IBC bund sizing: what AS 1940 actually requires

The widely repeated "a bund must hold 110% of the largest container" is a myth — it isn't what AS 1940 says. Under AS 1940:2017, a spill containment compound must hold at least 100% of the largest single package, plus 25% of the aggregate storage on the bund (up to 10,000 L), with reduced percentages above that. For a single IBC the difference is small; for a bund holding several, the real formula demands meaningfully more capacity than the rule of thumb — and getting it wrong is exactly how a split container reaches the drains.

What does AS 1940 actually require?

It requires 100% of the volume of the largest single container, plus a percentage of the total aggregate stored — added together, not chosen between. The relevant provision is the spill-compounding requirement in AS 1940:2017, the Australian Standard for the storage and handling of flammable and combustible liquids, which is referenced as the means of compliance under Work Health and Safety dangerous-goods duties (safeworkaustralia.gov.au). The required containment capacity is the sum of:

• 100% of the volume of the largest single container in the bund, plus
• 25% of the aggregate storage capacity up to 10,000 L, plus
• 10% of the aggregate between 10,000 L and 100,000 L, plus
• 5% of the aggregate above 100,000 L.

Notice this is additive. The "largest container" portion and the "percentage of aggregate" portion stack on top of each other — it is not "the greater of" the two, which is how the 110% shorthand is often mis-applied. The same additive logic carries into corrosives storage under AS 3780, so the method below holds whether you are bunding diesel, oils or many process reagents.

Two practical consequences follow from the additive rule. First, the aggregate is everything that drains into that one compound — so if you put more containers behind a single kerb, the required catch volume climbs with the total, not just with the biggest unit. Splitting a large store into two separate, lower-aggregate bunds can be the cheaper and safer answer, especially where it also lets you keep incompatible substances apart. Second, the number you calculate is a retained-liquid volume, which means the physical bund — kerb height, floor area and sump depth — has to deliver that volume after displacement and rainfall are accounted for, not before. Those two ideas, aggregate-drives-capacity and retained-volume-not-geometric-volume, are where most under-sized bunds go wrong.

Worked sizing table: where the myth breaks

The gap between the rule of thumb and the standard widens fast as you add containers. The table below works the AS 1940:2017 figure for common IBC arrangements (all 1,000 L IBCs, under the 10,000 L tier) against the "110% of the largest" rule of thumb. Every figure is computed as 100% of the largest + 25% of the aggregate, before any displacement subtraction or rain margin.

Read the single-IBC row and you can see why the myth survives: 1,250 L required vs 1,100 L from the rule — close enough that nobody gets caught. Read the four-IBC row and the rule under-sizes the bund by almost half. That is precisely the multi-container yard where a failure has the most to leak and the most chance of reaching a drain or watercourse.

Where you can, the cleanest way to shrink the sum is to cut the number of separate liquid packages — decanting reagent powder or non-hazardous liquid into one large rigid container instead of several drums lowers both the "largest" term and the aggregate. A single tall, solid-walled bulk container does the work of a stack of drums and is far easier to bund and inspect.

How much does displacement cost you?

It costs you whatever volume the solid objects standing in the bund take up — and on a multi-IBC base that can be hundreds of litres. Containment capacity is the volume the bund can actually hold as liquid, and anything solid inside it occupies space that liquid then can't. Drums, IBC cages, pallets and plinths displace liquid volume, so the usable capacity is less than the geometric volume of the empty bund.

Work it the same way every time: take the empty bund's internal volume to the overflow point, then subtract the submerged volume of everything that stands inside it up to that level. The remainder is your real, compliant capacity. A bund that meets the formula on a spreadsheet but ignores displacement can quietly fall short in an actual spill — which is the one moment it has to work.

A quick worked subtraction makes the point. Suppose two 1,000 L IBCs sit on a shared spill base, each on a 1165 × 1165 mm pallet roughly 150 mm tall. The AS 1940 target from the table above is 1,500 L (100% of 1,000 L + 25% of 2,000 L). Two timber or plastic pallets occupy on the order of 0.4 m³ between them once you allow for deck and bearers below the overflow line — call it ~400 L of displacement. To actually retain 1,500 L of liquid, the empty sump must therefore be built to about 1,900 L of geometric volume. Size only to 1,500 L and the first burst container will overtop the wall. The displacement subtraction is not a rounding detail; on a busy base it is the difference between a contained spill and a reportable one. When you brief us, tell us how many containers sit on the bund and on what (skids, feet or a pallet), and we'll size the catch volume net of displacement.

Which liquid class can a plastic bund hold?

A plastic (HDPE) bund suits Class II and III combustibles and many corrosives — but not Class I flammables. AS 1940 sorts liquids by flash point into classes, and that classification drives what the bund may legally be made of. Australia's dangerous-goods classes follow the UN/ADG system, where Class 3 covers flammable liquids and Class 8 covers corrosives (national poisons / ADG class references). In containment terms:

• Class I — flammable (low flash point, e.g. petrol, many solvents). These need fire-rated, non-combustible containment. A plastic (HDPE) bund is not appropriate for flammable liquids.
• Class II and III — combustible (higher flash point, e.g. diesel, many oils). This is where polyethylene bunded pallets and IBC spill bases suit — chemically resistant, won't corrode, and unaffected by the water and mud of a working yard.
• Class 8 — corrosives (acids, caustics). These fall under AS 3780 (the storage and handling of corrosive substances), which has its own containment and compatibility requirements — confirm the bund material is rated for the specific reagent and concentration.

In short: match the bund to the dangerous-goods class. HDPE is excellent for diesel, oils and many reagents, and the wrong choice for petrol-class flammables. On remote and outdoor sites, also specify UV-stabilised polymer so the containment doesn't go brittle in the sun — a cracked sump is a failed bund. For the reagent and diesel side of a gold or base-metals operation, the wider method is in our mine-site spill containment guide.

Do outdoor bunds need a rain allowance?

Yes — an open outdoor bund must carry extra capacity for rainfall, or that water steals the containment you sized for a spill. Every litre of captured rain sitting in the sump is a litre that isn't available to catch a leak, so an uncovered bund needs either a roof, a managed drain valve, or additional freeboard sized to a credible rainfall event for the region. Australia's wet-season north and the storm-prone east coast make this non-trivial: a Pilbara or Bowen Basin yard can take a year's "design storm" in a single afternoon.

The practical rule is simple: inspect and drain. Keep the drain valve closed by default, check the sump on a schedule, and pump out captured water (checking it for contamination first) before it eats into capacity. A bund that's half full of rainwater when a container splits has effectively been de-rated to half its design.

How do you keep a bund compliant in service?

A bund only counts if it stays liquid-tight, drains to nowhere it shouldn't, and is chemically compatible with what it holds — and those three things degrade over time if no one checks them. Sizing to the formula is the easy half; the harder half is keeping the compound performing through years of forklift traffic, UV exposure and reagent contact. Three failure modes account for most non-compliant bunds:

• Loss of integrity. Cracks, perished seals, corroded steel kerbs or a UV-embrittled polymer sump all let liquid escape below the rated level. A spill base that has gone chalky and brittle in the sun is no longer a spill base. Inspect for crazing and stress cracks, and choose UV-stabilised polymer outdoors so the catch volume you sized is the catch volume you keep.
• Drainage to the wrong place. A bund must not gravity-drain to stormwater, soak-away or any uncontrolled point — that defeats the entire purpose. Any outlet should be a normally-closed valve discharging to a controlled point, and the surrounding area should fall toward the bund, not away from it.
• Material incompatibility. The polymer or coating has to suit the specific liquid and concentration, not just the broad class. A bund fine for diesel may not suit a concentrated acid; confirm compatibility against AS 3780 for Class 8 corrosives and the substance's safety data sheet before you commit.

Put those on a simple inspection schedule — a walk-around that checks the sump is empty of rain, the valve is closed, the walls are sound and the labelling is legible — and the bund you sized on paper keeps protecting the site in practice. If you'd rather we shortlist compatible containers and spill bases for your reagents, use the guided product selector and tell us the substance.

What containers go on the bund?

Whatever you're storing — reagent, diesel, oil or non-hazardous liquid — the container on top of the bund should be solid-walled, rigid and easy to inspect. Vented bins are for produce and airflow, not liquids; for anything that can leak you want a closed, solid-walled bulk container or IBC that sits squarely on the spill base. Folding solid IBCs are popular on multi-site operations because they collapse for return freight when empty, cutting the cost of getting containers back to a remote yard.

A folding solid ISO-footprint bulk container is a good default for reagent decanting and non-hazardous liquids on a bunded base: it nests onto a standard spill pallet, stacks when full, and folds flat when empty.

Browse the full range of solid and folding IBCs in IBCs and bulk containers, see bunded and reagent options across the mining range, or compare the load figures that decide what can be stacked and racked in plastic pallet load ratings: static vs dynamic vs racking. Tell us your dangerous goods on the quote form and we'll spec compliant containment.

Practical sizing checklist

• Add, don't pick. 100% of the largest container plus the aggregate percentage — never the greater of the two.
• Count everything on the bund when you total the aggregate, not just the biggest unit.
• Subtract displacement of drums, frames, pallets and plinths inside the bund.
• Add rain margin on open outdoor bunds, and drain captured water before it steals capacity.
• Match the material to the class — HDPE for Class II/III combustibles and rated corrosives; non-combustible for Class I flammables.
• Separate incompatibles — keep corrosives and flammables on different bunds.
• Specify UV-stabilised polymer for any bund that lives outdoors.

Common questions

Is the "110% of the largest container" bund rule correct?

No. It is a rule of thumb that happens to land near the right answer for a single container, but AS 1940:2017 actually requires 100% of the largest container plus 25% of the aggregate storage (up to 10,000 L). For a bund holding several IBCs, the real requirement is significantly higher than 110%.

How big does a bund for one 1,000 L IBC need to be?

Under AS 1940:2017, at least 100% of 1,000 L plus 25% of the 1,000 L aggregate — so about 1,250 L of usable containment, before adding any rain margin for outdoor units and subtracting the displacement of the IBC frame and pallet inside the bund.

Can I use a plastic (HDPE) bund for flammable liquids?

Not for Class I flammable liquids such as petrol — those need non-combustible, fire-rated containment. HDPE bunded pallets and spill bases are suited to Class II/III combustible liquids like diesel and oils, and to many corrosives where the polymer is rated for the substance (check against AS 3780 for Class 8).

Does the IBC and pallet inside the bund reduce its capacity?

Yes. Any solid object standing in the bund — the IBC cage, its pallet, drums or plinths — displaces liquid volume, so the usable containment is less than the empty bund’s geometric volume. Always subtract that displacement when you size the bund.

Does the 25% proportion change for larger storage?

Yes. The 25% applies to aggregate storage up to 10,000 L. Above that it steps down — roughly 10% of the aggregate between 10,000 L and 100,000 L, and about 5% above 100,000 L — always on top of 100% of the largest single container. Confirm the current tiers against the standard for your storage volume.

Source: AS 1940:2017 The storage and handling of flammable and combustible liquids (spill-compounding provision), Standards Australia; Safe Work Australia hazardous-chemicals guidance; AS 3780 for corrosive substances; Australian Dangerous Goods (ADG) / UN class system. Capacity tiers and worked figures are general guidance for sub-10,000 L storage — always verify against the current standard and your state's dangerous-goods regulations. This is not a compliance certification or a quote.