Which figure matters for a stackable container, one case or the whole stack?

The bottom one. Whether a stack stands is decided by pressure under the lowest case, so confirm any single-case figure was applied over a plate, not a point.

How tall must a lid boss be to interlock?

Height follows horizontal displacement in transit, not case size: cover the largest expected shift plus a wear allowance, and cut the recess deeper so a warped base still drops in.

Why do two suppliers quote different layer counts for the same case?

Almost always a different basis: temperature, duration, permitted deflection, or how area was measured. Align all four before comparing.

Can a published five-tier rating be used as it stands?

Only if its support surface and load spread match yours. Floor, pallet and shelf present different effective areas, so one stackable container earns a different count on each surface. ---

Stackable Container Stacking: Load, Interlock, Stability

Quick answer: A stackable container holds a stacking rating only when three quantities are fixed together at once: contact pressure under the bottom layer, constraint between layers, and the stack centre of gravity. A usable sheet states the allowable static load at a named temperature, duration and deflection, names the contact area basis, and names the support surface the count assumes. Remove any one of these and the published figure is only an estimate.

Three variables in stacking design: load, interlock, gravity
Interlock geometry between lid and base
Reading static load and creep values
Checking the layer count
Stacking fields a specification should carry
FAQ

1. Three variables in stacking design: load, interlock, gravity

Stacking capacity is not one load figure but three variables resolved together. Wall thickness serves the first alone. A lid and base with no shape match leave the second at zero, and a bottom case set off centre spends the margin of both.

Shallow case foam shows four round pockets beside square cut-outs

The bearing face sits on the rim of a stackable container. Here the rim is a stepped edge and only that narrow land meets the base above, while a relief cut for the catch interrupts the ring. A wider cut shortens the perimeter and raises pressure at the same load.

2. Interlock geometry between lid and base

A stackable container interlocks through three dimensions: lid boss height, base recess depth, and clearance per side. Together they set how far a layer can shift horizontally, a different question from how many layers may be stacked.

Black tool case opened to a moulded tray, every recess left empty

The stackable container base boss is not a part to move freely. In the photograph one moulded tray runs almost to the wall, so shifting the boss would force a new mould and the liner can only fill the space left. Boss height follows the largest transit displacement plus a wear allowance, and clearance is quoted per side or total, never bare.

3. Reading static load and creep values

A stackable container static load means nothing without temperature, duration and permitted deflection. Change one and two suppliers stop being comparable; a short-term test figure used as a warehouse value is the commonest misreading here.

Five blue cases stacked straight on the floor, an open one holds bearings

A press test runs hours; warehouse stacking runs months. Plastic creeps under standing load, quickly then slowly, so conversion needs a creep curve rather than a straight line. Five stackable containers stand directly on the floor in the photograph, so the lowest carries four; area comes from the feet, not the footprint, and a pallet deducts for board gaps.

4. Checking the layer count

The stackable container layer count reduces to one inequality: pressure at the bottom must stay below the allowable pressure for the stated temperature and duration.

White case closed on a steel table, shelves of bagged packs behind

Measure before calculating. The case shown sits closed on a bench, its folding handle and two catches carrying no load, and corner scuffs show that corners touch first, so a worn pad gives less area than the drawing.

Gross weight at its permitted maximum.
Area as the smaller of feet or rim land.
Layers above multiplied by gross weight.
Allowable pressure with its three qualifiers.
Divide, round down, drop one tier.

On a ramp, a vehicle or a pallet, drop one more tier.

5. Stacking fields a specification should carry

A stackable container sheet should let a reader run the check unaided; eight fields suffice.

Field	Unit	Entry must state
Allowable static load	kg	Temperature, duration, deflection
Case weight	kg	Same build as the load
Contact area	cm²	Feet or rim land
Lid boss height	mm	From the highest point
Base recess depth	mm	Same datum as the boss
Clearance	mm	Per side or total
Layer count	tiers	Support surface, load spread
Permitted deflection	mm	Measuring point

Ask of any figure: at what temperature, for how long, with what deflection, over which surface. Two of four answers make a row decoration.

Conclusion

Confirm the qualifiers behind the load, confirm how area was measured, divide bottom pressure by allowable pressure, then deduct margin for the support surface. A stackable container figure without its basis is not yet a figure.

This article covers container-side specifications and design implications only.