Quick answer: DIM weight = length × width × height ÷ divisor. In inches and pounds most US couriers use 139; in centimetres and kilograms, air freight uses 6000 and international courier often 5000. You are billed on the greater of DIM and actual weight.
Divisors by carrier and service
| Service | Units | Divisor | Notes |
|---|---|---|---|
| UPS / FedEx US domestic | in → lb | 139 | Standard published rate |
| Some retail / lighter services | in → lb | 166 | More generous — lower billed weight |
| International courier | cm → kg | 5000 | Common express default |
| IATA air freight | cm → kg | 6000 | Standard air cargo |
| Air freight (alternative) | cm → kg | 5000 | Some forwarders and lanes |
| Road / LTL freight | in → lb | Varies | Often density-based classing instead |
The divisor is not a law of physics — it is a commercial term. A lower divisor produces a higher billed weight, so it works in the carrier's favour. Negotiated contracts often specify a better divisor than the published rate, and it is one of the more valuable things to ask for if you ship regularly.
Worked example
A 20 × 16 × 12 inch box holding 15 lb of goods, shipped US domestic at divisor 139:
- Volume: 20 × 16 × 12 = 3,840 in³
- DIM weight: 3,840 ÷ 139 = 27.6 lb → rounds up to 28 lb
- Actual weight: 15 lb
- Chargeable: the greater — 28 lb, nearly double the real weight
The break-even density is worth knowing: at divisor 139, anything lighter than about 10.4 lb per cubic foot gets billed on volume rather than mass. Books, tools and liquids sit above that line; clothing, foam, lampshades and most e-commerce packaging sit well below it.
Reducing what you get billed
- Right-size the box. Dropping a 20 × 16 × 12 carton to 18 × 14 × 10 cuts DIM weight from 28 lb to 19 lb — a third off, for the same goods.
- Mind the rounding. Most carriers round each dimension and the final weight up. A box measuring 12.1 inches is billed as 13.
- Compress soft goods. Vacuum bags on textiles and bedding attack the exact problem DIM pricing punishes.
- Test splitting the shipment. Two smaller cartons sometimes total less chargeable weight than one large one, particularly when the large one crosses an oversize threshold.
- Measure the packed carton. Bulging sides count. Carriers measure at the widest point, including any distortion from over-packing.
Frequently asked questions
What is volumetric weight?
Volumetric weight, also called dimensional or DIM weight, is a pricing figure derived from a parcel's size rather than its mass. Carriers charge on whichever is greater — actual weight or volumetric weight — because a light but bulky parcel occupies space that could have carried heavier freight.
How do I calculate dimensional weight?
Multiply length × width × height and divide by the carrier's divisor. In inches with a divisor of 139, a 20 × 16 × 12 in box gives 3,840 ÷ 139 ≈ 27.6 lb, rounded up to 28 lb. In centimetres, air freight commonly uses 6000: a 50 × 40 × 30 cm carton gives 60,000 ÷ 6000 = 10 kg.
What divisor does my carrier use?
Common values are 139 for UPS and FedEx domestic in inches and pounds, 166 for some retail and lighter-density services, 5000 for international courier in centimetres and kilograms, and 6000 for standard IATA air freight. Divisors change with contracts and service level, so check your rate card — a lower divisor means a higher billed weight.
Why do carriers charge by volume?
Because aircraft and trucks run out of space long before they run out of weight capacity for light cargo. A trailer full of pillows weighs a fraction of its limit but is completely full, so pricing purely by mass would leave carriers unable to cover the cost of that journey.
How can I reduce dimensional weight charges?
Use the smallest box that safely fits the goods, since a single spare inch on each side can push you over a billing threshold. Avoid oversized voids filled with air, consider vacuum packing soft goods, and check whether splitting one large carton into two smaller ones lowers the total chargeable weight — sometimes it does.