Dimensional Weight Explained
What dimensional weight is, why carriers bill on the greater of actual and volumetric weight, and how the divisor decides your chargeable weight.
Dimensional weight (often shortened to DIM weight, or called volumetric weight outside the US) is a billing figure that converts the space a parcel occupies into a stand-in weight. Carriers use it because a trailer or aircraft fills up by volume long before it hits its weight limit. A box of pillows weighs almost nothing but still takes a slot that a dense box could have used, so carriers charge on whichever is larger: the real weight on the scale, or the volume expressed as weight.
How dimensional weight is calculated
The formula is the same everywhere; only the divisor changes:
Dimensional weight = (L x W x H) / divisor
Measure length, width and height in the unit that matches the divisor, multiply them for the cubic size, then divide. An imperial divisor is in cubic inches per pound (in3/lb); a metric divisor is in cubic centimetres per kilogram (cm3/kg). A smaller divisor produces a larger dimensional weight, which is why courier divisors (139) bill more aggressively than the generic 166.
The number you are actually billed on is the chargeable weight:
Chargeable weight = max(actual weight, dimensional weight)
Work the dimensional side first, then compare it with what the parcel really weighs and take the higher value.
A worked example
Take a carton measuring 20 x 15 x 10 in, the default on our dimensional weight calculator.
- Cubic size: 20 x 15 x 10 = 3,000 in3
- Generic divisor (166): 3,000 / 166 = 18.07 lb
- Courier divisor (139, e.g. FedEx/UPS domestic): 3,000 / 139 = 21.58 lb
If that carton actually weighs 12 lb on the scale, its chargeable weight on a 166 divisor is max(12, 18.07) = 18.07 lb, and on a 139 divisor it jumps to 21.58 lb. The lighter the contents relative to the box, the more the dimensional figure dominates.
Divisors by carrier
Divisors are stable conventions, not prices, so they make a safe reference. Pick the one your service actually uses.
| Carrier / mode | Imperial (in3/lb) | Metric (cm3/kg) |
|---|---|---|
| Generic / standard | 166 | 5000 |
| FedEx (US domestic) | 139 | 5000 (intl) |
| UPS (daily rate, domestic) | 139 | 5000 (intl) |
| USPS (parcels over 1 ft3) | 166 | - |
| DHL Express | ~139 | 5000 |
| Air freight (IATA) | ~166 | 6000 |
The metric and imperial divisors are independent: 5000 cm3/kg is the common courier standard worldwide, while air freight uses the looser IATA 6000 cm3/kg. To see how the same box behaves under several divisors at once, use the chargeable weight calculator, and if your freight moves by the pallet rather than the parcel, density rather than DIM weight governs the rate, see how freight density affects LTL class.
A practical takeaway: the way to lower dimensional weight is not to lighten the contents but to shrink the box. Cutting void fill and right-sizing the carton reduces cubic size directly, and since dimensional weight scales with volume, every inch removed from a dimension counts. The same volume math underpins what CBM is, measured in cubic metres rather than as a billed weight.