Technical Resource

What Is a Conex Box? Conex Containers Explained

In the United States, "conex", "conex box" and "conex container" are everyday names for a steel shipping container. On a farm, a construction site or a military base, a conex is the same 20ft or 40ft box that carries cargo across the ocean. This guide explains where the name comes from, which sizes exist, how conex boxes are graded and what to check before buying one.

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The name

Where the word conex comes from

CONEX stands for Container Express, a steel box system introduced by the US Army in the early 1950s to move military supplies without unpacking them at every transfer. The original CONEX was much smaller than today's containers: about 8.5 ft long, 6 ft 3 in wide and 6 ft 10.5 in high, with double doors at one end.

The ISO containers standardised in the late 1960s replaced the military boxes, but the name stayed in American usage. Today a seller, a contractor or a farmer who asks for a conex almost always means an ISO shipping container, most often a 20ft or a 40ft high cube.

Conex or container

Is a conex box the same as a shipping container?

Yes. A modern conex box is an ISO intermodal container: built to ISO 668 dimensions, with corner castings at the eight corners for lifting and stacking, a corrugated steel shell, a hardwood or bamboo floor and lockable cargo doors. It carries an ISO 6346 unit number (four letters, the last one U, then seven digits) and a CSC safety approval plate.

Other names point to the same box: sea can (common in Canada), Connex (a frequent misspelling), steel storage container, ISO container or simply box. What differs between two conex boxes is the size, the type and the condition, not the name.

Sizes

Conex box sizes and dimensions

Three sizes cover most of what is sold in the US. All are 8 ft wide; the high cube is one foot taller. Interior figures vary by a few centimetres between manufacturers.

20ft standard (20DC): outside 20 ft x 8 ft x 8 ft 6 in; inside about 19 ft 4 in x 7 ft 8 in x 7 ft 10 in; about 1,170 cu ft; empty weight about 4,850 lb (2,200 kg).
40ft standard (40DC): outside 40 ft x 8 ft x 8 ft 6 in; inside about 39 ft 5 in x 7 ft 8 in x 7 ft 10 in; about 2,390 cu ft; empty weight about 8,330 lb (3,780 kg).
40ft high cube (40HC): outside 40 ft x 8 ft x 9 ft 6 in; inside about 39 ft 5 in x 7 ft 8 in x 8 ft 10 in; about 2,690 cu ft; empty weight about 8,690 lb (3,940 kg).
Less common: the 20ft high cube (20HC), and 10ft units, which are usually cut down from a 20ft or built for site storage rather than for ocean transport.

Types

Dry, high cube, refrigerated and open types

Most conex boxes are dry vans: closed steel boxes with doors at one end, used for general cargo and storage. Specialised types exist for other jobs: refrigerated containers (reefers) with a built-in cooling unit, open tops loaded from above under a removable tarp, flat racks with no side walls for oversized loads, and side-door units for loading along the length.

For storage, a workshop or a site office, the dry 20ft and the 40ft high cube are the usual choices. The high cube's extra foot of height matters once shelving, lighting or insulation go in.

Condition

Conex grades: new, cargo worthy, wind and watertight

A conex box is sold by grade, which describes its condition. The grade decides what the box can be used for more than its age does.

New or one trip: built recently and shipped once with cargo. Uniform paint, straight panels, clean floor. The choice for a long life, a visible location or a conversion.
Cargo worthy (CW): used, and still fit to carry cargo by sea. Dents and rust within limits, sound structure, doors that seal.
Wind and watertight (WWT): used and no longer meant for ocean transport, but it keeps wind and rain out. The usual grade for storage on site.
As-is: sold without a watertight guarantee, for parts, a project that will rebuild the box, or a buyer who has inspected it.

Before buying

What to check on a used conex box

Photos and a grade name are not enough on a used unit. These checks take a few minutes on site, or can be asked of the seller as specific photos.

The unit number on the doors and on the CSC plate match, and the number passes the ISO 6346 check digit.
Doors open and close by hand, the locking bars turn, and the rubber gaskets are whole.
Stand inside with the doors closed on a bright day: any point of light in the roof or walls is a hole.
The roof has no standing-water dents, and the floor is firm, with no soft or delaminated boards.
The bottom rails and corner castings are straight, with surface rust only.
The seller can be verified: a registered company, a depot address and a written offer naming the unit number.

Delivery

Getting a conex box on site

A conex box usually arrives on a tilt-bed truck that slides it to the ground, or on a truck with a crane when the site is tight or the box goes on a foundation. The site needs firm, level ground and straight access for the truck; a 40ft unit needs about 100 ft of room behind the truck.

Set the box on gravel or concrete pads rather than bare soil, level it so the doors keep closing, and leave an air gap underneath to slow corrosion. MG-Atlantic supplies the container from a partner depot; delivery is quoted for the site's address and access.

Frequently Asked Questions

Conex boxes, answered

CONEX stands for Container Express, the steel box system the US Army introduced in the early 1950s. The name stayed in American usage and now means any steel shipping container.

Yes. Today a conex box is an ISO shipping container: same dimensions, corner castings, unit number and CSC plate. Sea can, Connex and ISO container are other names for the same box.

Mainly 20ft and 40ft, both 8 ft wide. Standard units are 8 ft 6 in high and high cubes 9 ft 6 in. The 40ft high cube has about 2,690 cu ft inside, the 20ft standard about 1,170 cu ft.

Empty, a 20ft standard weighs about 4,850 lb (2,200 kg), a 40ft standard about 8,330 lb (3,780 kg) and a 40ft high cube about 8,690 lb (3,940 kg). The exact tare is printed on the doors.

Yes, if it is wind and watertight or better. Check the doors, the gaskets and the roof, place it level on a firm base, and control condensation with vents or desiccant.

MG-Atlantic trades and leases 20ft, 40ft and 40ft high cube containers from partner depots in US port and rail cities. Request a quote with the size, the grade and the delivery address.

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