Carriers sell room in the aircraft as well as weight capacity, so a big, light box can cost more than its weight suggests. Volumetric weight is how they turn that space into kilograms: the box’s length × width × height in centimeters divided by a fixed number, called the divisor. You’ll also see it called dimensional weight (the usual US term), DIM weight or volume weight. The carrier compares it with the actual weight on the scale and bills the higher of the two, the chargeable weight. A 12 kg box of pillows, for example, can be billed at 50 kg by air and 60 kg by express. This guide shows where those numbers come from and what you can do about them.
How to calculate volumetric weight
The formula is the same everywhere; only the divisor changes.
Volumetric weight (kg) = L cm × W cm × H cm ÷ divisor
| Mode | Divisor (cm³/kg) | 1 m³ counts as |
|---|---|---|
| Air freight (IATA general rule) | 6,000 | About 167 kg (166.67) |
| Express (e.g., UPS) | 5,000 | 200 kg |
| Ocean freight LCL | 1,000 equivalent (weight-or-measurement rule, not a divisor) | 1,000 kg |
Take a box of pillows measuring 100 × 60 × 50 cm and weighing 12 kg. Its volume is 300,000 cm³, or 0.3 m³.
- Air freight: 300,000 ÷ 6,000 = 50 kg, about 4.2 times the actual weight. Run it in the calculator and the chargeable weight shows 50.0 kg.
- Express: 300,000 ÷ 5,000 = 60 kg, five times the actual weight. The express version shows 60.0 kg.
Same box, different bill. A smaller divisor always produces a higher volumetric weight: dividing by 5,000 instead of 6,000 makes every box 1.2 times heavier on paper, a 20% increase. In both cases the volumetric weight beats 12 kg, so the carrier charges on size. If the box had been heavier than its volumetric weight, the scale would have won instead. The chargeable weight guide covers that comparison rule in more detail.
Why airlines charge for space
An aircraft runs up against two limits on every flight: how much weight it can carry and how much room there is in the hold. Dense cargo, like steel, hits the weight limit first. Light, bulky cargo, like feathers, fills the hold long before it gets anywhere near the weight limit. IATA makes the point with a ton of feathers and a ton of steel: same weight, very different amounts of space. Converting volume into kilograms lets the airline charge on whichever limit your shipment uses up first.
Space in the hold isn’t sold by the loose cubic meter, either. Air cargo is loaded onto unit load devices (ULDs), the aircraft pallets and containers that fit the hold, and ULD build-up (loading cargo onto the ULD) works within those fixed shapes. A bulky box takes up ULD space no matter how little it weighs.
For air freight, the divisor of 6,000 cm³/kg is what IATA describes as the general rule, and airline tariffs follow it; one airline tariff states it as 6,000 cubic centimeters to 1 kg. Express carriers set their own figures in their service guides, and 5,000 is the figure UPS publishes in the UK.
The density where volume stops mattering
Every divisor implies a cutoff density: the density at which volumetric and actual weight come out equal.
- Air freight: about 167 kg/m³ (166.67)
- Express: exactly 200 kg/m³
- Ocean freight LCL: 1,000 kg/m³
If your cargo is denser than the cutoff, you pay on actual weight. If it’s less dense, you pay on volumetric weight. Right at the cutoff, the two numbers are the same. Don’t mix up the units: the divisor is in cm³ per kg, while density is in kg per m³.
This is the most useful number to know before you ask for a quote. Work out your product’s density and you can tell right away whether size or weight will drive the bill. The pillow box above is 12 kg in 0.3 m³, or 40 kg/m³, far below either cutoff, which is why its volumetric weight came out four to five times its actual weight. Pillows, foam, textiles, plastic parts, assembled furniture and consumer goods in bulky packaging tend to land on the low side. Metal parts usually land on the high side.
How carriers measure (and remeasure) your boxes
Most surprise charges come from the tape measure, not the formula.
- Measure at the extreme points. Carriers take the greatest length, width and height, including bulges, the pallet and any overhang. If a box bulges 2 cm on each side, the dimensions are taken at the bulge, not at the flat edge.
- Dimension rounding varies by carrier. One airline tariff rounds each dimension to the nearest centimeter, up or down. UPS’s US guidance says to round up to the next whole number in one place and to the nearest whole number in another. Check the rule in your carrier’s own guide instead of assuming.
- Weight rounding varies too. One airline tariff rounds the chargeable weight up to the next 0.5 kg. UPS in the UK rounds up to the next 0.5 kg, and UPS in the US to the next 1 lb. The CargoSum calculator rounds air and express results up to the next 0.5 kg.
- Shipments can be remeasured. A carrier can reweigh and remeasure your shipment, and if its numbers differ from yours, the charge changes and additional, often unexpected, costs can follow. Weigh and measure accurately before you book, declare what you measured, and pack with as little empty space as possible.
For a shipment with several packages, add up the volume of every piece (or the volumetric weight of every piece; with one divisor the total is the same) and compare that total with the total actual weight. The higher figure is what you pay on. The calculator works this way. Some express carriers handle multi-package shipments package by package, and the calculator doesn’t model that.
Palletized freight is measured as the whole loaded pallet, so the pallet’s own height counts. A euro pallet (120 × 80 cm) is 144 mm tall, and CargoSum’s 48 × 40 in standard pallet preset uses a 14.5 cm deck height, so plan on roughly 14–15 cm of extra height before the first box goes on. Overhang is measured at its widest point, which adds to the volume you pay for, and boxes hanging over the edge also lose strength; FedEx puts the loss in compression strength at up to 32%. The boxes-per-pallet guide covers stacking patterns that keep loads inside the pallet footprint.
When the divisor isn’t 6,000 or 5,000
US carriers work in inches and pounds, so they publish a DIM factor in cubic inches per pound instead. UPS in the US uses 139 for Daily Rates and 166 for Retail Rates. 139 in³/lb comes to roughly 5,022 cm³/kg, very close to the metric 5,000. 166 is close to the air freight standard, since 6,000 cm³/kg is about 166.1 in³/lb.
Divisors also differ by carrier, service and country, and carriers can change them. When a factor changes, your volumetric weight changes with it, even if the boxes stay exactly the same. High-volume shippers may have a divisor written into their contract, so check your contract terms rather than assuming the published figure applies.
Before comparing quotes, find out which divisor each one uses, from the carrier’s service guide (for example, FedEx publishes its own) or from your contract. Then rerun the same dimensions with each divisor. The chargeable weight calculator has a Custom option where you can enter any divisor in cm³/kg.
Worked example: vacuum-packing 40 boxes of bedding
This is a made-up scenario. A bedding brand ships 40 boxes by express, each 60 × 40 × 40 cm and 4 kg.
- Before: actual weight 160 kg. Each box is 96,000 cm³, so the shipment totals 3,840,000 cm³, or 3.84 m³. Volumetric weight is 768 kg, 4.8 times the actual weight, at a density of about 41.7 kg/m³. Check it in the calculator: 768.0 kg.
- After vacuum-packing: the bedding goes into vacuum-packed bags (air drawn out to compress it), and box height drops from 40 cm to 24 cm. Each box is now 57,600 cm³, and the shipment totals 2,304,000 cm³, or 2.304 m³. Volumetric weight falls to 460.8 kg, exactly 40% less, because the height ratio is 24/40 = 0.6. The calculator rounds that up to a chargeable weight of 461.0 kg; see the compressed version.
Even compressed, the shipment’s density is only about 69.4 kg/m³, still well under the 200 kg/m³ express cutoff, so it’s still billed on volume. Compression keeps paying off only as long as volumetric weight is higher than actual weight. Once the shipment reaches the cutoff density, the carrier bills actual weight and further squeezing saves nothing. Here the cutoff is 0.8 m³ in total (160 kg ÷ 200 kg/m³), which would mean boxes only about 8.3 cm tall.
Five ways to lower volumetric weight
- Right-size your boxes. Empty space and void fill (the paper or air pillows padding out a box) are billed like product. A box that fits the item cuts volume directly.
- Compress what can be compressed. Textiles and bedding shrink a lot when vacuum-packed. How much depends on the product; the 40% in the example above comes from its assumed box sizes.
- Ship it knocked down. Furniture shipped knocked down (KD), or flat-pack, takes far less space than assembled pieces, and if the shipment gets dense enough, it can switch to being billed on actual weight.
- Rethink the mode. If your cargo is far less dense than 167 kg/m³ and isn’t urgent, look at LCL ocean freight, where a cubic meter counts as 1,000 kg. Whether it works out cheaper per cubic meter depends on the rates for each mode; the sea vs. air freight guide walks through the trade-offs.
- Consolidate small boxes. Packing many small boxes into an outer box (a case that holds several inner packs) can reduce the empty space between them.
The same logic shows up in US less-than-truckload (LTL) freight, where density drives the freight class and therefore the rate. The freight density guide explains how that works.
What is volumetric weight and how is it calculated?
Volumetric weight is a box’s size expressed as a weight. Multiply length × width × height in centimeters and divide by the carrier’s divisor: 6,000 for air freight under IATA’s general rule, or 5,000 for express services such as UPS. The carrier then charges whichever is greater, the volumetric weight or the actual weight.
What’s the difference between volumetric weight and chargeable weight?
Volumetric weight is one of two inputs; chargeable weight is the result. Dimensional weight, DIM weight and volume weight are other names for volumetric weight. Chargeable weight is the figure the carrier actually bills, and it equals the higher of the actual weight and the volumetric weight.
Why is my volumetric weight so much higher than the actual weight?
Because your cargo weighs less per cubic meter than the carrier’s divisor assumes. Air freight treats every cubic meter as about 167 kg and express services as 200 kg, so anything less dense than that is billed on its size. Pillows, foam, textiles and assembled furniture usually fall well below those levels; metal parts usually sit above them.
Why do express carriers use 5,000 instead of 6,000?
The smaller the divisor, the more kilograms the same box converts to. A box divided by 5,000 comes out 1.2 times heavier, or 20% more, than the same box divided by 6,000. That means a lightweight shipment sent by express will usually have a higher chargeable weight than the same shipment sent as regular air freight.
What DIM factor does UPS use in the US?
UPS in the US divides cubic inches by 139 for Daily Rates and by 166 for Retail Rates to get pounds, then rounds up to the next whole pound. 139 in³/lb works out to roughly 5,022 cm³/kg, close to the metric 5,000, and 166 in³/lb is close to the 6,000 cm³/kg air freight divisor. Other carriers set their own factors and can change them, so check each carrier’s current service guide.
Does ocean freight use volumetric weight?
Not with a divisor, but the same idea applies. LCL ocean freight is charged per cubic meter or per 1,000 kg (one metric ton), whichever is greater. That treats 1 cubic meter as 1,000 kg, the equivalent of a divisor of 1,000 cm³/kg, so light cargo converts to far fewer kilograms than it would by air or express.
Why was I charged for more weight than I declared?
Carriers can reweigh and remeasure shipments, and if their figures differ from what you declared, the charge changes and extra costs can follow. They measure each package at its extreme points, including bulges, pallets and overhang, and rounding rules for dimensions and weight vary by carrier. Weighing and measuring yourself before booking is the best protection.