Boxes are not all the same. The flute profile, number of walls, face-paper weight and edge-crush strength determine load, stacking and impact resistance — the wrong board makes a heavy box blow out at the bottom or crush the cartons underneath.
How corrugated board is built
A corrugated sheet is face paper plus a wavy fluted core (and an inner liner). Most boxes are single wall (3-ply) or double wall (5-ply); heavy items use triple wall (7-ply).
Common flute profiles
| Flute | Thickness (approx.) | Character | Typical use |
|---|---|---|---|
| E-flute | 1.5–2mm | Finest flutes, flat surface for printing, lower load | Retail/colour boxes, inner packs, small items |
| B-flute | ~3mm | Puncture-resistant, thin and rigid | Cans, bottles, light-to-medium shipping |
| C-flute | ~4mm | Balanced cushioning and stacking, most versatile | The majority of shipping cartons |
| BC double wall | ~6–7mm | Two flutes: strong load, stacking and impact | Heavy, fragile, long-haul and sea freight |
3-, 5- or 7-ply
| Walls | Best for | Indicative load (approx.)* |
|---|---|---|
| 3-ply (single wall) | General merchandise | Up to ~10–15kg (size-dependent) |
| 5-ply (double wall) | Heavy, fragile, stacking | ~15–25kg |
| 7-ply (triple wall) | Bulky items, wood-crate replacement | ~25kg+ |
Strength specs to read
- ECT edge crush test (kN/m): stacking and crush resistance; a common single-wall carton is ~32 ECT, heavy duty is 44–55 ECT.
- Mullen burst (psi): puncture and rupture resistance; North American boxes are often dual-rated “200# / 32 ECT”.
- Face-paper weight (gsm): for the same flute, higher gsm is stronger.
- In Australia, just ask the supplier for flute, walls, ECT, max load and gsm.
What reduces real load capacity
- Dampness: a wet board can lose over 30% of its strength; keep cartons off the floor and protect against moisture in sea freight.
- Bigger boxes and shorter walls carry less; the same grade in a large box fails sooner.
- A bottom that is not H-taped, corners left hanging, or too much void all make the box fail early.
- The higher you stack, the more the bottom carton carries; put heavy boxes at the base and limit stack height.
Do not default to thicker is better: use single wall for light goods to save cost and freight, double wall only for heavy or fragile. Settle 2–3 core sizes from 30 days of weight data, then ask the supplier to match the flute to the load.
* Loads are indicative bands and vary greatly with box size, flute, gsm, sealing, moisture and stack height — this is not a load guarantee; use the supplier spec and your own testing. See Sources & method.
Choosing sizes and converging to 3–4 boxes is in boxes & poly mailers; full heavy/oversized requirements are in the heavy & oversized spec; the bottom H-seal is in tape & sealers.