Poly bag or box? How FedEx measures and bills soft packaging
Poly mailers often bill at a lower weight than boxes, but a loose bag or an oversized one can cost more. How bags get measured and when to use one.
For soft goods like clothing, a poly bag is usually the cheapest way to ship. There is no cardboard adding size, and the package ends up only as thick as what is inside. But bags get measured too, and a sloppy one can bill like a box.
How a bag gets measured
FedEx applies dimensional weight to bags the same way it does to boxes: length × width × height, each side rounded up to the next inch, divided by the dim divisor.
The difference is what counts as the length, width and height. A bag is not a neat shape. Plan on it being measured at its furthest points, as if a box were drawn around it. That includes the loose tail of the bag if you leave it flapping.
A useful rule of thumb: a flat bag that is W inches wide and L inches long, filled to a thickness of T inches, ends up measuring about (W − T) × (L − T) × T.
A T-shirt, bag vs box
One folded T-shirt, about 10 × 8 × 1 inches and under half a pound.
- In a 9 × 12 poly bag: measures about 12 × 9 × 2. That is 216 cubic inches, a dim weight of 2 lb.
- In an 8 × 6 × 4 box: the outside bills as 9 × 7 × 5. That is 315 cubic inches, a dim weight of 3 lb.
On 2026 FedEx Ground list rates to zone 5 that is $15.66 in the bag and $16.81 in the box, and the bag itself costs a fraction of what the box does.
The tail matters
A hoodie in a 14.5 × 19 bag, left loose, measures roughly 16 × 12 × 4: 768 cubic inches, billed at 6 lb. Fold the extra bag over and tape it tight to the contents and it measures closer to 14 × 12 × 3: 504 cubic inches, billed at 4 lb.
Ground list to zone 5 is $18.60 at 6 lb and $17.73 at 4 lb. Tape is cheap. Use the smallest bag that fits, and fold down what is left.
Watch the size limit on bags
FedEx charges an additional handling fee for packaging that is not a corrugated box. Parcel-audit firms report that soft packs such as poly bags are charged it once they exceed about 18 inches on the longest side, 13 on the second, or 5 inches thick. In 2026 that fee is reported at roughly $25 to $35 per package depending on zone, which wipes out any saving from the bag.
Confirm the current limits and amounts in the FedEx Service Guide or on an invoice. The practical rule: keep bags small and flat, and put big or bulky orders in a box.
When a box is still the right call
- Anything fragile or crushable.
- Orders too big to stay under the soft-pack size limit.
- Dense items, where the price is set by actual weight and the packaging size does not matter.
Compare them yourself
The free calculator has a box and poly bag switch. Enter the filled size of each and see the billed weight and cost side by side.
Rates quoted here are FedEx 2026 standard list rates, effective January 5, 2026, before fuel and surcharges.