How to measure a box for shipping

Length, width and height

1. Pack the box and tape it shut. Measure it the way it will ship, with any padding that pushes the sides out.
2. Set the box on a flat surface and measure each side on the outside, edge to edge, at its widest point. Canada Post takes measurements where the dimensions are greatest.
3. Call the longest side the length. UPS puts it plainly: “The longest side of your package is your length.” The other two are the width and the height.
4. Round each side up to the next whole inch or centimetre.
5. Weigh the packed box, labels and filler included, and round up to the next pound or kilogram.

Inches and centimetres both work, as long as you keep to one unit through the whole calculation. Canadian carriers publish their limits in both.

How to measure girth

Girth is the distance around a parcel at its thickest part, measured at a right angle to the length. FedEx and Canada Post both define it that way. For a box, you don’t need to wrap anything: add the width and the height and double it.

Girth = 2 × width + 2 × height

Carriers use girth in one figure, length plus girth, which caps how big a parcel can be. UPS gives the formula as length + 2 × width + 2 × height, with the longest side as the length. To measure it directly, run a soft tape measure around the middle of the box, over the two shorter sides, and pull it snug.

How each carrier rounds

Rounding up is why a side of 12.1 inches counts as 13. It matters most near a size line and for dimensional weight, where each side is multiplied by the other two.

Length plus girth limits by carrier

A parcel over these lines can be refused, or accepted with extra charges, so check the box before you buy a label. The length plus girth limit runs from 118 to 165 inches depending on the carrier and the service.

FedEx Ground stops at 70 lb to Newfoundland and Labrador and the territories. Canada Post’s own limits are 30 kg, 2 m on any side and 3 m in length plus girth, shown here in pounds and inches rounded down.

A worked example

A packed box measures 17.5 × 12.25 × 9.6 inches and weighs 6.3 lb.

1. Round each side up: 18 × 13 × 10 inches. The length is 18 inches.
2. Girth: 2 × 13 + 2 × 10 = 46 inches.
3. Length plus girth: 18 + 46 = 64 inches, well inside every published limit.
4. Round the weight up: 7 lb.
5. Dimensional weight: 18 × 13 × 10 = 2,340 cubic inches. Divided by 139, the divisor most Canadian services use, that’s 17 lb once rounded up.

The carrier bills the higher of the two weights, so this 7 lb box bills at 17 lb on a service that divides by 139, about 15 lb on Canada Post Regular Parcel at 166, and 21 lb on a UPS air service at 115. Our dimensional weight calculator does these sums for every carrier at once, and shipping box sizes lists what common boxes bill at.

Irregular items, tubes and soft packs

An item that isn’t a box is measured as if it were packed in the smallest rectangular box that holds it. UPS says to measure it “from its extreme points”, FedEx to assume it’s “enclosed in a six-sided, rectangular container”, and Canada Post uses “the smallest box within which the parcel can be entirely contained”.

• A tube or a rolled poster counts as a box as wide and as high as its diameter, so its girth is four times the diameter.
• Something with a handle or a part sticking out is measured to the tip of that part.
• A padded mailer or poly bag is measured full, at its thickest point. Since July 19, 2026, Purolator treats soft-sided packs longer than 18 inches as Flat or Soft Packages, a special handling category.

Shape can add a handling charge on top of the size. Canada Post counts an “irregular, cylindrical or round” item, or one not fully encased in a box, as unpackaged; Purolator lists “cylindrical-like or irregular-shaped items” such as mailing tubes and tires for additional handling; FedEx Ground adds charges for packages not fully encased in an outer container. Boxing an odd-shaped item usually keeps it to a plain size.

Bulging boxes and re-measurement

Carriers measure parcels again on their own equipment. Canada Post audits with devices approved by Measurement Canada, Purolator may reweigh and cube any shipment “notwithstanding that a weight and/or dimensions has been declared”, and FedEx notes that a package’s shape and size can change in transit, which can change its dimensional weight and surcharges.

A box stuffed until its sides bow out measures bigger than the size printed on it, and the carrier bills what it measures. To keep the label and the parcel in agreement:

• Use a box big enough for the item and its padding without forcing the flaps shut.
• Measure after you seal it, at the widest point of any bulge.
• Round up yourself, so the carrier’s rounding doesn’t add to the size you entered.
• Weigh it on a scale, with the label and filler on, rather than guessing.

If a carrier’s figures come out higher than yours, it bills the difference as an adjustment. With ProtonShip, an adjustment is posted with the carrier’s reason, and its figures next to yours when it reports them, before anything is charged. How adjustments work

Questions about measuring a package