Multi-item carts

Mixed-cart billable weight estimator.

Enter the items in a cart and the box you would pack them in. See the USPS Ground Advantage price and FedEx billable weight for one shared box against one box per unit.

Ground Advantage retailFedEx per package and per shipmentSources cited

Cart and packing

Result

What this page answers

A carrier never sees your cart. It sees boxes, and it prices each box on its own weight and size. So the question behind a multi-item order is not what the items weigh but how many boxes they make and how big each one is. This page prices the same cart two ways, as one shared box and as one box per unit, so you can see which packing the published rules reward.

The result often runs against instinct. Two heavy, dense items often ship cheaper together, because one parcel pays one base price where two parcels pay two. Two light, bulky items can ship cheaper apart, because a large shared box crosses the dimensional-weight line and the cube fee.

How it is calculated

Rules and prices as of 2026-10-26: USPS Notice 123 effective October 4, 2026; FedEx Service Guide effective January 5, 2026, updated September 11, 2026.

USPS Ground Advantage retail. Postage is the greater of actual weight and dimensional weight. Dimensional weight only applies to a parcel over 1 cubic foot (1,728 cubic inches): round each dimension up to a whole inch, multiply, divide by 139, round up to a whole pound, capped at the 70 lb price. The price comes from the Notice 123 retail parcel table for your zone. Fees are added for a longest side over 22 in ($4.50) or 30 in ($10.00) and for a parcel over 2 cubic feet ($21.00); a length + girth over 108 in and up to 130 in pays the oversized price.

FedEx billable weight. Each dimension is rounded up to a whole inch, the three are multiplied and divided by 139, and any fraction of a pound is rounded up. FedEx says dimensional weight applies per package or per shipment depending on the service; for a per-shipment basis it adds the volumes of every piece before dividing. The page shows both. It does not price FedEx, because FedEx prices depend on your account.

Each unit in the separate-box option gets the extra inches you enter on every dimension and the packaging weight you enter. The shared box gets the product weight of the whole cart plus one packaging weight.

Worked example: two bulky pillows

Cart: two foam pillows, each 20 x 14 x 6 in and 2 lb. Zone 5. Shared box 22 x 16 x 14 in; 1 inch of extra room and 0.5 lb of packaging when each pillow ships alone.

Shared box: 4.5 lb on the scale, but 22 x 16 x 14 = 4,928 cubic inches. 4,928 / 139 = 35.45, which rounds up to 36 lb. The zone 5 price for 36 lb is $87.25, and the parcel is over 2 cubic feet, so the $21.00 cube fee applies: $108.25. FedEx billable weight is also 36 lb.

Separate boxes: each box is 21 x 15 x 7 = 2,205 cubic inches, 2,205 / 139 = 15.86, so 16 lb. The zone 5 price for 16 lb is $32.95 per box, $65.90 for both, and billable weight drops to 32 lb in total. Separate boxes are $42.35 cheaper here.

Reverse the cart: a 3 lb item (9 x 6 x 4) and a 2 lb item (10 x 7 x 5) in a 12 x 9 x 7 shared box with 0.4 lb packaging is 756 cubic inches, under the dimensional line. Zone 5, 5.4 lb prices as the 6 lb tier at $19.80. Shipped apart they are 4 lb ($17.80) plus 3 lb ($15.70) = $33.50. Together saves $13.70.

Where this stops

This page prices USPS Ground Advantage retail only. Priority Mail, UPS and commercial or cubic prices are not covered, and no discount, residential or fuel surcharge is applied. UPS is left out because its dimensional-weight page could not be opened when this page was built, and a rule we did not read is not one we publish. Packaging weight and padding are your estimates. Platform checkout rates (Shopify, Etsy, eBay, Amazon) may use their own box assumptions; this page does not model how any platform packs a cart.

Sources

Carriers revise prices and rules during the year. Confirm against the live documents before relying on a figure for a contract or a dispute. This is a calculator, not a carrier quote.

Common mix-ups

Often assumedEvery bulky box is charged on dimensional weight instead of what it weighs.

What the source saysFor USPS Ground Advantage, section 1.4.1 applies dimensional weight only to parcels exceeding 1 cubic foot (1,728 cubic inches), and then bases postage on the greater of actual and dimensional weight. At or under that size the actual weight is used. ^[1]
Often assumedPutting two items in one box always costs less than sending two parcels.

What the source saysSection 1.4.2 divides the rounded box volume by 139 and rounds up, so a roomy shared box can be priced on size rather than weight. Light, bulky items can cost more together than in two small boxes, as the worked example on this page shows. ^[1]

USPS DMM 133, section 1.4 Dimensional Weight Price for Low-Density Parcels

Checked against these sources on 2026-10-26.

FAQ

Why can shipping cost more after I add a second item?

A carrier prices each box by weight and size. A second item can push a box into the next weight tier, over the 1 cubic foot line where dimensional weight starts, or past the 2 cubic foot line where a $21.00 USPS fee applies. The worked example shows a shared box that costs $108.25 against $65.90 for two smaller boxes.

Is it cheaper to ship two items in one box?

Not always. Dense items often are, since one parcel pays one base price where two parcels pay two. Light, bulky items often are not, because the larger box is billed on dimensional weight. Enter your own sizes to see which side wins.

How does USPS dimensional weight work for Ground Advantage?

It applies to a parcel over 1,728 cubic inches. Round each dimension up, multiply, divide by 139, round up, and pay the greater of that and actual weight, capped at the 70 lb price. This follows DMM 133 section 1.4, text marked 7-12-26.

Does FedEx add the boxes of a shipment together?

The FedEx Service Guide says dimensional weight applies per package or per shipment. On a per-shipment basis it adds the volume of each piece, then divides by 139. This page shows the per-package sum and the per-shipment figure so you can compare them with your rate card.