Carton Stacking Strength in the Warehouse: BCT, ECT, Humidity and Why Imported Cartons Collapse

Walk through many importers’ godowns in Bangladesh during the monsoon and you will see it: bottom cartons bulging, corners folded, stacks leaning against each other. The goods inside may still be fine, but crushed cartons cause damage, make stock counting harder, and in the worst case bring a whole stack down onto a worker. The usual reaction is to blame the Chinese supplier for weak boxes. Sometimes that is right. Often the carton was adequate for the voyage but not for weeks of storage in humid air under a tall stack, and the problem could have been prevented by specifying the right board and stacking the right way.

This guide explains how corrugated carton strength works, what the common test numbers mean, and how to specify cartons with Chinese suppliers and stack them sensibly in Bangladeshi warehouses.

How a carton carries load

A corrugated carton carries stacking load mainly through its vertical walls, especially at the corners, where the board is folded and supported. The fluted inner layer of corrugated board acts like a row of tiny columns between the outer liners. Squash those flutes, or weaken the paper, and the box loses its ability to hold weight.

Several things reduce stacking strength in real warehouses, and their effects combine:

• Humidity: paper absorbs moisture from the air and loses stiffness. Corrugated board in very humid conditions can lose a large share of its dry compression strength. Bangladesh’s monsoon humidity is exactly the condition that does this.
• Time under load: a carton that holds a weight for a short test may slowly creep and fail when the same weight sits on it for weeks.
• Stacking pattern: cartons stacked directly on top of each other in columns, corner over corner, are far stronger than interlocked or brick-pattern stacks, where corners sit over the weaker middle of the box below.
• Pallet overhang: cartons hanging over the pallet edge lose support at their strongest point, the corners and walls.
• Gaps between pallet boards: cartons sitting over wide gaps sag and weaken.
• Handling damage: a single dented corner can greatly reduce what that carton can carry.

BCT, ECT and burst: what the numbers mean

When you ask a Chinese carton supplier or product manufacturer about strength, you may hear several test names:

Test	What it measures	Why it matters to you
—	—	—
BCT, box compression test	The load a complete empty box withstands before collapsing	Most direct measure of stacking strength
ECT, edge crush test	The strength of the board on its edge	Main input to predict BCT; good for comparing boards
Burst (Mullen) test	Resistance of the board to rupture from pressure	Older spec, more about puncture and rough handling
Board grade and flute	Paper weights and flute type, such as A, B, C or E, single or double wall	How the supplier describes the construction

Packaging engineers often estimate BCT from ECT, board thickness and box perimeter using the McKee formula, a well-known relationship in the corrugated industry. You do not need to calculate it yourself, but it explains why a larger box made from the same board is not automatically weaker per box, and why thicker double-wall board makes such a difference.

How Chinese suppliers describe carton specifications

Chinese carton specs are often written in a shorthand of paper and flute types, for example describing a five-layer (double-wall) board, the kind of kraft liner used, and the flute combination. Many product factories simply order “export carton, five-ply” from their local carton maker without any strength figure. That is not enough if your goods will be stored for long periods in humid conditions.

When strength matters, ask your supplier to state in the contract or packing specification:

• Board construction: single or double wall, flute type, and liner type.
• Either a target BCT or ECT value, or at least the board grade the carton maker uses, so you can compare between shipments.
• Maximum gross weight per carton.
• Box dimensions that fit your pallets without overhang.

What strength you need depends on the product weight, how high you stack, how long goods stay in the warehouse and how humid it gets, so there is no universal number; engineers apply safety factors for exactly these reasons. If you are unsure, send us the product weight, carton size and your storage conditions, and ask us for a packing specification built around your goods.

Product support: not every load is carried by the box

Some products support their own weight, such as cans, rigid bottles or metal parts in tight cartons. Others offer no support at all: soft goods, bagged powders, loose plastic items. If the product carries part of the load, the carton can be lighter. If it carries none, the carton must do all the work, and a slightly underfilled carton with a gap at the top will crush quickly. Ask suppliers to fill cartons fully or add inner dividers and top pads where products are soft.

Stacking rules for a Bangladeshi warehouse

Good warehouse practice can recover much of the strength lost to humidity:

• Stack in columns, corner over corner, rather than interlocking, whenever stability allows. Use stretch film or straps for stability instead of interlocking.
• No overhang on pallets; choose carton sizes that fit your pallet footprint.
• Set and mark a maximum stack height for each product, based on experience and the carton strength. Many cartons carry a stacking limit symbol from ISO 780 showing the maximum number or mass that may be stacked.
• Keep cartons off the floor on pallets, away from walls where condensation forms, and out of any area that floods. See our monsoon preparedness guide.
• Improve ventilation and, where products justify it, reduce humidity in the storage area.
• Rotate stock so cartons do not sit under load for months. See FIFO vs FEFO.
• Use racking for slow-moving products so cartons carry only their own pallet load. Compare options in pallet racking vs floor stacking.

Receiving check: spotting weak cartons on arrival

The receiving dock is the best place to catch a carton problem before it becomes a warehouse problem. When a container is unloaded, staff can quickly check a few cartons per lot: press the top centre and corners to feel whether the board is soft or damp, look for crushed corners and bulging sides in the bottom tiers of the container, and note any cartons that are wet or stained. If the board is already soft on arrival, the lot should be stacked lower than usual or racked, and the problem reported to the supplier with photos so the next order can be packed in stronger board. Our guide to warehouse goods receiving covers the full inbound check.

Common mistakes and what they cost

• Interlocked stacking of weak cartons for months: bottom layer collapses and stacks lean.
• Pallet overhang to fit more cartons: corners unsupported, cartons fail early.
• Accepting “export carton” with no specification: board quality varies between shipments.
• Storing against damp walls: cartons absorb moisture and lose strength.
• Underfilled cartons of soft goods: top collapses under the next layer.
• Stack heights set by ceiling height instead of carton strength: avoidable damage and safety risk.

Bringing it together

Carton strength is partly a sourcing question and partly a warehouse question. Specify the board and maximum weight with your supplier, design carton sizes around your pallets, then store with column stacking, sensible heights and humidity in mind. Costs of upgraded packing vary with the product and order size, so ask us for a quote built around your specific shipments.

DE International helps importers specify export packing with Chinese suppliers, inspects packing before shipment and offers warehousing and door-to-door delivery in Bangladesh. Explore our services, browse the shop, use our China sourcing and buying agent service, or contact us. Related: carton size and CBM optimisation and warehouse layout planning.