Conex Box: Sizes, Prices & History

The story starts before the name. In 1948 the US Army Transportation Corps fielded the Transporter: a rigid corrugated-steel box 8 ft 6 in long, 6 ft 3 in wide and 6 ft 10 in high, rated to carry 9,000 lb (about 4,080 kg). It proved a simple thesis — cargo sealed in a reusable steel box at origin arrives faster and with less theft and damage than cargo handled loose at every transfer.

In late 1952, mid-Korean War, the Army scaled the idea into the Container Express program — CONEX. The first shipment ran engineer repair parts from the Columbus General Depot in Ohio to Korea, and the results made the case in one voyage: transit time roughly halved, pilferage and weather damage collapsed, because the box travelled sealed from depot to front line. CONEX containers had a door at one end, forklift pockets underneath, and stacked three high — every idea the commercial container would later standardise.

By Vietnam the program had grown into the largest containerised logistics operation yet attempted: roughly 200,000 CONEX containers were in service moving US military cargo by the late 1960s. Thousands never came home — left in-country as storage sheds, bunkers and field offices, the first mass demonstration that a retired container is useful real estate.

Commercial shipping ran on a parallel track: trucking entrepreneur Malcom McLean sailed 58 containers from Newark to Houston on the Ideal X in 1956, and ISO standardisation between 1968 and 1970 settled the world fleet on 8 ft wide, 20 and 40 ft long boxes with common corner castings (the full standard is decoded in our ISO guide). The military’s small boxes were retired — but American English kept the name. “Conex” transferred to the ISO container and never left.

No. A conex box, a shipping container, an ISO container and a sea can are the same physical object — an ISO 668 steel box with eight corner castings in eight standardised positions. The names map to regions and industries, not to different products:

Which name to use when: booking freight, say “20 GP” or “40 HC” — carriers think in size plus type codes. Shopping for storage in the US, search “conex box” and “shipping container” both; inventory hides under each. In Canada, “sea can”. In contracts and standards, “intermodal container” or the ISO 6346 code itself.

Conex boxes come in the standard ISO lengths — 10, 20, 40 and 45 ft — in two heights: standard (8′6″ / 2.591 m) and high cube (9′6″ / 2.896 m). Every one is exactly 2.438 m (8 ft) wide. The 20 ft and 40 ft dominate the market; the table gives the figures that matter when you plan a purchase:

Tare varies a few hundred kilograms by manufacturer and build year — the CSC plate riveted to the door of a specific unit is always authoritative. Full internal dimensions, feet-and-inches conversions and pallet counts are on the 20 ft and 40 ft pages and the master dimensions chart.

Beyond the ISO set, North America runs domestic 48 ft and 53 ft units (wider at 2.591 m, rail and road only), and the military still builds fractional boxes — the Tricon at one-third and Quadcon at one-quarter of a 20 ft footprint, which pin together into a single 20 ft slot.

Grade moves the price as much as size — the same physical 20 ft box spans a 2× range between wind-and-water-tight and one-trip condition. Observed US market ranges, 2025–26:

Renting instead of buying runs $75–150 per month for a 20 ft and roughly $100–250 for a 40 ft, which puts the buy-vs-rent breakeven at about 18–30 months of use. Delivery is always extra: typically $150–500 within ~50 miles of the depot, $2–4 per mile beyond that — and since conexes pile up at import ports, the same used 20 ft that costs $1,300 in Houston can cost $2,400 in Denver. The full market picture, including what drives the ranges, is in the container prices guide.

Construction site storage is the classic use and the reason the word survives in contractor vocabulary: tools, fasteners and materials locked in 2 mm corrugated steel overnight, moved to the next site by tilt-bed when the job ends. A lockbox — the welded shroud that shields the padlock from bolt cutters — is the standard upgrade and often an insurance condition.

On-property storage is the volume market: a 20 ft conex puts 33 m³ of dry, rodent-proof space in a yard for less than two years of self-storage-unit rent, with no loading dock queue. Sizing guidance is in the storage container sizes guide.

Jobsite offices and conversions use the box as a weather-tight shell: a 20 ft unit converts to a site office with a door, window, insulation and electrics for a few thousand dollars over the box price. Longer-term builds — workshops, farm stores, pop-up retail, container homes — usually start from one-trip units, because remediating an old floor and rust costs more than the grade difference.

Decide the grade before shopping: wind-and-water-tight is enough for static storage, cargo-worthy (valid CSC plate) if the box might ever ship, one-trip for conversions and anything long-term. The full ladder — one-trip, IICL, CW, WWT, as-is — is decoded in the container grades guide.

Then inspect — fifteen minutes, six points: roof dents that pool water, door gaskets (the daylight test: close yourself inside; any light is a leak), floor delamination and chemical stains, cracked corner castings (structural — walk away), the CSC plate, and rust type (surface rust on Corten steel is cosmetic; scale rust that flakes in sheets is not). The step-by-step checklist is in buying a shipping container.

Finally, confirm the site before paying: a tilt-bed delivery needs a straight, level approach of roughly 18 m for a 20 ft and 30 m for a 40 ft, with about 5 m of overhead clearance for the raised bed. Methods, costs and clearances are in moving a shipping container.