Three numbers decide whether your cargo fits a container: internal dimensions, usable volume and payload. Only the first is fixed. Usable volume is always less than the nominal figure, and the weight you can actually load is usually set by road law rather than by the container.
Figures below are typical for standard ISO containers. Individual boxes vary by manufacturer, age and shipping line, and every container carries its real tare and maximum gross weight stencilled on the door. Confirm with your carrier before planning to the last centimetre.
Dry containers
| Type | Internal L x W x H (m) | Nominal volume | Plan to | Typical max payload |
|---|---|---|---|---|
| 20ft standard (20GP) | 5.90 x 2.35 x 2.39 | ~33.2 CBM | ~28-30 CBM | ~28,000 kg |
| 40ft standard (40GP) | 12.03 x 2.35 x 2.39 | ~67.7 CBM | ~58-60 CBM | ~26,700 kg |
| 40ft high cube (40HC) | 12.03 x 2.35 x 2.70 | ~76.4 CBM | ~66-68 CBM | ~26,500 kg |
| 45ft high cube (45HC) | 13.56 x 2.35 x 2.70 | ~86 CBM | ~74-76 CBM | ~27,700 kg |
Why "plan to" is lower than nominal
Nominal volume assumes a perfect solid block of cargo to the ceiling. Real loads lose space to carton dimensions that do not divide evenly into the container, pallet footprints and the height pallets add, dunnage and bracing, and the gap left at the door. Planning at 85-90% of nominal is realistic; planning at 100% produces a booking that does not close.
The container load estimator and cartons per container calculator work from your actual carton spec rather than a rule of thumb.
Special equipment
| Type | Typical internal | Use it when |
|---|---|---|
| 20ft reefer | ~5.45 x 2.29 x 2.27 m, ~28 CBM | Temperature control; insulation costs you roughly 15% of the volume |
| 40ft high cube reefer | ~11.58 x 2.29 x 2.50 m, ~67 CBM | The default for perishables and pharma |
| Open top | 20ft or 40ft, tarpaulin roof | Cargo craned in from above, or over-height by a small margin |
| Flat rack | 20ft or 40ft, collapsible or fixed ends | Out-of-gauge width or height; machinery, vehicles, project cargo |
| Tank container | ~21,000-26,000 litres | Bulk liquids, food grade or hazardous |
Open top and flat rack cargo that exceeds the container envelope becomes out-of-gauge and is priced entirely differently — expect surcharges for the slots it blocks, and a routing check before booking.
The payload trap
A 20ft container can legally carry roughly 28 tonnes, but you will rarely be allowed to. The binding limit is usually the road weight limit at each end, which varies by country, state and axle configuration and is frequently in the 18-22 tonne range for the container plus chassis plus tractor. Heavy, dense cargo therefore fills a 20ft on weight long before volume — which is why dense goods ship in 20ft boxes and light goods in 40ft high cubes.
Two consequences worth planning around:
- Two 20ft containers often beat one 40ft for dense cargo, despite costing more in freight, because one 40ft may be undeliverable by road at full weight.
- VGM is mandatory. Under SOLAS you must declare a verified gross mass before loading, and an incorrect declaration stops the container. Use the VGM calculator and confirm the method your terminal accepts.
Choosing between them
- Calculate real volume with the CBM calculator, then add pallet height if palletised.
- Check weight against the road limit at both ends, not the container plate.
- Under ~15 CBM, compare LCL — see FCL vs LCL shipping and the cost comparison tool.
- Between 28 and 33 CBM, price both a full 20ft and LCL; the crossover moves with the market.
- Confirm equipment availability on your lane. High cubes and reefers are not equally available everywhere, and a cheaper rate on unavailable equipment is not a rate.
Post your cargo dimensions and let forwarders quote the equipment, or find forwarders on your lane.
