Guide / Materials & board grades / Chapter 3.2
How strong is my box? Flutes, walls and load.
Box strength isn't one number. It depends on the flute, the number of walls, the liners and how the box is used. Here's how to judge it in plain terms.
The short answer
A box's strength comes from the flute (E, B, C or BC), the number of walls (single or double) and the liners on each side. Flute describes the board's structure, not a fixed load rating. We describe our board by flute as a practical starting point: E flute for light items and printing, B and C flute for everyday shipping and heavier contents, and BC double wall for heavy and stacked loads. Industry tests such as edge crush and burst strength measure different kinds of strength, but the best test is your real product on its real journey.
Three things that make a box strong
The flute
Taller flutes cushion better and, with similar liners, usually resist crushing better when stacked. Shorter flutes are thinner, fold neatly and print well. Our flute chapter compares E, B, C and BC flute.
The walls
Double wall board, with two flutes, is much stronger than single wall in stacking and crush resistance. It's also heavier and thicker.
The liners
The flat paper layers on each side of the flute carry much of the load. Heavier and stronger liners make a stronger board, even with the same flute.
Matching board to load
A general guide. Your product, how it's packed and how it travels all play a part.
| Board | Typical load | Typical use |
|---|---|---|
| E flute | Light to medium | Mailer and product boxes, retail packaging |
| B flute | Medium | Everyday shipping cartons and larger mailers |
| C flute | Medium to heavy | Stacked cartons, heavier contents |
| BC flute (double wall) | Heavy | Heavy goods, removals, freight, high stacks |
Strength terms you might hear
Packaging buyers and specifications sometimes use test names. These are general industry terms, and different suppliers report different ones.
- Edge crush test (ECT): how much force the board can take on its edge before it buckles. It's closely linked to how well a box stacks.
- Burst strength: how much pressure the board's surface can take before it ruptures. It relates to resisting punctures and rough handling.
- Box compression: how much weight a complete, empty box can carry on top before it collapses. It depends on the board, the box size and the design.
We describe our board by flute rather than by test values. If a retailer or customer asks for test results, talk to us before you order.
What weakens a box
- Moisture and humidity, which soften the board and reduce stacking strength.
- Overhang on pallets, where cartons hang over the edge and lose corner support.
- Empty space inside, which lets the box walls cave in. Use suitable inserts or filler to support the walls; fragile products shouldn't have to carry stacking loads.
- Handholes and cut-outs, which remove board from the walls.
- Poor sealing, which lets seams open under load.
What it means for you
The same question, answered for where your business is today.
Choose your stage at the top of the page to open the advice for you.
Start-up · under 100 units
You don't need test numbers. Choose a stock box in the flute that suits your product's weight, and make sure it fits well.
Pack your heaviest order and lift the box by its sides. If it bows or the base sags, choose stronger board.
Growing · 100 to 2,499 units
Look at where damage happens. If boxes crush in transit, try a stronger flute or better fill before moving to double wall.
Try this: stack five packed boxes for a day in your storage area and check the bottom one.
Enterprise · 2,500 units and more
Write the board by flute and wall, the product code and any agreed performance details or tolerances into your specification for every product and route, and test with your real loads. If your customers require a test value, agree it before ordering.
Account for humidity and stacking time in storage, not only transit.
Common mistakes
- 1Judging strength by thickness. Thickness helps, but liners and construction matter as much.
- 2Ignoring storage. Boxes stacked for weeks in a humid space lose strength.
- 3Overhanging pallets. Cartons that hang over the pallet edge lose much of their stacking strength.
- 4Upgrading board instead of fixing fit. A box that fits well with good filler often solves damage without heavier board.
Box strength questions
How is cardboard box strength measured?
How do you describe your board strength?
What does ECT mean?
Which board is strongest?
How do I know if my box is strong enough?
Keep reading
Shop by strength
Need help choosing board?
Send us your product's weight, how it's packed and how it travels. Our Melbourne packaging team will recommend the right flute, usually within 1 business day.