Before choosing a corrugated board grade, find out how your current boxes fail. If returns come back with crushed corners and bowed lids, the problem is compression, and the number to improve is edge crush. If they come back punctured or split along a panel, it is liner strength and burst resistance. If neither happens but postage is high, the board may be heavier than the job needs. Corrugated board grades are a set of trade-offs between those three outcomes, and a good specification names the one that matters.
Below: flute profiles, liner and fluting papers, the two strength tests that appear on most specifications, and how to write a grade into a quotation request. Box styles, print and ordering are covered on the main page for custom printed mailer boxes.
What makes up a corrugated board grade?
A corrugated board grade is defined by three things: the flute profile (the height and pitch of the wave), the papers used for the liners and the fluting medium (type and grammage), and the construction (single wall or double wall). Together they set thickness, stiffness, print surface, weight and strength.
Single-wall board has one fluted medium glued between two liners. Double-wall board has two fluted layers and three liners. Most mailers are single wall; double wall is used for heavy contents or long, rough transit.
Flute profiles for mailer boxes
| Flute | Typical thickness | Flutes per metre (approx.) | Character | Mailer use |
|---|---|---|---|---|
| F | 0.8-1.0 mm | 400 or more | Very fine; close to a thick carton board | Small premium boxes, letterbox depth |
| E | 1.1-1.8 mm | 290-330 | Fine; smooth print surface; good stiffness | Default for most mailers |
| B | 2.5-3.2 mm | 150-185 | Coarser; more cushioning and crush resistance | Heavier or fragile goods, larger mailers |
| C | 3.5-4.5 mm | 120-145 | Standard for shipping cartons | Rare for die-cut mailers |
| EB double wall | 4-5 mm | E plus B | Strong, thick, heavier | Heavy contents, export cartons |
Figures vary between corrugator lines and paper mills; use them to compare, not as tolerances.
Finer flutes have more waves per metre, which supports the outer liner more evenly. That is why E and F flute take direct print and fine detail better than B flute, where the flute tips can show as stripes ("washboarding") under solid colour. Taller flutes add thickness, which increases bending stiffness and stacking strength for the same paper weight, and give more cushioning against impacts.
Liner and fluting papers
The paper choice often matters as much as the flute. Main liner types:
- Kraftliner (K): mainly virgin fibre, brown, high strength per gram and good moisture resistance. The strongest option for a given grammage.
- Testliner (T): recycled fibre, brown or grey-brown, lower strength per gram, lower cost. Widely used, especially on the inside of boxes.
- White-top liner: a bleached white layer on a kraft or recycled base. White-top testliner is a common economical white print surface, sometimes slightly mottled; white kraftliner and coated white liners give cleaner, brighter print.
Fluting media are typically semi-chemical fluting (stiffer, often more virgin fibre) or recycled fluting (sometimes called Wellenstoff in German supply chains). Grammages for liners commonly run from around 100 to over 200 gsm and for fluting from around 90 to 150 gsm, though the range offered depends on the mill.
A typical E-flute mailer combines a white or brown outer liner chosen for print, a recycled fluting and a testliner inside. Upgrading only the outer liner improves appearance; upgrading liner grammage or moving from testliner to kraftliner raises strength.
How is the edge crush test (ECT) measured?
ECT measures the force needed to crush a small strip of combined board standing on its edge, with the flutes vertical, between two platens. The result is reported in kilonewtons per metre (kN/m) under ISO 3037, or in pounds per inch (lb/in) under TAPPI T 811 in North America. One lb/in equals about 0.175 kN/m, so a common US single-wall grade of 32 ECT is about 5.6 kN/m.
Board samples are conditioned before testing, normally at 23 °C and 50 % relative humidity, because moisture changes the result substantially. A board's ECT depends mainly on the compressive strength of its liners and fluting, which is why recycled papers with lower short-span compression strength give lower ECT at the same grammage.
Why ECT predicts stacking strength
In a stack, a box carries load down its vertical walls, so the board is loaded edgewise, exactly as in the ECT. Box compression strength (BCT, measured on a whole box) is therefore closely related to ECT, board thickness and box perimeter. The widely used McKee formula expresses this as BCT ≈ 5.87 x ECT x √(board caliper x box perimeter), in consistent units.
Three practical consequences follow:
- Doubling ECT roughly doubles predicted box compression strength.
- Thicker board (taller flute) raises compression strength even at the same ECT, through the caliper term.
- Larger boxes, with longer perimeters, carry more load in total, though not per unit of wall.
McKee is an estimate for regular boxes; die-cut mailers with rolled double side walls behave differently, usually stronger at the ends than a simple calculation suggests. For critical applications, test finished boxes for compression rather than relying on the formula.
How is burst strength (Mullen) measured?
Burst strength is the pressure needed to rupture the board when it is pushed from one side through a circular opening. In the Mullen-type test, a rubber diaphragm is inflated hydraulically under a clamped sample until the board breaks. Results are reported in kilopascals (kPa) under ISO 2759 or in pounds per square inch under TAPPI T 810.
Burst reflects the tensile strength and stretch of the liners more than the structure of the board. It correlates with resistance to puncture and to bursting from heavy contents shifting during handling, but says little about how a box behaves in a stack. North American carrier classification rules (UFC Rule 41 and NMFC Item 222) allow boxes to be certified by either a minimum burst test or a minimum ECT, and the box maker's certificate printed on brown cartons shows which method was used.
ECT vs burst: which should a mailer specification use?
For most e-commerce mailers, specify ECT as the main strength requirement, because stacking in pallets, cages and vans is the dominant risk. Add a burst or puncture requirement only where sharp or dense products, or rough sortation, make punctures a known problem.
| Question | ECT | Burst (Mullen) |
|---|---|---|
| What it measures | Edgewise compressive strength of the board | Pressure to rupture the board face-on |
| Main driver | Paper compression strength, flute and caliper | Liner tensile strength and stretch |
| Predicts | Stacking and crush resistance (via BCT) | Puncture and rupture resistance |
| Rewards | Efficient fibre use; recycled-content designs can still perform | Heavy, strong liners |
| Best for mailers when | Boxes are stacked or loaded in cages and vans | Contents are dense, sharp or heavy for the box size |
Humidity, time and safety factors
Corrugated board absorbs moisture from the air and loses compression strength as humidity rises. Between standard test conditions and very humid storage, a box can lose a large part of its stacking strength, often cited as a third or more. Load applied over days or weeks also causes creep, so a box that survives a short laboratory test may fail under long-term stacking.
Packaging engineers allow for this with a safety factor: the box's tested compression strength divided by the load it will actually carry. Factors of around 3 to 5 are commonly used for warehouse storage, higher for humid climates, long storage or pallets with overhang. For mailers that move quickly from fulfilment to delivery, lower factors are often acceptable; the decision should reflect your own supply chain.
Which board grade for which mailer?
| Use case | Suggested starting point | Notes |
|---|---|---|
| Apparel, soft goods under about 1 kg | E flute, testliner inside, white or kraft outer | Main risk is crushed corners in vans |
| Cosmetics, glass bottles, candles | E or B flute with insert | Insert controls movement; B flute adds cushioning |
| Letterbox boxes | F or E flute | Thickness limit dominates; see sizing guide |
| Heavier mailers (around 3-5 kg and above) | B flute or EB double wall | Consider kraftliner for strength per gram |
| Premium full-colour graphics | E or F flute, or litho-lam onto E/F | Fine flutes avoid washboarding |
| Long export transit, humid destinations | Higher-ECT grade, kraftliner, or double wall | Allow for humidity in the safety factor |
These are starting points. Final grades should be confirmed with samples and transit testing on your products. For pizza boxes, where heat, steam and grease change the choice, see E flute vs B flute pizza boxes. For letterbox depth constraints, see mailer box sizes and letterbox dimensions.
How to write a board grade into a quotation request
European suppliers usually describe board by flute and paper make-up, while US suppliers often quote ECT. To compare quotations, ask every supplier for both: the paper composition and a tested or calculated ECT, at the same conditions.
Include in your specification:
- Flute (for example E or B) and single or double wall.
- Outer liner type and colour (kraftliner, testliner, white-top, coated white) and grammage if known.
- Inner liner type and colour, especially if you plan inside print.
- Minimum ECT (kN/m or lb/in) and, if relevant, minimum burst (kPa).
- Any finished-box requirement: compression test result, drop test or transit test standard you use.
- Storage and transit conditions, including destination climate.
A supplier can then propose equivalent grades from the papers available to them without changing performance. For how print method interacts with board surface, see flexo vs offset vs gravure vs digital printing. When the specification is ready, send it with your size range through the custom printed mailer boxes page or contact us for a technical review.