Interactive model
Dim weight break-even
The box size at which dimensional weight overtakes actual weight, and what a packaging change is worth across a real order profile.
Rule set
Divisor 139, each side rounded up to the next inch, applied to every package regardless of size. The break even density is 12.4 lb per cubic foot.
One package
Enter the outside dimensions as the carrier would measure them. The default is a 14×12×8 carton with a quarter inch of wall on every side, which is exactly the kind of box the round up rule catches.
| Rule set | Measured as | Cubic in | Dim wt | Billed | Cost |
|---|---|---|---|---|---|
| UPS and FedEx, before August 2025 | 14×12×8 | 1,344 | 10 lb | 10 lbdim | $12.93 |
| UPS and FedEx, from August 2025 | 15×13×9 | 1,755 | 13 lb | 13 lbdim | $14.01 |
| USPS, before July 2026 | 14×12×8 | 1,344 | exempt | 4 lbactual | $10.71 |
| USPS, from July 2026 | 15×13×9 | 1,755 | 13 lb | 13 lbdim | $14.01 |
Under UPS and FedEx, from August 2025, this package bills 13 lb for 3.2 lb of product and box. It keeps paying for space until the contents push it past 12 lb. At 3.2 lb, the largest box that bills on actual weight is 556 cubic inches, against 1,755 measured here. The package density is 3.8 lb per cubic foot, against a break even of 12.4.
Packaging program
The same 20,000 synthetic orders, packed three ways. The product never changes, only the container around it. Air premium is what each order paid above the same package billed on actual weight.
| Program | Billed on space | Actual lb | Billed lb | Air premium | Cost per order | Delta |
|---|---|---|---|---|---|---|
| Three stock cartons | 98% | 4.8 | 20.0 | $5.45 | $16.35 | baseline |
| Twelve carton set | 85% | 4.6 | 13.9 | $3.41 | $14.23 | −13.0% (−$2.12) |
| Twelve cartons plus mailers | 80% | 4.2 | 7.8 | $1.34 | $12.01 | −26.5% (−$4.34) |
Under UPS and FedEx, from August 2025, the average order weighs 4.8 lb in stock cartons and bills 20.0. Moving to the twelve carton set and bagging the soft goods cuts the cost of every order by 26.5%, and no one touched a rate. On this synthetic table that is $4.34 an order, or $86,702 across 20,000 orders.
Where the break even sits
One package in every 25, plotted by measured cube and actual weight. Switch the program and watch the cloud slide left toward the line.
Where the air is
Air premium per order by category under UPS and FedEx, from August 2025. Dense goods barely notice dimensional weight. Light, bulky goods pay for it on almost every order, and they are the ones a mailer can fix.
| Category | Share | Three stock cartons | Twelve carton set | Twelve cartons plus mailers |
|---|---|---|---|---|
| Apparel | 32% | $5.71100% | $3.71100% | $0.6685% |
| Beauty and small goods | 22% | $1.68100% | $0.6292% | $0.6292% |
| Home goods | 21% | $7.4598% | $3.2590% | $3.2590% |
| Bedding and pillows | 12% | $12.59100% | $11.71100% | $2.58100% |
| Dense goods | 13% | $1.5186% | $0.1211% | $0.1211% |
The small figure is the share of that category billed on space.
What the rule changes cost
Cost per order for each program under each rule set. Nothing about the boxes changes between columns.
| Program | UPS/FedEx before | UPS/FedEx now | USPS before | USPS now |
|---|---|---|---|---|
| Three stock cartons | $15.14 | $16.35 | $13.35 | $16.18 |
| Twelve carton set | $13.36 | $14.23 | $12.24 | $13.65 |
| Twelve cartons plus mailers | $11.55 | $12.01 | $10.96 | $11.35 |
The UPS and FedEx round up raised the cost of the average order in stock cartons by 8.0%, or $1.21 on this table. The USPS change, a lower divisor and the same round up, raised it by 21.2%, or $2.83. Under both, the right sized program with mailers costs less than stock cartons did before either rule moved, which is the argument for treating packaging as a transportation decision.
What the model assumes
- 20,000 single item orders across five synthetic categories, shipped from Kansas City, KS, with the same destinations and residential mix as the zone optimizer.
- Every item goes in the smallest carton it fits with an inch of clearance on each dimension. Cartons add a quarter inch of wall to every outside dimension and weight by board area plus dunnage.
- Only apparel and bedding are allowed in a poly mailer, and bagging compresses their height. Fragile and dense goods stay boxed.
- Rates are the site's synthetic ground table, set at roughly half of list, with synthetic fuel and residential surcharges. Read the percentages as the result and the dollars as illustration. One rate table is used for every rule set, so the columns isolate billing mechanics and say nothing about which carrier is cheaper.
- Additional handling and oversize surcharges are not modeled. They would widen the gap between the stock cartons and everything else.
How it works
Carriers bill the greater of actual weight and dimensional weight, where dimensional weight is the carton volume divided by a published divisor. Past a certain box size the carrier stops charging you for what you shipped and starts charging you for the air around it, and most shippers cross that line without noticing, because the packaging decision is made by a different team than the one holding the transportation budget.
The line is a density. At a divisor of 139, anything lighter than about 12.4 pounds per cubic foot bills on space, and most direct to consumer goods sit well under that. The rules around the divisor matter as much as the divisor itself: when UPS and FedEx started rounding every fractional inch up before the math runs, a box with a quarter inch of wall on each side gained a full inch in every direction, and nothing about the box changed.
The tool above starts with one package, then runs the same synthetic order book through three packaging programs and four rule sets. The product never changes between them, only the container and the billing rule, which is the point. Every dollar of difference in those tables is a packaging decision or a rule change, not a rate.
Data provenance
- Orders
- Synthetic. Generated by seeded formula, not sampled from any real catalog or order file. Five categories with stated share, size range, and weight range, drawn uniformly inside each range and rounded to the quarter inch and tenth of a pound. Destinations and the residential flag reuse the zone optimizer's synthetic order book, shipped from Kansas City, KS so the zone mix sits mid range.
- Dim rules
- Divisors, side rounding, and the one cubic foot threshold as published in UPS, FedEx, and USPS service guides. These are public billing mechanics, and no negotiated term is modeled.
- Rates
- Synthetic. Every figure was generated by formula. rate = 4.00 + 0.31 * (zone - 2)^1.25 + weightLb^0.92 * (0.275 + 0.0425 * (zone - 2)) No carrier rate card, no negotiated agreement, no employer pricing. Replace this file to plug in published list rates with their own provenance.
The billing rules, the packaging programs, and the rate table are separate modules, so a new divisor or a real carton catalog is a data change, not a model change. The code is on GitHub.