Flute is the wavy middle layer of a piece of corrugated board. Peel back the outer sheet of any shipping box and you will see it: a continuous row of arches glued between two flat liners. Those arches are the whole trick. They hold the liners apart, resist crushing, and let a sheet of paper behave like a structural material.
Why the arch matters
Each flute is a small arch, and arches carry load by transferring it along the curve rather than bending. That is why a thin sandwich of paper can support a stack of boxes on a pallet. It also explains the two ways corrugated fails: crush the arches flat and the board loses most of its strength permanently, and let the board get damp and the arches soften. Neither is reversible, which is why crushed cartons in a warehouse are not simply cosmetic damage.
The flute letters
| Flute | Approximate thickness | Flutes per foot (approximate) | Common use |
|---|---|---|---|
| A | About 3/16 in | Around 36 | Maximum cushioning, fragile and light bulky goods |
| C | About 5/32 in | Around 41 | The general shipping carton, good all round strength |
| B | About 1/8 in | Around 49 | Good crush resistance, prints well, common for mailers |
| E | About 1/16 in | Around 90 | Retail ready boxes, printed mailers, folds neatly |
| F | About 1/32 in | Around 130 | Small retail packaging where the board must feel thin |
The letters are not a ranking. They were assigned roughly in the order each profile came into use, which is why A, B and C are not in size order. A finer flute gives a smoother printing surface and a tighter fold. A coarser flute gives more cushioning and more thickness for the same weight of paper.
Single, double and triple wall
- Single wall. One fluted layer between two liners. Most boxes you handle.
- Double wall. Two fluted layers with three liners, usually pairing two profiles such as B and C. Used for heavy contents, long transit and stacking.
- Triple wall. Three fluted layers, for industrial loads that would otherwise need a crate.
Combining profiles is deliberate. A BC double wall gets the printable, crush resistant surface of B on the outside and the cushioning of C beneath it.
Flute direction, and why box makers care
Corrugated is much stronger in compression along the flutes than across them. Stand a piece on end and press down and it resists. Lay it flat and press and it collapses. Boxes are therefore laid out so the flutes run vertically up the walls when the box is upright, which is what carries a stack. It also means a die cut panel that runs the wrong way to the flutes will bend more easily, a detail that shows up on large display panels and long lids.
What to ask for instead of a flute letter
Flute alone does not specify a box. Board is also described by an edge crush value or a burst rating, and those numbers, not the letter, tell you what the box can carry. The practical approach is to tell your supplier the packed weight, how the boxes are stacked, and how they travel, then let the spec come back in writing. We do exactly that on every quote. For structure and sizing, see custom corrugated boxes, and for the difference between corrugated and plain cardboard, read corrugated versus cardboard.
If you would rather skip the theory, send your product weight and dimensions to our quote form and we will recommend a flute and a board grade with the price.
