Box guide
Corrugated Board Flute Guide
E, B, C and BC flutes explained in millimetres, with the jobs each board does best.
What is a flute in corrugated board?
Cut a corrugated box and look at the edge. Between the two flat sheets of paper you will see a wavy layer. That wavy layer is the flute, and the flat sheets on either side are the liners.
The flute is formed by running paper through heated, ridged rollers, then gluing it to the liners at the tip of each wave. The arches it creates are what give the board its stiffness and its spring.
Flutes are named by letter. The letter tells you the height of the wave, which sets the thickness of the board. The names came about in the order the flutes were invented, so they do not run in size order.
Flute height is only half the story. The number of waves in each metre also matters: more, smaller waves give a flatter face and better puncture resistance, while fewer, taller waves give more cushioning.
The flutes we use, compared
These are approximate figures. Exact thickness varies a little with the paper used for the liners and the flute.
Flutes per metre is counted along the board, across the waves. Double wall has two separate layers, so it is described by the pair of flutes rather than a single count.
| Flute | Approx. thickness | Flutes per metre (approx.) | Print surface | Cushioning |
|---|---|---|---|---|
| E flute | 1.5 mm | About 295 | Very smooth | Low |
| B flute | 3 mm | About 155 | Good | Medium |
| C flute | 4 mm | About 130 | Fair | Good |
| BC double wall | 7 mm | Two layers | Fair | High |
E flute: thin board for neat small boxes
E flute is the thinnest board we use, at about 1.5 mm. It has many small waves packed close together, so the surface is flat and smooth.
That smooth face makes E flute the best choice for detailed print and for boxes that must fold crisply. It is the usual board for small mailers, retail cartons and gift boxes, where the box is seen as much as it is used.
It gives less cushioning than thicker flutes, so pair it with a close fit or an insert for anything delicate. Our small mailing boxes in thin E flute show it at its best.
Because it is so thin, E flute also saves space in storage. A bundle of 100 flat E flute mailers takes up far less shelf room than the same number in C flute.
B flute: the all-round parcel board
B flute is about 3 mm thick. It sits in the middle of the range and balances stiffness, print quality and weight.
It folds well enough for mailers and lidded boxes, yet it is tough enough for everyday parcels. Many online shops use B flute for nearly all their orders.
B flute also resists puncture well because the waves are fairly close together. It is a sound default when you are not sure which board to pick, and our everyday shipping boxes made to order are often quoted in it.
A 305 × 229 × 152 mm parcel box in B flute, packed with around 3 to 5 kg of books or clothing, is a very typical order. It goes through a courier network without fuss when the fit is close.
C flute: more cushioning for medium loads
C flute is about 4 mm thick, with taller and wider waves than B flute. Those bigger arches give more cushioning and better resistance to being crushed from above.
It is the common choice for medium parcels, shipping cartons and storage boxes. The surface is less smooth, so fine print can show the line of the flutes. One or two colour logos print well on it.
For slotted cartons and warehouse stock, C flute is a good middle ground between B flute and double wall. Our regular slotted cartons in C flute are widely used for bulk packing.
The extra height of the waves also traps more air, which helps a little with knocks from the side. That makes C flute a sensible upgrade for items like boxed kitchenware that are not quite fragile enough for double wall.
BC double wall: two flutes for heavy jobs
BC double wall glues a B flute and a C flute together with a liner between them and one on each face. The result is about 7 mm thick.
Two layers of arches make the board far stiffer than any single wall. It handles heavy contents, stacking and rough handling much better, and it gives more protection to breakables.
The trade-off is weight, bulk and cost. Expect about 14 mm more on every outside measurement, and a board that does not fold as tightly on small boxes. Our guide on choosing single or double wall covers when the extra board pays off.
A good test is a stack. If the box will sit under four or five others of similar weight for weeks, BC is the board to start with.
How to pick a flute for your box
Work through these questions in order. The first one that clearly applies usually decides the board.
- Is the box heavy, stacked or carrying breakables? Choose BC double wall.
- Is it a medium parcel or a storage carton? Choose C flute.
- Is it an everyday parcel or a lidded box? Choose B flute.
- Is it small, printed in detail or sold on a shelf? Choose E flute.
- Still unsure? Tell us the weight and contents and we will name a board on your quote.
- Before a big run, pack a handful of samples with real stock and send them on a test trip.
Flutes, print and finishes
Print sits on the outer liner, so the flute underneath affects how sharp it looks. Smaller flutes give a flatter surface and crisper detail.
Full colour photographs and small text look best on E flute. B flute takes full colour well too. On C flute and double wall, bold logos and simple shapes work better than fine detail.
Finishes such as lamination, spot UV and foil also sit best on a flat surface. If you want a premium finish on a shipping box, consider E or B flute with an insert rather than double wall. Our custom printed boxes in any flute can be quoted in more than one board so you can compare.
Signs you have chosen the wrong flute
Feedback from customers and couriers will tell you when a board is not right. Look out for these signs, and change the flute on your next order if you see them.
Keep notes on damage reports by box size. A pattern of crushed corners on one size, but not others, points to that size needing a thicker board or a better fit.
- Corners arrive crushed or split: try a thicker flute
- Boxes bow when stacked in storage: move to C flute or double wall
- Fine print looks ridged or blurred: move to E or B flute
- Small boxes look bulky and fold badly: use a thinner flute
- Parcels fall into a dearer postage band: check if a thinner board will do
- Goods rattle even in a strong box: improve the fit, not the flute
Kraft and white liners
Every flute can be made with a brown kraft or a white outer liner. The flute stays the same; only the colour and finish of the surface change.
Brown kraft looks natural and pairs well with one or two inks. White liners make colours brighter and closer to what you see on screen. Some customers pick kraft outside and white inside, so printed messages inside the box stand out.
Liner colour does not change the thickness or the cushioning of the board. Choose it for the look you want, then pick the flute for the job the box has to do. If you are ordering plain boxes with no print at all, kraft is the usual choice.
Inside print works on either liner. A kraft box with a white inside gives a plain, low-key outside and a bright reveal when the lid opens.
