Every mode has a mechanism for the same problem: a truck or plane runs out of space before it runs out of payload. Parcel and air solve it with dimensional weight; LTL solves it with density and freight class. If you ship across modes, you need both dialects.
The DIM formula
Imperial: inches ÷ divisor → pounds. Metric: cm ÷ divisor → kilograms.
Chargeable weight = max(actual weight, dimensional weight).
The divisor is an assumed density. 139 in³/lb says “we treat freight as if it were at least 12.4 lb/ft³ dense”; 6,000 cm³/kg says “at least 166.7 kg/m³.” Ship anything fluffier and you're billed for the missing density.
The divisors you'll actually meet
| Context | Divisor | Implied density | Notes |
|---|---|---|---|
| US parcel (FedEx, UPS, retail rates) | 139 in³/lb | 12.4 lb/ft³ | Applies to nearly all boxes; negotiated accounts sometimes get 166 or DIM waivers |
| USPS (large packages) | 166 in³/lb | 10.4 lb/ft³ | Applied over 1 ft³ on many services |
| International air cargo (IATA convention) | 6,000 cm³/kg ≈ 166 in³/lb | 166.7 kg/m³ | The standard “volumetric weight”; express products sometimes 5,000 cm³/kg |
| LTL freight | — (uses class) | — | Density → freight class; plus cubic-capacity & linear-foot rules per tariff |
Worked example, both dialects
One pallet, 48 × 40 × 48 in, 260 lb of foam products (light and bulky — the worst case):
- Parcel logic: 92,160 in³ ÷ 139 = 663 lb dimensional weight. Chargeable weight: 663 lb, not 260. (This is why palletized foam doesn't ship parcel.)
- LTL logic: 53.3 ft³; density 260 ÷ 53.3 = 4.9 lb/ft³ → class 175 on the 2025 scale. The high class plays the same economic role as the DIM penalty.
- Air logic: 1.51 m³ × 166.7 = 252 kg volumetric vs 118 kg actual → charged on 252 kg.
Cutting the DIM bill
DIM charges are packaging charges in disguise. Right-size cartons (an inch of air on each face of a 14-inch cube adds ~23% cube), switch pillowy void fill for paper, and for parcel shipments over ~2 ft³, price LTL against parcel — the crossover where class beats divisor comes sooner than most shippers expect. When your density lands near a class boundary, the same packaging discipline pays twice.