What Is a Conex Box? Sizes, Uses, and Buying Guide

A conex box is a standardized, heavy-steel intermodal shipping container used worldwide for freight transport, on-site storage, and everything in between. If you’ve heard the terms conex box, shipping container, and storage container used interchangeably, they mostly mean the same thing. This guide clears up the terminology, walks through conex box sizes and real-world uses, and explains what container condition grades actually mean so you know what you’re getting before you buy.

Conex Box or Shipping Container?  Understanding the Terminology

“Conex” is short for Container Express, a standardized military shipping system developed by the U.S. Army in the early 1950s. The name stuck, especially in government and military contexts, and civilians still use it today when searching for a conex container for sale.

Term	Origin	When it’s used
Conex box	U.S. military logistics, 1950s	Civilian and government purchasing
Shipping container	Commercial freight industry	Units in active ocean service
Intermodal container	ISO/international standards	Technical term across ship, rail, and truck
Storage container	Repurposed ground use	Units retired from ocean service
Sea can	Informal slang	Agriculture, construction, rural markets

One distinction worth knowing: “storage container” usually means a unit no longer certified for ocean transport. For buyers planning to ship goods overseas, that difference matters.

Conex Box Sizes and Dimensions

Conex box dimensions are standardized globally by the ISO, making intermodal containers interchangeable across ships, rail, and trucks worldwide.

Size	Exterior dimensions	Interior floor space	Door opening
20′ standard	20′ L × 8′ W × 8’6″ H	~160 sq ft	7’8″ W × 7’5″ H
40′ standard	40′ L × 8′ W × 8’6″ H	~320 sq ft	7’8″ W × 7’5″ H
40′ high cube	40′ L × 8′ W × 9’6″ H	~320 sq ft + headroom	7’8″ W × 8’5″ H
45′ high cube	45′ L × 8′ W × 9’6″ H	~360 sq ft	7’8″ W × 8’5″ H

High cube units add approximately one foot of interior height, a real difference when storing tall equipment or planning modifications. E&S also carries 10-foot containers for tight spaces, plus specialty configurations like open-side and double-door units. Our specialty cargo container guide covers those configurations in detail. 

What Are Conex Boxes Used For?

Storage for Farms, Job Sites, and Residential Properties

This is the most common use for a portable steel storage container. Farmers store hay, seed, fertilizer, and equipment. Construction crews use conex boxes for tool security. Residential buyers use them during renovations and for seasonal items. The appeal is universal: weatherproof, vermin-proof, lockable, and movable. Corten steel construction resists rust naturally and holds up well in Florida’s humid climate.

Check out 15 ideas for agriculture uses for shipping containers

Overseas Freight and Intermodal Shipping

Certified units still move cargo across oceans. If you need to ship goods to the Bahamas, the Caribbean, or other international destinations, a properly certified intermodal container is required. E&S’s founder is a licensed container surveyor who can inspect and issue CSC certification for units going into ocean service, a capability most sellers and every broker cannot offer.

Modifications for Workshops and Custom Access

Standard conex containers accept a wide range of modifications. E&S has performed in-house modifications for 15 years: man doors, roll-up doors, windows, wall A/C units, custom paint, and ramps. 

Our complete buying guide covers how to plan for modifications before you purchase.

Understanding Conex Box Condition Grades

Grade is one of the most important factors when comparing used conex boxes for sale, and where many buyers get burned.

Grade	What it means	Best for	E&S warranty
New one-trip	One voyage from factory, like-new condition	Modifications, customer-facing locations	5 years
Cargo Worthy (CW)	Inspected and certified for ocean shipping	Long Term Storage, freight, most applications	2 years
Wind and Water Tight (WWT)	Keeps out weather but not ocean-certified	Budget domestic short term storage only	90 days
As-Is	No guarantees	Not recommended	None

Why Some Companies Mislabel WWT as Cargo Worthy

This happens regularly in the conex box market. A seller lists a container as “cargo worthy” and prices it $300–$500 less than it should be. What the buyer receives is a Wind and Water Tight unit, structurally inferior and ineligible for ocean transport. A WWT unit taken to port for export will fail inspection, and that cost falls on the buyer.

E&S does not sell As-Is containers under any circumstances. Every used conex box we deliver is Cargo Worthy or New One-Trip, inspected before delivery.

CSC Certification

If a conex container is going onto a cargo ship, it must carry a valid CSC plate, certification issued by a licensed surveyor confirming the unit meets structural requirements for ocean transport. Only licensed surveyors can issue these certificates. E&S’s owner holds that license.

For a deeper look at grades and what to verify before delivery, start understanding shipping container grades.

How Much Does a Conex Box Cost?

Conex box cost shifts with market conditions, and the right number depends on size, grade, location, and current inventory. The price difference between a well-graded container and an inferior one is often a few hundred dollars, but what you receive can differ significantly.

Rent-to-own is available for New One-Trip and Cargo Worthy units, with terms up to 48 months, no credit check, and no complicated paperwork. It’s a straightforward way to buy a conex box without a large upfront payment.

For current pricing, request a quote or call (800) 995-2417.