Ocean freight runs out of space long before it runs out of weight, and international moving shipments are almost all bulky, light cargo. Bill by weight and a container packed with quilts and mattresses collects freight on a few tonnes while consuming the space of an entire container. The carrier never recovers the slot cost, and two outcomes follow: refuse the cargo, or hide the space cost inside a unit rate so you pay more. Volume-based billing puts that space cost on the table.

1. The physical starting point of billing: every mode of transport has two ceilings

Every cross-border shipment runs into two limits at once: how much space it can hold (volume) and how much weight it can carry (payload). Whichever fills first stops the shipment. That forces billing to measure both space and weight, and different modes of transport hit a different ceiling first.

1.1 The dual limits on containers and aircraft holds

A 20ft container (20GP) holds roughly 25 to 26 cubic metres; a 40ft high cube (40HQ) holds roughly 55 to 60. Payload capacity sits in the twenty-something tonne range. An aircraft hold runs the other way: weight is the hard limit, volume comparatively generous.

1.2 Ocean fills volume first, air fills weight first

This table explains a commonly misunderstood pattern. For the same customer shipping the same goods, ocean quotes come per cubic metre and air quotes come per box or per kilogram. The two companies are not playing by different rules. The two modes hit different bottlenecks, and the billing unit follows the bottleneck.

2. The density profile of moving goods: why weight-based pricing fails at sea

Understanding the two ceilings is half the picture. Look at the goods themselves. Personal moving shipments share one trait: large volume, light weight.

2.1 The industry's standard density: 200 kg per cubic metre

Seapoe's billing rules include one line: ocean export shipments cap at 200 kg per cubic metre, and going over may trigger an overweight surcharge. That number is more than a ceiling. It is the industry's accepted density for moving goods, where one cubic metre of luggage and furniture comes to roughly 200 kg. This baseline is what makes space and weight comparable.

2.2 The maths on one container: space full, payload a third used

The conclusion is direct. Under weight-based pricing the carrier collects freight on 5 tonnes of cargo and burns an entire container of space. That space cost has nowhere to go.

2.3 How far apart densities run

The point of this table: moving goods span a wide density range. A piano sits at or above standard density; a mattress or a PC tower is a fraction of it. Weight alone cannot show how differently those two items use capacity, which is exactly why volumetric weight exists.

3. Volumetric weight: the industry standard for converting space into weight

If the cost lands on space, someone has to convert space into a billable unit. The industry uses volumetric weight.

3.1 The formula and the greater-of rule

Volumetric weight (kg) = length (cm) × width (cm) × height (cm) ÷ 5000. Compare that figure with the scale weight and bill on whichever is greater. That figure is the chargeable weight.

One detail ties the whole logic together. A cubic metre is one million cubic centimetres, and dividing by 5000 gives exactly 200 kg. The ÷5000 formula is the same statement as "treat one cubic metre as 200 kg", the same source as the 200 kg per cubic metre cap. One is the ruler for light cargo, the other the ceiling for heavy cargo, and both point at the same standard density.

3.2 Two conversion factors coexist

The same box can produce different chargeable weights on different channels. When in doubt, ask which divisor the line uses.

3.3 Packing adds roughly 20% to volume

Once goods are packed into cartons, volume typically runs about 20% above the unpacked figure. That is the ordinary reason a pre-move estimate of 5 cubic metres measures 6 on the day, not a measurement error. [Source: same as above]

4. The counter-test: what the invoice would look like under weight-based pricing

The fastest way to test whether a billing rule holds up is to imagine life without it.

4.1 Two methods on the same shipment

4.2 The carrier's three options, and who ends up paying

If the industry billed purely by weight, a carrier facing a container full of light cargo would have three options: refuse light cargo, raise prices, or keep losing money.

The first two land on the moving customer, because personal moving shipments are almost all light cargo. Refusal means furniture, mattresses and bedding cannot ship at all. A price rise means paying an unexplained premium for the space you occupy. Volume-based billing does the opposite: it puts that space cost on the table and settles it, rather than hiding it in a unit rate.

4.3 A comparison: why consolidation companies can quote per kilogram

Commercial consolidators ship through commodity channels, where items are small, few and dense, so a per-kilogram rate covers cost. But they declare goods as merchandise rather than clearing them as personal effects. That requires material certificates and purchase receipts for every item, and anything with a complicated description becomes unworkable.

Personal moving shipments are furniture, mattresses and pianos, large and light, clearing through the personal effects channel. Only volume-based billing covers the cost, and only a personal effects declaration gets them out.

5. Why different service lines use different rulers

5.1 How billing units split across four service lines

5.2 Three boundaries people mix up

First, per-box pricing on return express is not really weight-based. It packs the space-versus-weight trade-off into a single box price, so what you put in the box decides whether it pays off.

Second, the ocean rules of charging a full cubic metre under 1 and setting a 2-3 cubic metre minimum both amount to a minimum shipment floor. An LCL container only sails once it is full, so anything below the minimum bills at the minimum.

Third, some return ocean routes waive the minimum. Europe to China, for example, offers a waiver from 3-4 cubic metres. Reaching a volume tier can cost less in total than shipping less; on return ocean freight, less volume does not always mean a smaller bill.

6. FAQ: the six questions customers ask most about billing

Q1: I weighed it at home and got 20 kg. Why do you calculate 26?

Billing takes the greater of volumetric weight and scale weight. Fill a carton with soft items and it bulges, the outer dimensions grow, and volumetric weight passes scale weight. This is not a markup. It is a different ruler, and that ruler is the same standard as the 200 kg per cubic metre cap. To check it yourself, measure the carton's length, width and height in centimetres, multiply them, divide by 5000, and you have the volumetric weight.

Q2: Why not bill by actual weight? My things barely weigh anything.

Because you occupy slot space. Your light cargo fills the capacity, other cargo cannot take it, and the carrier loses that revenue. International ocean freight has long billed on W/M (Weight or Measurement, whichever is greater). That is an industry convention, not a rule one company invented. The parts worth checking are not the greater-of rule but three things: whether measurements come with photos, which divisor the line uses, and whether surcharge triggers appear in the quote up front.

Q3: Does billing by volume put me at a disadvantage?

Not necessarily, and it depends on your density. A piano sits near or above 200 kg per cubic metre, so if the volumetric figure falls below the scale weight you pay on scale weight and never pay for space you did not use. Volume tiers on ocean freight also lower the unit rate for customers who reach them. Adding small items to a shipment costs nothing when the volume has not changed, and topping up to the minimum can work out cheaper than shipping less.

Q4: The warehouse measured larger dimensions than I did. What now?

The proper practice is for the warehouse to supply measurement photos and the calculation for the customer to check, and for the company to absorb any price difference caused by a measurement error. [Source: Seapoe Business White Paper, Section 10.16, Handling volumetric weight measurement disputes] Ask about this before signing. One question about measurement photos and calculations screens out a share of sloppy quotes.

Q5: I hear some channels quote by weight. Is that cheaper?

It depends on what you ship. For dense, compact items such as a few boxes of books or small appliances, a per-kilogram channel can cost less. But those channels declare goods as merchandise and clear them through trade procedures, which rarely works for a household of furniture and mattresses. For whole-home furniture, mattresses and pianos, a low weight-based quote seldom survives the measurement. You end up billed for the space you actually occupy, or forced onto a worse clearance route.

7. Turning the principle into savings: three actions

First, compress the volume. Use vacuum bags for soft items and vacuum the bedding, leaving no gaps in the carton. Dismantle furniture where you can, pack bed frames and table legs separately, and fill gaps with clothing and towels. What you cut is billable volume.

Second, watch the per-item limits. Keep each item's weight and dimensions inside the lower tier so one oversized piece does not push you into the next price band.

Third, send the inventory before the warehouse. An itemised estimate from your coordinator lands closer to the final bill than a measurement taken after packing. Before signing, confirm that the quote lists what is and is not included, and that measurements come with photos and calculations.

8. Seapoe Relo's commitment to transparent billing

If you are unsure how many cubic metres your goods come to, send room photos or an inventory to Seapoe Relo's customer service. We estimate item by item, then supply photos and the calculation after measuring, so every cubic metre is accounted for.