Heel · IMDG 4.2

ISO tank heel calculation

Catch the heel-contamination risk before the next cargo loads.

The heel is the residual an ISO tank cannot drain on discharge. Load an incompatible next cargo on top of a heel and you contaminate the whole parcel, or worse, trigger a reaction. The cost is the lost cargo plus the tank clean plus the customer's downstream batch. IMDG Code Chapter 4.2 governs the tank, but compatibility is the operator's call.

Pick the tank build and the prior cargo.

Last updated 2026-05-09. Math runs in your browser, no data leaves your computer.

Pick the build that matches the actual tank, since heel depends on discharge geometry. Bottom-outlet T11 drains to about 0.5 percent; top-discharge and older shells leave far more.

The rated volume from the tank data plate. T11 is usually 24,000 L, T14 around 25,000 L, T20 around 21,000 to 23,000 L.

What the tank carried last. This is the chemistry sitting in the heel that the next load will sit on top of.

What you want to load next. The tool checks this against the heel for any reaction or contamination trigger.

Optional. Add the heel cargo's SDS Section 9 density and the tool also reports the residual in kg, which is what the carrier bills or credits.

General guidance only. Verify against the cited primary sources before you commit to a shipment, declaration, or contract.

Heel and cleaning in chemical-trade context

An ISO tank does not empty completely. The discharge geometry, the bottom-outlet valve seat, and any internal baffles or steam coils trap a small residual volume of liquid that the next cargo will be loaded on top of. For a clean haul (same cargo, same supplier, same grade), the heel is acceptable and saves a wash cycle. For a switch to a different cargo, the heel becomes a chemical question: is the residual compatible with the new cargo, or does it trigger a mandatory tank wash?

The compatibility table follows industry practice for tank-container operators. The hazardous pairs (acid plus caustic, acid plus cyanide, oxidiser plus organic flammable, isocyanate plus amine, etc) are universally treated as cleaning triggers; the compatible pairs (same cargo, or commodity chemicals with the same chemistry) are typically cleared with a single rinse or no rinse at all. Food-grade cargo is its own category: any non-food previous load triggers a triple wash with a food-grade cleaning certificate.

EFTCO (European Federation of Tank Cleaning Organisations) codes A through J are the standard reference for cleaning protocols. EFTCO A is a single hot-water rinse (commodity to commodity, same chemistry). EFTCO C is a caustic-and-acid double wash (typical chemical change). EFTCO H is a steam-and-detergent food-grade wash. The depot issues an EFTCO Cleaning Certificate (ECC) at completion, which travels with the tank to the next loader and which the loader checks before loading.

For chemical-by-chemical detail, read the supplier SDS and consult the carrier's cleaning matrix.

A worked example

You have a 24,000 L T11 that just discharged sodium cyanide solution, with a backhaul of sulphuric acid waiting at the next loader. The bottom-outlet heel is only 0.5%, about 120 L, which feels like a rounding error.

It isn't. Acid on a cyanide heel releases hydrogen cyanide gas, fatal at low concentration, and 120 L is far more than enough. The booking is refused outright until the depot issues an HCN-free atmosphere certificate. The rule here is absolute: no acid into a cyanide tank without a clean-air cert.

Send the tank through a deep clean, get the EFTCO certificate and the HCN-free reading, and the same backhaul loads safely. The 120 L heel cost a wash cycle and a day, not a fatality and a cargo claim.

Need to match a wash to an EFTCO code before you accept the cleaning certificate? The tank cleaning code lookup maps the protocols A through J at /tools/containers-and-logistics/tank-cleaning-code-lookup.

Sizing the fill on the same tank before you load? The ISO tank loading calculator runs the binding IMDG fill limits at /tools/iso-tank-loading.

Frequently asked

What is "heel" on an ISO tank?

The volume of liquid that cannot be drained from a tank container after discharge. For a standard T11 with bottom-outlet discharge, the industry-standard heel is 0.5 percent of nameplate capacity (so on a 24,000 L T11, the heel is roughly 120 L). For top-discharge-only tanks the heel is 1.5 to 2 percent because the dip-pipe foot does not reach the very bottom.

Why does heel matter for the next cargo?

Whatever liquid is left in the tank after discharge becomes part of the next cargo unless the tank is washed. For compatible cargo (same product, same supplier) heel is acceptable and saves a wash. For incompatible cargo (acid after caustic, oxidiser after organic flammable, etc) heel is a chemical hazard and triggers a mandatory tank wash with a certificate of cleaning. EFTCO codes A through J cover the common wash protocols.

Is heel always paid for by the next shipper?

Industry practice varies. Some carriers absorb the heel as part of the haul; some bill the heel as residual freight. For high-value cargo (food-grade, pharma intermediates), heel is usually purged during loading and credited back to the previous shipper. For commodity chemicals (caustic, urea, glycol), heel is typically left in and absorbed.

Related tools