL, the same volume divided by 166 gives 45.5, rounded up to 46 pounds. Either way, the carrier bills you for the dimensional weight rather than the 12 pounds on the scale.
Running these numbers before you choose a box size can save real money. A slightly smaller box that fits the item snugly could drop the dimensional weight by several pounds, which means a lower shipping cost on every package you send.