Corrugated boxes explained: 3-ply vs 5-ply vs 7-ply, BF, GSM and burst strength

Corrugated boxes are the most common shipping package in the world, and also one of the easiest to get wrong. Order a box that is too weak and products get crushed in stacked storage or rough transport. Order one that is too strong and you pay extra on every shipment, plus extra freight for the weight.

The key is to describe the box in the same terms the box maker uses. This guide explains those terms.

What a corrugated box is made of

Corrugated board is a sandwich of paper layers:

• Liners: the flat outer and inner sheets.
• Fluting (medium): the wavy layer glued between liners, which gives the board its stiffness and cushioning.

The paper is usually kraft paper, made from virgin fibre, recycled fibre or a mix. Virgin kraft is stronger and more moisture-resistant; recycled paper is cheaper.

3-ply, 5-ply and 7-ply

"Ply" is the number of paper layers:

• 3-ply (single wall): two liners and one flute. For light products, retail shipping cartons and inner packs.
• 5-ply (double wall): three liners and two flutes. For heavier goods, export cartons and stacking in warehouses. This is the most common industrial choice.
• 7-ply (triple wall): four liners and three flutes. For very heavy or fragile goods such as machinery parts, auto components and bulk industrial items.

Flute types

Flutes come in different heights, labelled by letter:

• A flute: tallest, best cushioning and stacking strength.
• C flute: a common all-rounder.
• B flute: lower, with good puncture resistance and a flatter surface for printing.
• E flute: thin, used for retail boxes and printed cartons.

Double-wall boards often combine two flutes, such as B and C (BC flute).

Paper GSM and BF

Two numbers describe the paper used for each layer:

• GSM (grams per square metre) is the paper's weight. Heavier paper is generally stronger.
• BF (bursting factor) describes the paper's strength relative to its weight. Higher BF paper is stronger at the same GSM.

A box specification often lists each layer, for example "top liner 150 GSM 22 BF, flute 120 GSM 18 BF..." Asking suppliers to quote on the same GSM and BF for each layer is the easiest way to compare quotes fairly.

Burst strength vs compression strength

These are two different kinds of strength:

• Bursting strength (kg/cm²) measures how much pressure the board withstands before it ruptures. It relates to rough handling and puncture.
• Box compression strength (BCT) measures how much weight a box can carry on top before it collapses. It relates to stacking in warehouses and containers.
• Edge crush test (ECT) measures the board's resistance to crushing on its edge, a key input to compression strength.

For most shipping and warehousing, compression strength is the more important number, because boxes fail from being stacked. Humidity matters too: corrugated board loses a large share of its strength in damp conditions, so allow a safety margin for monsoon storage and sea freight.

Working out what you need

Answer these questions before you ask for quotes:

1. What does the packed box weigh?
2. How high will boxes be stacked, and for how long?
3. What transport will it go through: local truck, courier, sea container?
4. What humidity and temperature will it face?
5. Does the product support the load itself (like canned goods) or does the box carry it all (like soft goods)?

From the stacking height and weight, the box maker can calculate the compression strength you need and suggest a board.

Box styles

• RSC (regular slotted container): the standard box with four flaps top and bottom. The most economical.
• Die-cut boxes: shaped boxes with lock bottoms, handles or windows, often for retail or e-commerce.
• Telescopic boxes: separate lid and base.
• Inserts and partitions: to separate bottles, glassware or parts.

Printing

Corrugated boxes can be printed by flexo printing directly on the board (economical for one to three colours) or with litho-laminated sheets for high-quality graphics. More colours and full coverage raise the cost.

Tests to ask for

• Bursting strength test
• Edge crush test and box compression test
• Moisture content of the board
• Dimensional check (inside dimensions, which is what matters for fitting your product)
• GSM check of each layer

A specification template

1. Inside dimensions (length × width × height) in mm.
2. Box style (RSC, die-cut, etc.).
3. Ply and flute combination.
4. Paper for each layer: GSM, BF, virgin or recycled.
5. Required bursting strength and/or compression strength.
6. Printing: colours and coverage.
7. Quantity per order and annual volume.
8. Packing (bundles, pallets) and delivery location.

Frequently asked questions

Is 5-ply always stronger than 3-ply?

Generally, a 5-ply board is stronger than a 3-ply board made from similar paper. But a well-specified 3-ply with strong paper can outperform a 5-ply made from weak paper, which is why GSM and BF matter as much as ply.

What is a good bursting strength for export cartons?

It depends on the product weight, stacking and transport. For export, compression strength usually matters more than burst strength. Give the box maker your packed weight and stacking height and ask them to calculate the board you need.

Why do boxes collapse in the monsoon?

Corrugated board absorbs moisture from humid air and loses much of its strength. Use stronger board, moisture-resistant coatings, or better storage conditions in humid seasons.

Should I specify inside or outside dimensions?

Inside dimensions. That is what determines whether your product fits. Box makers work from inside dimensions and add allowances for board thickness.

Source boxes on GlobalMart

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