Heel · IMDG 4.2

ISO tank heel calculation

Heel is the residual liquid an ISO tank cannot drain on discharge. This tool returns the heel volume by tank build and flags whether the next cargo is compatible or triggers a mandatory wash.

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

General guidance only, not legal or professional engineering advice. Verify against the cited primary sources (IMDG, REACH, ChAFTA, RCEP, Customs Tariff Act, supplier SDS, etc.) before committing to a shipment, declaration, or contract. Sourzi assumes no liability for outcomes based on these calculators.

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 a quick tank-to-tank compatibility check, use Sourzi /tools/containers-and-logistics/tank-cleaning-code-lookup. For chemical-by-chemical detail, read the supplier SDS and consult the carrier's cleaning matrix.

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.

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