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Measure the outside of the sealed, packed box: length by width by height, with the longest side as the length. Use a tape measure, measure each side at its widest point, and round up to the next whole inch or centimetre. Carriers scan dimensions with lasers in their sorting facilities, so the numbers you enter need to match the box they will measure.
| Dimension | How to measure | Common mistake |
|---|---|---|
| Length | The longest side of the box | Measuring the side facing you instead of the longest side |
| Width | The second-longest side | Measuring the inside of the box instead of the outside |
| Height (depth) | The shortest side | Forgetting that bulging adds height |
| Girth | Twice the width plus twice the height (2W + 2H) | Not knowing carriers use length plus girth for oversize limits |
Carriers define length as the longest side, full stop. If you list dimensions in a different order, rating engines may misread the parcel's shape, and dimensional-weight calculations assume the standard ordering. It also matters for surcharges: oversize rules are usually written as "length plus girth exceeds X," so a box measured in the wrong order can look compliant on your label and oversized on the carrier's scanner.
Measure after packing and sealing, never before. An empty flat box and a packed, taped, slightly bulging box are different parcels. If the box bulges, measure at the bulge: the scanner will.
Girth is the distance around the thickest cross-section of the parcel: twice the width plus twice the height. You will see it in oversize rules written as "maximum length plus girth." A long skinny box (a poster tube, a ski bag, a curtain rod) can pass the length limit on its own but fail the combined limit once girth is added. This is the number that surprises people shipping long items, so compute it before you buy the label: L + (2W + 2H), and compare it against the carrier's maximum.
Lay the sealed mailer flat and measure its length and width; the height is effectively the thickness of the contents. Poly mailers compress, which is part of their appeal: they dodge dimensional-weight charges that a box would trigger. But do not understate: if the mailer bulges around a chunky item, measure the bulge. And keep the mailer smooth when you attach the label, since a wrinkled label over a bulge can fail to scan.
Rounding down is the costliest habit. Carriers round up fractional measurements, so a box you call 11 inches that scans at 11.4 gets rated at 12. Entering 11 buys you a postage adjustment later. Round up yourself and the label matches the scan.
Second: measuring the item instead of the box. The label covers the parcel as tendered, packaging included. Third: reusing a box without removing old labels and markings, which can confuse scanners and send the parcel on a detour. Fourth: the bulge problem mentioned above, especially with overstuffed boxes taped into a pillow shape. If the box will not sit flat, it is the wrong box: repack into the next size up rather than fighting it.
Often not, which is one of flat-rate's quiet advantages. LetterAgent shipping labels are flat-rate Canada Post Xpresspost labels (Canada-only, up to 5 kg): the price is quoted up front and does not depend on the box's measurements, so a careful dimension entry is one less thing to get wrong. You still need the parcel to be within the service's size and weight limits, and the destination must be in Canada, but the per-shipment math disappears.
Ready to ship it? Get a shipping label with LetterAgent: describe it to your AI assistant, approve the exact quoted price, and get your label.
LetterAgent is a working name. Prices include print, postage, and our service fee. US prices in USD, Canadian prices in CAD (Canadian prices exclude applicable sales tax, added at checkout). Currently in private testing.
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