Sump bulge
The lower container's bottle takes load through the base. Permanent deformation; the container is a rebottle candidate from then on.
Sizes / stacking & load ratings
Stacking rules, and why they exist
One principle, seven limits, four failure modes. This page exists because we take in dozens of containers a year that were destroyed by a stack somebody thought would be fine.
- Sheet
- 46
- Revision
- 2026-10
- Scale
- 1:1
- Drawn at
- Peoria IL
3High, empty
2High, filled & rated
1High, filled in transit
3–5×Dynamic vs static load
A composite IBC stacks on its cage, never on its bottle
Every stacking rule in this industry is a restatement of that single sentence. Once you internalise it, you can derive the rest yourself.
| Situation | Limit | Why |
|---|---|---|
| Empty, aligned cage posts | 3 high | Cage carries load in compression through the posts |
| Empty, misaligned | 1 high | Load path goes through the bottle, which deforms |
| Filled, rated units | 2 high | Static load rating on most 31HA1 designs |
| Filled, unrated or unknown | 1 high | No rating means no stacking, full stop |
| In transit, filled | 1 high | Dynamic loads are several times static |
| In transit, empty | 2 high with securement | Strapped and corner-boarded only |
| Racked on beams | Per rack rating | Check the pallet type supports beam loading |
- Static rating is measured standing still in a warehouse. Road and rail impose dynamic loads that can be three to five times higher.
- A bulged bottle has already yielded. It never goes back in a stack, at any height.
- Mixed heights in a stack is a guarantee of misalignment. Keep 275s with 275s.
- Rack decking matters: some one-piece plastic pallets are rated for floor stacking only.
What a bad stack actually does
Cage racking
The frame goes out of square, so nothing above it sits flat and the whole stack walks over a few weeks.
Popped welds
Load concentrated at a corner post joint. Repairable on a jig, but the container is out of service until it is.
Progressive creep
The failure nobody notices: a filled double stack slowly shortens over a month, then fails all at once.
If you only remember one number
Empty: three high. Filled: two high, and only if the unit is rated. In transit: one high filled. Everything else is somebody rationalising a shortcut.
Rated loads, and what the plate is actually telling you
| Rating | Typical 275 gal | Typical 330 gal | Where to find it | Applies when |
|---|---|---|---|---|
| Max gross mass | 1,100–1,250 kg | 1,350–1,500 kg | UN plate | Always |
| Stacking test load | 1,800–2,400 kg | 2,200–2,800 kg | UN plate or separate plate | Static, warehouse only |
| Permitted stack, empty | 3 high | 3 high | Convention, not the plate | Posts aligned, same height |
| Permitted stack, filled | 2 high | 2 high | Only if the plate states it | Static, rated units only |
| Permitted stack, in transit | 1 high filled | 1 high filled | Carrier practice | Always |
| Hydrostatic test pressure | 20–25 kPa | 20–25 kPa | Design type record | Type approval only |
- Dynamic load multiplier, road
- 3–5× static through potholes and hard braking
- Dynamic load multiplier, rail
- Higher still on coupling impacts
- Rack beam load, two full 275s
- ≈ 4,840 lb per bay — confirm the beam rating
- Floor loading, full 275
- ≈ 181 lb per square foot
- Floor loading, full 330
- ≈ 217 lb per square foot
- Typical mezzanine rating
- 125 lb per square foot
Rack, floor or ground — and the mistake in each
| Method | Capacity | Advantage | The mistake people make |
|---|---|---|---|
| Pallet rack, beam supported | 2 per bay | Dense, valve access on every unit | Pallet not rated for beam support |
| Pallet rack, full decking | 2 per bay | Works with any pallet type | Deck rating ignored |
| Floor stack, empty | 3 high | Free, fast | Mixed heights, misaligned posts |
| Floor stack, filled | 2 high max | No equipment needed | Unrated units, or creep unnoticed |
| Bare ground outdoors | 1–2 high | None | Base-rail rust, the one unrepairable failure |
| Dedicated tote rack | 2–3 high | Designed for the job | Rarely cost-justified under 100 units |
- Store valve-side out. Finding a weep from the aisle is a gasket change; finding it behind a stack is a day.
- Never store a bulged container in a stack, at any height. A bulged bottle has already yielded and will not carry load again.
- Walk a filled double stack weekly and look for creep — a stack that has shortened an inch is a stack about to fail.
- Keep containers off bare ground. Base-rail corrosion climbs the posts and takes the UN plate with it.
- Never slot a container into a rack on an unsupported pallet edge. Full decking or a rated pallet, nothing in between.
Can I stack two filled containers if the plate says 2,400 kg?
In a warehouse, standing still, on level concrete, with the posts aligned — yes, that is exactly what the rating is for. In a trailer, no, because dynamic loads run three to five times higher and the rating was never measured under those conditions.
How do I know whether my container is rated for stacking?
The stacking load appears on the UN plate or on a separate plate near it. If it is not stated, treat the container as unrated and single-stack it when full.
Is three high safe for empties?
With matching heights and aligned posts, on level ground, yes — it is standard practice everywhere. Misalign them and the third tier becomes a lever on the bottom container's bottle.
What about stacking on top of drums or crates?
No. A composite IBC needs its corner posts landing on something that can take point loads at those four positions. Drums and crates do not provide that.
Three high, posts aligned
This is the rule working: identical heights in every stack, corner posts landing on corner posts, valves facing the aisle so a weep is visible without moving anything. Mix a 330 into a column of 275s and the upper container's posts land between the lower one's — straight into the bottle shoulder, which is how a sump gets permanently deformed.