"Ply" is the single most important spec on a corrugated box, and it is often misunderstood. It does not describe how thick the paper is — it counts the number of paper layers in the board. Getting it right means your goods arrive intact without paying for strength you do not need. This guide breaks down how ply construction works, the role of flutes, GSM and bursting strength, and which ply suits courier, export and heavy-industrial shipping.
How ply construction works
Corrugated board is built from two elements: flat sheets called liners, and wavy sheets called fluting (or the medium). The fluting is glued between liners, and the arches of the flutes are what give the board its stiffness and cushioning. Ply simply counts these layers:
- 3-ply (single wall): two liners with one fluted layer between them — the everyday shipping box.
- 5-ply (double wall): three liners with two fluted layers — stronger and stiffer, for heavier or export loads.
- 7-ply (triple wall): four liners with three fluted layers — a near-crate level of strength for very heavy industrial goods.
Flute basics
Flutes come in standard profiles graded by height. Common ones are A-flute (tallest, best cushioning), C-flute (a versatile all-rounder), B-flute (shorter, firmer, better for printing and puncture resistance) and E-flute (very fine, for retail cartons). Taller flutes cushion better and resist stacking loads; shorter flutes give a smoother surface and hold print more crisply. Double-wall boards often combine two profiles — a BCcombination pairs C-flute cushioning with B-flute rigidity and print quality. At KR Thermopack, our corrugated boxes use B, C and BC flute combinations selected to match the box's job.
GSM and bursting strength
Two numbers usually accompany a ply spec. GSM (grams per square metre) is the weight of the paper used for the liners — heavier kraft paper generally means a stronger, more puncture-resistant box. Our corrugated boxes typically use kraft liners in the 100–180 GSM range, with heavier grades for export and heavy-duty work. Bursting strength measures the pressure the board withstands before it ruptures (reported in kgf/cm²), and is matched to the load the box must carry. A related measure, edge crush test (ECT), gauges stacking strength — how much weight can sit on top without the box collapsing. Rather than defaulting to a higher ply, the right approach is to specify GSM and bursting strength to the actual load; we can supply test reports so the spec is verifiable.
Which ply for which job
| Construction | Walls | Typical load | Best for |
|---|---|---|---|
| 3-ply | Single wall | Light — a few kg | Courier, e-commerce, retail, light FMCG |
| 5-ply | Double wall | Medium to heavy | Export shipments, stacking, heavier goods |
| 7-ply | Triple wall | Very heavy | Machinery, auto parts, industrial and bulk goods |
3-ply is the workhorse for courier and e-commerce — lightweight, low cost and strong enough for products of a few kilograms travelling a normal parcel route. Step up to 5-ply when goods are heavier, when boxes will be stacked several high on a pallet, or for export, where longer transit, multiple handling points and humidity all add stress. Reach for 7-ply for genuinely heavy industrial loads — machinery, automotive assemblies and bulk goods — where the box is doing structural work close to a wooden crate. The right choice is rarely just about weight; stacking height, transit distance and handling roughness all push the decision.
Frequently asked questions
What does "ply" mean in a corrugated box?
Ply counts the paper layers in the board. A 3-ply board is one fluted layer between two flat liners (single wall). 5-ply adds a second fluted layer for double wall, and 7-ply has three fluted layers for triple wall.
Is a higher ply always better?
No. Higher ply means more strength but more material cost and weight. Over-specifying wastes money and pallet space, while under-specifying risks crushing. The right ply is matched to product weight, stacking height and how rough the journey will be.
What is bursting strength?
Bursting strength measures how much pressure the board can take before it ruptures, typically reported in kgf/cm². It is one of the common ways corrugated board is specified and matched to the load a box must carry.