Guide · Dimensions and units
ISO Container Standards
Yuri Kan Updated
Two standards make containers interchangeable: ISO 668 fixes outside sizes and ratings — every box is 2.438 m (8 ft) wide, a 20 ft 6.058 m (19′10½″) and a 40 ft 12.192 m (40′) long, read in IS 6566:2001 and ГОСТ Р 53350-2009 — and ISO 6346, read in IS 6928:2001 and BIC’s 2022 code table, defines the door markings: owner code, serial, check digit and a size/type code like 22G1 (20 ft standard dry) or 45G1 (40 ft high cube).
Two standards make global container shipping interchangeable. ISO 668 fixes external dimensions and gross ratings — that is why every box is 2.438 m wide and stacks on any ship. ISO 6346 defines the identification system: the owner code, serial number and size/type code stencilled on every container.
A container number is 11 characters in four parts. The size and type code is painted below it but is not part of the number.
- MSKOwner code3 letters
- Issued by the BIC, the worldwide register, and unique: MSK belongs to Maersk. It says whose box this is, not who is carrying it today.
- UEquipment category1 letter
- U is a freight container. J is detachable container-related equipment and Z is a trailer or chassis. In practice you will almost always see U.
- 307703Serial number6 digits
- Chosen by the owner and never repeated under its code. It carries no information about size or type.
- 8Check digit1 digit
- Computed from the ten characters before it. Mistype any single character and the digit no longer matches.
- 45G1Size and type code4 characters, separate
- 4 = 40 ft long, 5 = 9′6″ high (high cube), G1 = general purpose with passive vents. Not needed for tracking.
Where the 8 comes from
Each letter has a fixed value between 10 and 38 (11, 22 and 33 are skipped); digits are worth themselves. Each value is multiplied by a weight that doubles with every position.
| Character | M | S | K | U | 3 | 0 | 7 | 7 | 0 | 3 |
|---|---|---|---|---|---|---|---|---|---|---|
| Value | 24 | 30 | 21 | 32 | 3 | 0 | 7 | 7 | 0 | 3 |
| × Weight | 1 | 2 | 4 | 8 | 16 | 32 | 64 | 128 | 256 | 512 |
| = Product | 24 | 60 | 84 | 256 | 48 | 0 | 448 | 896 | 0 | 1,536 |
The products add up to 3,352. Divided by 11 that is 304 with 8 left over: the check digit is 8.
If the remainder is 10 the digit is 0. Owners usually avoid serial numbers that give that remainder.
Reading a container number
A marking like MSKU 307703 8 / 45G1 decodes as: three-letter owner code plus category letter U (MSKU — Maersk), six-digit serial, a check digit, then the size/type code. In “45G1”: the first character is length (4 = 40 ft, 2 = 20 ft; a 45 ft is L in the 1995 text and 5 in BIC’s 2022 table), the second is height (2 = 8′6″, 5 = 9′6″), and the last two are the type (G1 = general purpose, R1 = reefer, U1 = open top, P1 = flat rack, T = tank). Validate any number’s check digit with the container tracking checker.
| Position | Character | Meaning |
|---|---|---|
| 1st (length) | 1 | 10 ft |
| 2 | 20 ft | |
| 4 | 40 ft | |
| L (5 since 2022) | 45 ft | |
| 2nd (height) | 2 | 8′6″ standard |
| 5 | 9′6″ high cube | |
| 3rd–4th (type) | G1 | General purpose (dry) |
| R1 | Reefer | |
| U1 | Open top | |
| P1 | Flat rack | |
| T | Tank |
ISO 668 size designations
ISO 668 assigns each size a designation: 1CC (20 ft standard), 1AA (40 ft standard), 1AAA (40 ft high cube), 1EEE (45 ft high cube). US domestic 48 ft and 53 ft units sit outside the standard entirely.
Beside each figure, its evidence: ISO set by the standard · ✓ 16 sheets that agree, counted · range the sheets disagree · 1 sheet one source. A mark opens the sheets behind the figure.
| 20 ft container | 40 ft container | 40 ft high cube container | 45 ft high cube container | |
|---|---|---|---|---|
| ISO type code | 22G1 | 42G1 | 45G1 | L5G1 |
| External dimensions — Length | 6.058 m19′10½″ | 12.192 m40′ | 12.192 m 40 ft high cube container · Outside length 12,192 mm Set by the standard | 13.716 m 45 ft high cube container · Outside length 13,716 mm Set by the standard |
| External dimensions — Width | 2.438 m8′ | 2.438 m8′ | 2.438 m 40 ft high cube container · Outside width 2,438 mm Set by the standard | 2.438 m 45 ft high cube container · Outside width 2,438 mm Set by the standard |
| External dimensions — Height | 2.591 m8′6″ | 2.591 m8′6″ | 2.896 m 40 ft high cube container · Outside height 2,896 mm Set by the standard | 2.896 m 45 ft high cube container · Outside height 2,896 mm Set by the standard |
| Internal dimensions — Length | 5.898 m19′4″ | 12.032 m39′5½″ | 12.032 m39′5½″ | 13.556 m44′5½″ |
| Internal dimensions — Width | 2.352 m7′8½″ | 2.352 m7′8½″ | 2.352 m7′8½″ | 2.352 m7′8½″ |
| Internal dimensions — Height | 2.393 m7′10″ | 2.393 m7′10″ | 2.698 m8′10″ | 2.698 m8′10″ |
| Tare weight | 2,230 kg4,916 lb | 3,700 kg8,157 lb | 3,830 kg8,444 lb | 4,650 kg10,251 lb |
| Max gross weight | 30,480 kg67,197 lb | 32,500 kg71,650 lb | 32,500 kg71,650 lb | 32,500 kg71,650 lb |
| Max payload | 28,250 kg62,281 lb | 28,800 kg63,493 lb | 28,670 kg63,207 lb | 27,850 kg61,399 lb |
| Capacity (volume) | 33.2 m³1,172 ft³ | 67.7 m³2,391 ft³ | 76.4 m³2,698 ft³ | 86.0 m³3,037 ft³ |
| EUR pallets | 11 | 25 | 25 | 27 |
| TEU | 1 | 2 | 2 | 2.25 |
Checked : outside sizes set by ISO 668, read in IS 6566:2001, GOST R 51876-2008 and GOST R 53350-2009; every other figure against 32 published sheets of 20 lines, lessors and makers. A sheet agrees within ±5 mm, or ±2 % for a mass and ±1 % for a volume; a maximum gross is a rating and must be the same. Every figure and its sources
Data: ISO 668 (1995 text, read in IS 6566:2001) · CMA CGM Equipment Guide · Evergreen Container Specifications · Hapag-Lloyd Container Specifications · Maersk Container Specifications · ONE Container Specifications · Singamas product range · COSCO, Crowley, HMM, Matson, MSC, OOCL, Seaco, SeaCube, Textainer, TOTE, Trailer Bridge, Triton, Yang Ming, ZIM
The CSC plate
The safety approval plate that the International Convention for Safe Containers (CSC, 1972) requires on every approved box is the container’s passport. The Convention’s annex lists what it carries: the country and reference of approval, the month and year of manufacture, the manufacturer’s number, the maximum operating gross weight, the allowable stacking weight for 1.8 g and the transverse racking test load, each in kilograms and pounds, plus the maintenance examination dates (or an ACEP reference). The tare is not among them. When any figure on this site disagrees with the plate or the markings of your specific unit, the unit wins.
Frequently asked questions
What does 22G1 mean on a container?
22G1 is a general purpose dry container 20 ft (6.058 m) long and 8′6″ (2.591 m) tall: the first 2 is the length, the second the height, G1 the type, per ISO 6346 as read in IS 6928:2001. Likewise 45G1 is a 40 ft, 9′6″ (2.896 m) high cube dry box.
What is ISO 668?
ISO 668 is the international standard that fixes the outside of series 1 containers — 2,438 mm (8 ft) wide, 2,591 mm (8′6″) and 2,896 mm (9′6″) as the common heights, 6,058 mm (19′10½″) or 12,192 mm (40′) long for the 20 ft and 40 ft — with their tolerances and maximum gross ratings (30,480 kg, or 67,200 lb, for the 20 ft, 40 ft and 45 ft; ratings up to 36,000 kg since the 2016 amendment, per GOST R 51876-2008 and GOST R 53350-2009); we read it in IS 6566:2001 and ГОСТ Р 53350-2009. Those figures are why any container fits any ship, crane and chassis worldwide; the inside is left to ISO 1496, which sets only minimums.
How is the container check digit calculated?
By a weighted modulus-11 sum over the ten preceding characters: each letter maps to a numeric value, each position is weighted by a power of 2 (1 to 512), the total modulo 11 gives the digit (a remainder of 10 gives 0, and the standard advises against serials that produce it). Terminals recompute it on every gate move — a mistyped container number fails instantly.