The 20 ft ISO tank frame is the standard global ocean-shipping format and the equipment that 80% or more of the worldwide tank-container fleet operates on. Outer dimensions 6,058 mm x 2,438 mm x 2,591 mm per ISO 668:2020 Series 1 freight containers (designation 1CC or, for the high-cube 9’6” variant, 1AA). Corner castings per ISO 1161 at the four upper and four lower corners, accepting standard twist-lock fittings for vessel stowage. Stacking limit 9-high empty, 6-high loaded mid-ship per ISO 1496-3, with corner-post compressive load 86,400 kgf.
What 20ft is built for
Every UN cargo authorised for ISO portable-tank shipment runs in some configuration of the 20 ft frame. The cargoes split across T-code builds: T11 stainless workhorse for the broad chemical lane (24,000 L typical, 32 t payload), T14 stainless or lined for high-haz corrosives (21,000 L typical), T50 carbon steel for non-refrigerated liquefied gases (24,300 L typical, MAWP 22 to 34.4 bar by gas), T75 vacuum-jacketed cryogenic for refrigerated gases (21,500 L typical for 20 ft, 45,500 L for 40 ft variants). The full T-code ladder T1 through T22 plus T23 sits on the same frame footprint with the cylinder, shell metallurgy, lining, insulation, and pressure rating varying by code.
Construction and materials
The frame uses SPA-H Cor-Ten or Q345D / E36 weathering steel painted for corrosion resistance, with corner castings welded to the frame at all eight corners per ISO 1161. The cylinder mounts inside the frame on saddle suspensions that absorb thermal expansion and shock loading. Cylinder material varies by T-code: 316L stainless for chemical T11 / T14 builds, 304L stainless or aluminium 5083 inner with carbon-steel outer jacket for T75 cryogenic, carbon steel Q345R for T50 gas builds.
Tare ranges enormously by T-code build: 3,400 kg for the lightest beam-frame T11 stainless to 13,500 kg for a 40 ft T75 LH2 build. MGW typically 36,000 kg per ISO 1496-3 (some legacy fleets 30,480 kg, some swap-body builds 38,000 kg under EU road weight rules).
When 20ft is the right choice
20 ft is the right frame for almost every cargo. The fleet inventory is the largest of any frame size; lead times for new builds run 60 to 120 days for standard T11 / T14 builds, longer for specialty (T22 240 days, T75 cryogenic 6 to 12 months). The substitution rule of IMDG 4.2.5.2.5 lets a single 20 ft tank ride a wide range of cargoes; the same physical frame and cylinder can carry methanol on one rotation and 98% sulphuric on another (with cleaning between).
When 20ft is the wrong choice
20 ft is the wrong frame for cargoes that need the larger 40 ft frame: cryogenic LNG, LH2, and ethylene routinely need the 45,500 L typical of 40 ft T75 builds. Some bulk-food and bulk-chemical operations on intra-EU lanes prefer 30 ft swap bodies (28,000 to 35,000 L) for the per-tonne payload economics. For offshore service the 10 ft frame is right; for very high-volume light-density cargo the 40 ft 1AAA frame is right.
Frame interface specifications
Outer length 6,058 mm. Outer width 2,438 mm. Outer height 2,591 mm (1CC); 9’6” high cube 1AA at 2,896 mm exists but is uncommon for tank service. Corner castings per ISO 1161. Stacking 9-high empty, 6-high loaded. Lifting via top corner castings (preferred), bottom castings for fork-lift, side fork pockets on offshore variants.