The short answer
A standard 20 ft container is 6.058 by 2.438 by 2.591 m (19 ft 10.5 in by 8 ft by 8 ft 6 in). A 40 ft container is 12.192 by 2.438 by 2.591 m. High cube versions of both add 305 mm (1 ft) of height, reaching 2.896 m.
Internal dimensions are smaller by the thickness of the corrugated steel walls and the floor structure: about 5.898 m of usable length in a 20 ft and 12.032 m in a 40 ft. Those internal figures are the ones that decide whether a load fits, and they are the ones most quoted charts get slightly wrong.
The full ISO 668 specification
Container dimensions are set by ISO 668, which defines external dimensions as fixed and internal dimensions as minimums, so slight variation between manufacturers is normal.
| Spec | 20 ft standard | 40 ft standard | 40 ft high cube | 45 ft high cube |
|---|---|---|---|---|
| External length | 6.058 m (19 ft 10.5 in) | 12.192 m (40 ft) | 12.192 m (40 ft) | 13.716 m (45 ft) |
| External width | 2.438 m (8 ft) | 2.438 m (8 ft) | 2.438 m (8 ft) | 2.438 m (8 ft) |
| External height | 2.591 m (8 ft 6 in) | 2.591 m (8 ft 6 in) | 2.896 m (9 ft 6 in) | 2.896 m (9 ft 6 in) |
| Internal length | 5.898 m (19 ft 4 in) | 12.032 m (39 ft 5.7 in) | 12.032 m | 13.556 m |
| Internal width | 2.352 m (7 ft 8.6 in) | 2.352 m | 2.352 m | 2.352 m |
| Internal height | 2.393 m (7 ft 10.2 in) | 2.393 m | 2.698 m (8 ft 10.2 in) | 2.698 m |
| Door opening width | 2.343 m (7 ft 8.2 in) | 2.343 m | 2.343 m | 2.343 m |
| Door opening height | 2.280 m (7 ft 5.8 in) | 2.280 m | 2.585 m (8 ft 5.8 in) | 2.585 m |
| Internal volume | 33.2 m³ (1,172 cu ft) | 67.7 m³ (2,390 cu ft) | 76.3 m³ (2,694 cu ft) | 86.1 m³ (3,040 cu ft) |
| Tare weight | 2,200 kg (4,850 lb) | 3,800 kg (8,380 lb) | 3,900 kg (8,600 lb) | 4,800 kg (10,580 lb) |
| Max gross weight | 30,480 kg (67,200 lb) | 30,480 kg (67,200 lb) | 30,480 kg | 32,500 kg |
| Max payload | 28,280 kg (62,350 lb) | 26,680 kg (58,820 lb) | 26,580 kg | 27,700 kg |
The most important line in that table is the door opening height. A 40 ft standard container has 2.393 m of internal height but only 2.280 m of door clearance, because the door header cuts into the opening. Machinery that fits inside will not necessarily go through the door, and this catches out first-time shippers of tall equipment more than any other dimension.
Note also that a 20 ft container carries more payload than a 40 ft, despite being half the size. Both share the same 30,480 kg gross limit, and the 40 ft weighs 1,600 kg more empty. For dense cargo like tile, stone, or metal, two 20 ft containers move more mass than one 40 ft. For bulky light cargo, the 40 ft wins on volume.
Why a 20 ft container is not 20 feet long
The external length of a 20 ft container is 6.058 m, which is 19 ft 10.5 in. It is 38 mm (1.5 in) short at each end.
This is deliberate. Two 20 ft containers have to occupy exactly the same slot as one 40 ft container, on a ship, on a chassis, and in a stack. The arithmetic:
The 76 mm gap between two adjacent 20 ft boxes is what makes the whole intermodal system interchangeable. It gives the crane spreader room to land, and it gives the twistlocks room to engage the corner castings without the boxes fouling each other.
Those corner castings, specified in ISO 1161, are the real interface standard. Every container has eight of them, and every crane, chassis, lashing rod, and stacking cone in the world engages the same aperture geometry. The box is a distance-holding frame between eight standardized fittings, which is why the external dimensions can never be changed and why internal dimensions vary slightly by manufacturer.
For reference against values you can convert directly: a 40 ft container at 12.192 m is close to 12.26 m, and a 20 ft container at 6.058 m sits just below 6.4 m. Neither is exact, and for anything load-critical the meters-to-feet calculator is the right tool.
Fitting cargo: pallets and road limits
Two practical constraints govern what actually goes in a container.
Pallet count. The internal width of 2.352 m is the binding dimension, and it works out unevenly against both major pallet standards:
| Pallet type | Footprint | Per 20 ft | Per 40 ft |
|---|---|---|---|
| Euro pallet (EUR 1) | 1.2 by 0.8 m | 11 | 25 |
| Standard / GMA pallet | 1.219 by 1.016 m (48 by 40 in) | 10 | 21 |
| Euro pallet, double stacked | 1.2 by 0.8 m | 22 | 50 |
Eleven Euro pallets in a 20 ft is the number European freight forwarders quote from memory. The awkward part is that 2.352 m of internal width fits two 1.2 m pallets side by side with only 48 mm of slack, so loading has to be square or the last row will not go in.
Road height. In the US, the standard legal vehicle height is 13 ft 6 in (4.11 m) in most states. A 40 ft high cube at 2.896 m on a standard chassis with a deck height of about 1.27 m totals roughly 4.17 m, which exceeds the limit. Carriers solve this with a low-profile chassis that drops the deck to about 1.14 m. In Europe the standard limit is 4.0 m and high cubes require careful routing.
Rail double-stacking works only where clearances allow. US double-stack corridors are cleared to 6.15 m (20 ft 2 in), which accepts two high cubes. Most European and UK rail networks cannot double-stack containers at all, which is a structural reason European freight relies more heavily on road.
The specialized types
Dry boxes are the majority of the global fleet, but four other types share the same corner castings and the same external envelope.
| Type | External size | Internal height | Notes |
|---|---|---|---|
| Reefer (refrigerated) | 20 or 40 ft | 2.28 m (20 ft), 2.55 m (40 ft HC) | Insulation and the refrigeration unit cost 200 to 300 mm of internal length and height |
| Open top | 20 or 40 ft | 2.35 m to the rail | Removable tarpaulin roof, loaded by crane from above |
| Flat rack | 20 or 40 ft | No roof or sides | Collapsible ends, for over-width and over-height cargo |
| Tank | 20 ft frame | 21,000 to 26,000 L | An ISO frame around a cylindrical vessel |
Reefers are the type that most often surprises shippers. A 40 ft high cube reefer has the same external footprint as a dry 40 ft HC but only about 2.55 m of internal height and 11.58 m of internal length, because the evaporator unit occupies one end. Volume drops from 76.3 m³ to roughly 67 m³, a 12 percent loss inside an identical box.
Flat racks and open tops exist because the door opening is the binding constraint on a dry box. Anything taller than 2.28 m or wider than 2.34 m cannot pass through the doors regardless of how much internal space is available, so it has to be loaded vertically or carried on an open frame with out-of-gauge charges attached.
Containers as buildings
A 20 ft container gives 160 sq ft (14.9 m²) of floor area and a 40 ft gives 320 sq ft (29.7 m²), measured externally. Internally the usable figures are closer to 139 sq ft (12.9 m²) and 283 sq ft (26.3 m²).
The height is where container architecture lives or dies. A standard container’s internal height of 2.393 m sounds workable, but a habitable conversion needs floor buildup, insulation, and a ceiling. Spray foam at 50 mm on the ceiling, a 40 mm floor assembly, and you are at 2.30 m of finished ceiling, below the 2.44 m most people are used to and close to code minimums.
This is why nearly all container homes use high cubes. Starting at 2.698 m internal, the same buildup lands at about 2.60 m of finished ceiling, comfortably above the standard ceiling heights found in US housing. The extra 305 mm costs perhaps 10 percent more per box and is the difference between a room and a corridor.
Doors are the other conversion challenge. The cargo door opening at 2.280 m is taller than a standard residential door at 2.03 m, so cutting a conventional entry into the side wall means adding structural reinforcement around the opening. See standard door heights around the world for the target dimensions, and converting metric blueprints for handling a container drawing that mixes ISO millimeters with US framing inches, which nearly all of them do.
Sources and further reading: