If you ship liquid in bulk, you have two main alternatives to drums, IBCs and a tanker ship: an ISO tank container, a reusable pressure vessel built into a standard-sized steel frame, and a flexitank, a single-use bladder installed inside an ordinary 20ft dry container. They look like substitutes, but they are not. The first rule that separates them is cargo: the industry code for flexitanks excludes anything classified as dangerous goods, while ISO tanks are built to carry both dangerous and non-hazardous liquids. The rest of the decision (fill rules, cleaning, who owns the equipment, what happens when something leaks) follows from there.
Capacities, codes and test intervals below were checked against the sources named in the text as of October 2026. Regulations are amended on a cycle, so confirm the current edition and your operator's own limits before you book.
What each one is
ISO tank containers
An ISO tank container is a cylindrical tank (the "barrel") held inside a rectangular steel frame that matches the corner fittings and outer dimensions of a standard 20ft container, so it can be lifted, stacked and carried by ship, rail and truck like any other box. One tank-container leasing company gives the 20ft frame dimensions as 6,058 mm long, 2,438 mm wide and 2,591 mm high, in capacities of 21,000, 25,000 and 26,000 litres. Another lessor's T11 to T14 page puts the standard range at 21,000 to 26,000 litres, with a maximum working pressure of 4.0 bar and a tare weight of 3,540 to 3,830 kg. The "T" numbers are UN portable tank types, and which one you need depends on the cargo, so the choice is made against the dangerous goods list, not by preference.
Features that matter to a shipper: insulation and heating for cargoes that thicken in the cold (the second lessor's page lists eight heating runs for high-viscosity cargo, and the first describes steam heating capability), a bottom discharge valve, and a safety relief valve. The tank is reused. It is cleaned, inspected and sent out again.
Flexitanks
The Container Owners Association (COA) Code of Practice defines a flexitank as "a large bladder with a valve(s) that is designed to fit inside a general-purpose freight container and operates as part of a system which includes the container, flexitank, its fittings and restraining system. It is designed for single-use." The usual host is a 20ft general-purpose container. The Code sets a maximum of 24,000 litres and 24,000 kg, and the COA's own web page describes the capacity as "usually 24000 litres". Bladders can be made from polyethylene, polyethylene blends or PVC.
The "restraining system" is the bulkhead: a barrier fitted across the door end of the container to hold the filled bladder in. The Code lists bulkhead installation as part of the manufacturer's training manual and the COA notes that the metal bulkhead components can be serviced and re-used or recycled. The container is part of the system, which is why its condition matters so much more here than it does for an ISO tank.
Side by side
| Point | ISO tank container | Flexitank |
|---|---|---|
| What it is | Stainless-steel tank in a 20ft ISO frame, built to the UN portable tank rules | Single-use polymer bladder with a bulkhead, installed in a general-purpose container |
| Typical 20ft capacity | 21,000 to 26,000 litres (lessor product pages) | Up to 24,000 litres under the COA Code |
| Dangerous goods | Yes, in the right tank type for the substance | No: the COA Code states IMDG dangerous goods are not permitted |
| Non-hazardous liquids | Yes, including food-grade and temperature-controlled needs | Yes, if compatible with the bladder material |
| Reuse | Reused after cleaning, with periodic inspection and testing | Single use: bladder and fittings removed and disposed of after discharge |
| Heating or insulation | Available (heating runs, steam, insulation) | Temperature range set by the manufacturer's instructions |
| Fill rule | Governed by the filling-degree rules and, for dangerous goods, the tank's own data | Filled to the manufacturer's specified capacity, plus or minus 3 percent of nominal |
| Main failure risk | Surge in a part-filled tank, contamination from previous cargo, expired test | Bladder or bulkhead failure damaging the box and neighbouring cargo |
| Who you usually deal with | A tank-container operator, as well as the carrier | A flexitank operator or installer, as well as the carrier |
Which cargoes suit which
Flexitanks are for non-regulated liquids. The COA Code says the flexitank "is used only for the transport of liquids that are not classified or regulated as dangerous goods in accordance with the IMDG Code", and its section on cargo repeats that dangerous goods "are not allowed to be transported by flexitank systems". Suppliers commonly list edible oils, wine, fruit juices, glycerine, latex, base oils and non-hazardous industrial liquids as typical loads.
ISO tanks cover the same non-hazardous ground plus the dangerous goods that have a portable tank instruction. In the IMDG Code, the Dangerous Goods List assigns each substance a portable tank instruction and special provisions, and chapter 4.2 requires the tank to conform to the ones assigned to the substance you are shipping. A tank-container lessor's own T11 to T14 page lists hazardous chemicals, acids, wine, edible oils and spirits as examples, and adds that "the authorised Dangerous Goods Advisor should confirm fitness for purpose of any tank" before a specific cargo is loaded.
"Non-hazardous" is not the same as "compatible"
The most common mistake with flexitanks is to treat "not dangerous goods" as "safe for a bladder". The COA Code says directly that non-dangerous goods "might contain hazards and might not be compatible with the flexitank material or suitable for safe reliable transport in a flexitank system". It asks for a risk assessment before accepting any cargo, a safety data sheet from the shipper that stays available during carriage, and a check of the cargo temperature against the manufacturer's instructions. A liquid can sit outside the IMDG Code and still attack the bladder or react with its fittings. Ask the flexitank supplier for its compatibility advice on your specific product, in writing.
If your cargo is classified, the route runs through the dangerous goods rules instead. Our guides to shipping dangerous goods under the IMDG Code and IATA regulations and segregation and stowage of dangerous goods at sea cover what the shipper and the carrier each have to do.
Loading and discharge
ISO tanks. The tank is emptied through its bottom discharge valve assembly, with heating runs available for cargoes that need warming to flow. Filling is governed by the tank's rated limits and, for dangerous goods, by the filling-degree rules covered in the next section.
Flexitanks. The installer fits protective lining in the container, positions the bladder and the bulkhead, fills the bladder through its valve under controlled measurement, and fixes a warning mark to the outside of the container's left-hand door. The COA Code requires the manufacturer's manual to cover "filling methods, including filling capacity control by calibrated measuring equipment" and "action in the event of excessive container wall bulging or other incidents during filling", and it states that competent installation by properly trained personnel is "an essential requirement". The container should stand on a flat, level surface while it is filled so that the doors still close properly. After discharge, the bladder, linings and equipment must be removed completely and disposed of in line with the environmental instructions, and the container returned empty and in the condition it was received.
The real risks, and what the sources actually say
Part-filled tanks: the 20 and 80 percent rule
You will often hear that an ISO tank "must be at least 80 percent full". That is a simplification, and the exact rule is more useful. Paragraph 4.2.1.9.6 of the IMDG Code's chapter on portable tanks, whose wording matches the RID text published by OTIF, says portable tanks must not be offered for transport with a degree of filling of "more than 20% but less than 80%" for liquids with a viscosity below 2,680 mm²/s at 20 °C (or at the maximum temperature during carriage, for a heated substance), unless the shell is divided by partitions or surge plates into sections of not more than 7,500 litres. So the forbidden band is the middle: a low-viscosity liquid is acceptable at 20 percent or below, or at 80 percent or above, because a half-full barrel lets the liquid surge and slam against the walls as the vessel moves. A tank with surge plates dividing it into compartments of 7,500 litres or less is the exception. Liquids thicker than the 2,680 mm²/s viscosity are not caught by this paragraph.
The same chapter also limits how full a tank can be: for heated cargo it requires a temperature regulator so that the tank is "not more than 95% full at any time", and other paragraphs set a degree of filling that leaves room for thermal expansion. Operator pages commonly quote a working band of 80 to 95 percent. That is a practical range, not a single universal figure in the regulation, so ask the operator what limit applies to your cargo and your route. Note too that this chapter is a dangerous goods rule. For a non-regulated liquid the surge physics are the same, so ask the operator what fill range it requires rather than assuming the rule does not apply.
Flexitanks: the bulge and the leak
A flexitank has no 80 percent rule. The COA Code says the bladder must be filled "to the manufacturer's specified filling capacity and a tolerance of +/- 3% of the nominal capacity", with a ceiling of 24,000 litres and 24,000 kg. The hazard is different: the filled bladder pushes outward on the container walls and doors, and a failing bladder, valve or bulkhead puts cargo into the box and possibly onto neighbouring cargo. The Code quotes the CTU Code on this point: the contents of a flexitank are "subject to dynamic forces without significant retention from friction", which "may cause damage or complete failure", so the payload of the container "should be appropriately reduced", and the mass of liquid should not exceed a value agreed with the container operator, meaning the carrier. Do not assume a 24,000 litre fill is automatically accepted at the weight your cargo implies. Agree the figure with the carrier up front.
The Code also says that when something does go wrong, the parties may need to move the cargo to another flexitank or to an ISO tank container, IBCs or drums, with a surveyor appointed to investigate. That is a reminder to settle who pays for clean-up and recovery before the cargo moves. The Code requires its COA-member operators to hold all-risks liability insurance of not less than US$5 million, but that is a requirement on members, not a guarantee about the particular company you are dealing with. Ask for the insurance certificate.
Container condition, for flexitanks in particular
Because the container is part of the system, the Code asks for a 20ft general-purpose container rated to a minimum gross mass of 30,480 kg, conforming to ISO 1496 Part 1 and showing a valid CSC safety approval plate. Our guide to the container number and CSC plate explains what that plate tells you. The Code also sets criteria for dents, sharp edges, floor gouges and door hardware, and requires protective lining where the interior could snag the bladder. If a carrier will not supply a container to that standard, you want to learn that now, not on the day of loading.
Reuse, cleaning and the test plate
Flexitanks are single use, which removes cross-contamination between loads but creates a disposal step. The COA Code requires the bladder, linings and equipment to be removed and disposed of environmentally, and the bulkhead components can be recycled.
ISO tanks are reused, so what the tank last carried and how it was cleaned are part of your shipment. A UK tank-cleaning depot describes its process as high-pressure cleaning with food-grade detergents and food-grade drying, "to ensure the container or roadtanker is free from contaminants, dry & odourless", and it runs a separate food cleaning bay. Whether a tank must arrive with a written cleaning certificate and a record of its previous cargoes depends on your cargo and your contract, and no single rule covers every case. Treat a request for both as a standard question to put to the operator, not an optional extra, particularly for food-grade cargo.
Tank containers also keep a testing schedule. The International Tank Container Organisation (ITCO) states on its regulation page that "during service it is mandatory that each tank container undergoes testing every 2.5 years and 5 years", and that tanks failing the tests lose their approval for hazardous cargo. Trade press covering the RID-ADR 2023 amendments reports that a 2.5-year test performed after the expiry date on the last test certificate must be replaced by a full 5-year test, and notes that this applies to RID-ADR tank containers, with different rules for tanks approved only under the IMDG Code. An expired test date can therefore stop a booking, so read the rules that apply to your trade lane.
Who is responsible for the tank
With a standard dry container, you usually deal with the shipping line (see our guide to shipper-owned and carrier-owned containers). Bulk liquid equipment often adds another party. ITCO describes tank container operators as "third party logistics companies that provide a door-to-door service to shippers and others that require transport of bulk liquids, powders or gases", and leasing companies as providers of tanks to operators, shippers and others, usually on a contractual term. In its survey at 1 January 2023, ITCO counted more than 240 operators, more than 38 lessors, and a further 199,110 tanks held by shippers and other users, out of about 801,800 tanks worldwide. So a tank on your booking may be owned by a lessor, run by an operator, owned by the cargo owner, or carried on a vessel that belongs to a fourth company. The practical effect: one party holds the equipment, another provides the ocean carriage, and a cleaning depot is a third.
For flexitanks the structure is similar. The COA Code names the manufacturer, the operator (or installer) and the shipper as separate parties with separate obligations, and says shipping lines should refer to the COA's Flexitank Quality Management List and run their own risk assessment against their conditions of carriage. Check with the shipping line whether it accepts flexitanks, and on what conditions, before you commit to one.
Container return is a separate matter too. A tank or box that is not returned on time earns detention, and our guide to empty container return covers how that goes wrong. For flexitanks the box must come back completely empty, so removal and disposal of the bladder has to be planned into the discharge day.
A practical checklist before you book
- Classify the cargo first. Is it dangerous goods under the IMDG Code? If yes, flexitanks are out and you need a tank with the right portable tank instruction. Have the safety data sheet in hand.
- Test compatibility. For a flexitank, get the supplier's written compatibility advice for your product and temperature range. For an ISO tank, confirm the lining, seals and valves suit the cargo.
- Ask for the previous-cargo history and cleaning evidence on any ISO tank, and confirm the tank is suitable for food if the product is food.
- Check the tank's test status. Ask for the last periodic test date and the next due date, and make sure it will still be valid on the day of loading and for the whole voyage. Look at the tank's own plate data and certificate, not a verbal assurance.
- Agree the fill. For an ISO tank, confirm the permitted fill range for your cargo. For a flexitank, confirm the specified capacity and the maximum liquid mass the carrier will accept in the container.
- Check the flexitank installer. Ask whether the manufacturer holds COA quality compliance, whether the installer has been trained on that manufacturer's manual, and for the flexitank's serial number and capacity mark. Ask for the insurance certificate.
- Confirm the carrier accepts it. Carriers assess flexitank systems and the cargo against their own conditions of carriage. Ask in writing.
- Plan the other end. Who removes the flexitank, who cleans the ISO tank, and who is liable for detention if the discharge slips.
When to choose which
The sources point to a plain rule of thumb. If the liquid is dangerous goods, has to stay warm, or needs a food-suitable cleaning history, an ISO tank is the route that fits. If the liquid is non-regulated, compatible with the bladder, and you would rather not deal with tank equipment and its repositioning, a flexitank in a standard container is the usual alternative. Neither is automatically cheaper, and this guide quotes no rates: get both priced for your lane.
If you are comparing these options, it helps to speak to a freight forwarder with real bulk-liquid experience. You can browse logistics companies by country and service in the CargoLinked directory, where listings are published by the companies themselves, so check credentials and references before you rely on one. Or post your shipment on the requests board and compare the quotes eligible forwarders send back (each quote carries a validity date). For the container itself, our guides to container types and dimensions and special equipment: reefer, flat rack, open top and tank are good places to start.
Sources: the COA Code of Practice for flexitank operators (version V6-O, July 2019, the edition read for this guide); the IMDG Code chapter 4.2 text and the matching RID chapter 4.2 as published by OTIF (2009 edition), so check the current edition; the ITCO regulation page; and the ITCO Global Tank Container Fleet survey of January 2023. This guide is general information, not engineering or dangerous goods advice.
