An ISO tank is a reusable stainless steel container built to ISO standards for hazmat and non-hazmat bulk liquids, while a flexitank is a single-use polyethylene bladder fitted inside a standard dry container for compatible non-hazardous liquids only. A standard ISO tank container holds 5,000 to 6,340 gallons (21,000 to 24,000 liters) depending on product density and container type, while a flexitank typically carries 16,000 to 24,000 liters (4,200 to 6,340 gallons) and is cut out and discarded after discharge.
The two overlap on capacity and almost nowhere else. One is a certified asset built for repeat use and regulatory compliance, the other is consumable packaging built for one-way economics on freight that carries no hazard class. Guessing wrong on compatibility does not just cost money. It can mean a rejected load at destination or a cargo claim against a product that was never going to survive the trip in a liner.
ISO Tank vs Flexitank: The Quick Answer
If the product carries a UN number and a hazard class, the decision is already made. Flexitanks are approved for use only in 20-foot standard dry containers and are not permitted for hazmat cargo under the IMDG Code and most carrier policies. ISO tanks used for hazmat shipments must comply with IMDG portable tank instructions T1 through T23, matched to the UN number and packing group of the product. That single fact takes the flexitank off the table for acids, caustics, flammables and anything else carrying a hazard class.
For non-hazardous liquids, the decision turns on chemistry and lane economics. Food-grade oils, glycols and industrial lubricants that are compatible with a polyethylene liner and need no heating or pressure control move well in flexitank service. The flexitank skips the tank wash cycle entirely because the bladder is disposed of after a single use, which usually makes it the cheaper option on a one-way move.
ISO tanks are reusable assets that require tank wash and certification, such as an EFTCO cleaning document or an SQAS-assessed wash facility, between loads. On a repeat lane with real backhaul, spreading that reusable equipment cost across multiple cycles can flip the math back toward the tank.
What Is an ISO Tank Container
Construction and Capacity
An ISO tank is a cylindrical stainless steel vessel mounted inside a standard rectangular steel frame, built to the same dimensional standard as a shipping container so it can move on chassis, rail cars and vessels without special handling. Standard ISO tank containers typically hold 5,000 to 6,340 gallons (21,000 to 24,000 liters) depending on product density and the specific tank type. They can be insulated, heated or pressurized depending on the product, and many are lined for corrosive or food-grade service.
Typical Use Cases
ISO tanks are the default for hazmat liquids: acids, caustics, flammables and other regulated chemicals that require a certified portable tank. They also make sense for high-value products where contamination risk from a single-use bladder is unacceptable, and for any lane where the same product moves repeatedly and the shipper can plan around tank wash turnaround. Food-grade ISO tanks with kosher wash certification serve edible oils and liquid food ingredients on the same equipment used for industrial chemicals, with a documented cleaning chain between loads.
What Is a Flexitank
Construction and Capacity
A flexitank is a flexible polyethylene or polyethylene-laminate bladder installed inside a standard 20-foot dry container, strapped to the container walls and fitted with a discharge valve. Flexitanks typically carry 16,000 to 24,000 liters (4,200 to 6,340 gallons) of non-hazardous liquid such as food-grade oils, glycols or industrial lubricants. Once the cargo is discharged at destination, the bladder is cut out and disposed of rather than cleaned and reused.
Typical Use Cases
Flexitanks work best for one-way shipments of non-hazardous liquids where the destination has no need to return equipment and the shipper wants to avoid tank wash costs entirely. They are common for vegetable oils, wine, juice concentrate and non-hazardous industrial liquids moving from origin to a buyer with no backhaul relationship. Because the container itself is a standard dry box, flexitank loads also move through any depot or port capable of handling a normal 20-foot container, which can simplify routing on lanes without ISO tank depot access.
ISO Tank vs Flexitank: Side-by-Side Comparison
The decision factors that matter are narrow: hazmat eligibility, usable capacity, whether the equipment comes back, what the cost structure looks like across a lane rather than a single move, and what has to happen to the equipment between loads. The table below lines up both options against those five factors.
| Factor | ISO Tank | Flexitank |
|---|---|---|
| Hazmat eligibility | Approved for hazmat under IMDG portable tank instructions T1 through T23, matched to UN number and packing group | Not permitted for hazmat cargo under IMDG and most carrier policies |
| Typical capacity | 5,000 to 6,340 gallons (21,000 to 24,000 liters) | 4,200 to 6,340 gallons (16,000 to 24,000 liters) |
| Reusability | Reusable asset, requires tank wash and certification (EFTCO or SQAS) between loads | Single-use, disposed of after discharge |
| Cost structure | Cost spread across repeat cycles, strongest on lanes with backhaul | Consumable cost on every shipment, strongest on one-way non-hazmat moves |
| Equipment handling | Moves on standard chassis; hazmat loads require hazmat-endorsed drivers and placarding | Loaded into a standard 20-foot dry container, approved only in that container type |
Two lines in that table do most of the work. Hazmat eligibility is binary: a product with a UN number and packing group either has a matching IMDG portable tank instruction in the T1 through T23 range or it does not move in a tank at all, and in neither case does a flexitank qualify. Cleaning requirement is the other one. An ISO tank returns to a wash rack and comes back with a cleaning certificate before it loads again, which adds days to a turn and a line to the invoice. A flexitank bladder is cut out at destination and discarded, so there is no wash, no previous cargo history and no cleaning certificate to chase.
Capacity is closer than most shippers expect, since the ranges overlap at the top end. Volume alone rarely decides it. Density does: a heavy product hits the container payload limit long before it fills either vessel.
When to Choose an ISO Tank
Hazmat and Regulatory Triggers
If the product carries a UN number and a hazard class under Class 3, Class 5.2, Class 6.1 or Class 8, the ISO tank is the only tank option on the table. IMDG portable tank instructions T1 through T23 specify which tank design and fittings a given UN number and packing group require, and a flexitank is not built or certified against that code. Hazmat ISO tank moves also require hazmat-endorsed drivers and correct placarding under DOT and IMDG rules, so the equipment decision and the compliance paperwork travel together.
High-Value or Sensitive Cargo
- Products with strict purity requirements where any residual contamination from a single-use liner would trigger a rejected load or a quality claim at destination.
- Cargo moving on a lane with reliable backhaul potential, where the reusable tank spreads its cost across multiple cycles instead of one trip.
- Shipments that need heating, insulation or pressure control in transit, none of which a flexitank can provide.
When to Choose a Flexitank
Compatible Non-Hazardous Liquids
Flexitanks are the right call for non-hazardous liquids that are chemically compatible with polyethylene and do not require heating or pressure control in transit. Food-grade oils, glycols and many industrial lubricants fall into this category and move reliably in flexitank service without the tank wash cycle an ISO tank requires between loads. Compatibility is a product-by-product question, not a category question: check the SDS and the resin compatibility data for the specific grade, including additives, before booking. Viscosity matters as much as chemistry, since a product that needs heat to discharge is not a flexitank candidate no matter how benign its classification.
One-Way and Cost-Sensitive Lanes
- Single-direction lanes where the consignee has no use for returning equipment and no backhaul relationship exists to offset the cost of reuse.
- Shipments where landed cost per unit outweighs equipment flexibility, since skipping tank wash and repositioning removes two cost lines from the move.
- Routes through depots or ports without ISO tank handling infrastructure, where a standard 20-foot dry container keeps the routing simple.
The tradeoff is real: you give up heating, insulation and pressure control, and every shipment buys a new bladder instead of amortizing one certified asset across cycles.
Common Mistakes Shippers Make
Most equipment mismatches trace back to a handful of recurring errors, and every one of them is avoidable with a compatibility check and a lane review before the booking goes out.
- Assuming a non-hazardous classification on paper means flexitank-compatible in practice, without checking the product's reactivity with polyethylene or its need for temperature control.
- Booking a flexitank for a product that requires heating in transit, which flexitanks cannot provide and which leads to a congealed or unusable load at destination.
- Defaulting to an ISO tank for every hazmat-adjacent product without confirming the actual UN number and packing group, which drives up cost on freight that may ship fine in drums or IBCs.
- Overlooking tank wash lead time when scheduling a repeat ISO tank lane, which creates avoidable dwell at the depot and detention on the chassis.
- Failing to confirm the previous cargo history on an ISO tank before loading a sensitive product, risking cross-contamination that a single-use bladder avoids by design.
- Treating the container as the compliance decision. Placarding, driver endorsement and documentation follow the product's hazard class, not the vessel it rides in.
Getting the Right Equipment for Your Load
The call between an ISO tank and a flexitank comes down to three questions: does the product carry a hazmat classification, is it chemically compatible with a polyethylene liner, and does the lane support backhaul economics or run strictly one way. Answer those three and the equipment choice usually follows without much debate. The harder cases sit in the middle, a non-hazardous product with a tight purity spec, or a seasonal lane that runs one way for half the year and both ways for the rest. Those are conversations about a specific product on a specific lane, not rules.
Total Connection books both ISO tank and flexitank equipment inside a full liquid bulk and chemical logistics program, alongside the truckload, drayage, ocean and warehousing capacity that moves the rest of the supply chain, with tank wash coordination and compliance documentation on every move. For the wider picture, start with our complete guide to liquid bulk freight. For how previous cargo history affects a tank booking, read ISO tank previous cargo requirements. When you are ready to move the load, get a liquid bulk quote and we will match the equipment to your product before anything is booked.
Can flexitanks be used for hazmat chemicals?
Flexitanks are restricted to non-hazardous, chemically compatible liquids and are not approved under the IMDG Code for dangerous goods. Most carrier policies prohibit hazmat cargo in flexitanks outright, so any product with a UN number and hazard class needs an ISO tank instead.
Is a flexitank cheaper than an ISO tank?
For one-way, non-hazmat loads, a flexitank is generally cheaper because it avoids tank wash and repositioning costs. On repeat lanes with backhaul potential, an ISO tank can become more economical over multiple cycles since its reuse spreads the equipment cost across more shipments.
What products are not compatible with flexitanks?
Products that react with polyethylene, require heating or pressure control in transit, or carry a hazmat classification are not flexitank-compatible. Any product needing insulation, temperature maintenance or specialized venting also falls outside flexitank capability.
Do ISO tanks require special drivers or equipment?
ISO tanks move on standard chassis like any shipping container, so no special chassis is needed. Hazmat-loaded ISO tanks do require hazmat-endorsed drivers and correct placarding under DOT and IMDG rules, since the compliance requirements follow the product's hazard classification rather than the container type.
How do I know which option fits my shipment?
Start by confirming the product's UN number and hazard classification, since hazmat cargo eliminates the flexitank option immediately. From there, match product chemistry against polyethylene compatibility and weigh the lane's repeat volume and backhaul potential to decide whether the reusable ISO tank or the single-use flexitank makes more sense on cost.
