Give the calculator the outer carton and it prices what the carrier will actually bill.
The specification both carriers publish
UPS calls it the two-box method and FedEx calls it box-in-box, and the numbers match almost exactly. That agreement is unusual enough to be worth trusting.
FedEx states the trigger too, asking shippers to double-box fragile items with 3 inches of cushioning in and around the smaller box. Note the phrase in and around: the product is wrapped inside the inner carton first, with FedEx asking for at least 2 inches of air-cellular or foam material around each item, and only then does the inner carton get floated in the outer one.
Building it in five moves
- 1. Wrap the item, not the boxTwo inches of air-cellular material or foam directly against the product, taped so it cannot slide off. Hollow shapes get their cavities packed first, because an empty shell flexes and a filled one does not.
- 2. Seat it in a snug inner cartonThe inner box should be barely larger than the wrapped item, with fill closing the last gaps. FedEx asks you to restrict product movement inside the box using filler like crumpled newspaper, loosefill peanuts, or other cushioning material.
- 3. Seal the inner carton properlyFedEx asks for the inner box to be closed and taped using the H taping method, and says it helps prevent accidental opening. A loose inner lid defeats the whole arrangement when the outer box lands on a corner.
- 4. Float it in the outer cartonThree inches of fill on the bottom first, the inner box centred on that bed, then fill packed down the four sides and across the top until nothing shifts. The inner carton should not touch any outer wall at any point.
- 5. Close, shake and measureSeal the outer box with the same H pattern, rotate it slowly and listen for movement, then measure the outer carton and put it on a scale. The number the carrier bills comes from the outside of the second box, which is the part people forget.
Materials for step four are interchangeable within reason. Loose fill is cheapest per cubic foot but settles, and the void fill comparison covers what each option does when a carton lands hard.
What the outer carton adds
Packing guides describe the method and stop there. Here is the part they leave out, on a lane you can check.
| Carrier | Service | Price | Transit window returned |
|---|---|---|---|
| UPS | Ground Saver | $25.66 | Not returned for this row |
| FedEx | Ground Economy | $25.83 | 6-7 business days |
| UPS | Ground | $27.31 | 2-3 business days |
| FedEx | Ground | $37.40 | 5-6 business days |
| UPS | 3 Day Select | $38.60 | 5-6 business days |
Now set it against the same contents in a single carton. Six inches added in each dimension takes a 12 by 10 by 6 box to 18 by 16 by 12, which is 3,456 cubic inches instead of 720.
| Packing | Outer carton | Weight | Cheapest rate returned |
|---|---|---|---|
| Single box | 12 x 10 x 6 in | 5 lb | $13.52 |
| Double boxed to spec | 18 x 16 x 12 in | 6 lb | $25.66 |
The second carton cost $12.14 on this lane, close to doubling the single-box rate, and only one pound of that came from actual weight. The rest is volume, billed through the dimensional weight rule that applies once a parcel passes one cubic foot; the divisor walkthrough shows the arithmetic.
That number is the honest case for and against the method. For an inexpensive item, the extra volume can cost more than the goods are worth. For anything fragile and valuable, $12.14 is cheaper than one replacement plus one return label, which is exactly how the fragile guide frames it.
Shrink the inner box before the outer one
Six inches are added to whatever the inner carton measures, so every inch you trim inside saves three cubic inches of billed volume outside. Cutting a carton down to size is fifteen seconds with a blade.
A heavier grade is sometimes the better trade
For a sturdy item that just needs strength rather than isolation, moving up a corrugated grade costs cents and adds no dimensions at all. The grade chart shows what each rating carries.
Measure the outer box, always
Rates come from the carton the carrier can see. Anyone who prices the inner box and then double boxes it has bought the wrong label, and the correction arrives later as an adjustment.
Double boxing questions
What is the double boxing specification?
An outer carton six inches greater in depth, width and length than the inner one, with three inches of cushioning filling the space between them on the top, bottom and all sides. UPS and FedEx both publish those figures in their own packing guidance, read 2026-07-28.
How much does double boxing add to the shipping cost?
On our test lane it added $12.14. The same contents quoted $13.52 in a 12 by 10 by 6 inch carton at 5 lb and $25.66 double boxed in an 18 by 16 by 12 inch carton at 6 lb, Newark, NJ 07102 to Atlanta, GA 30303, both on 2026-07-28.
When is double boxing worth it?
When a replacement plus a return label costs more than the extra volume does. FedEx asks for fragile items to be double-boxed with 3 inches of cushioning in and around the smaller box, so anything breakable and worth more than the surcharge is a candidate.
Do I measure the inner box or the outer box for the rate?
The outer one. Carriers price the parcel they receive, so the dimensions that count are the outside of the second carton, and its volume is what the dimensional weight rule is applied to once the parcel passes one cubic foot.