Corrugated flutes and ECT: E, B, C and double wall explained
Corrugated board is named by its flute and rated by its strength. E-flute is about 1/16 inch thick and prints almost like a carton, B-flute is about 1/8 inch, C-flute about 5/32 inch, and BC double wall is a B and a C glued together. ECT, the edge crush test, says how much stacking load the board carries: under the carrier rules a 32 ECT single wall box is rated for up to 65 pounds and 44 ECT for up to 95. Pick the flute for how the box looks and packs, and the ECT for what goes in it.
Flutes, side by side
The flute is the wavy middle layer glued between two flat liners. More flutes per foot means a thinner, smoother board that prints and folds cleanly. Fewer, taller flutes mean more cushioning and more stacking height. The letters are historical, not a size order, which is why C sits between A and B.
| Flute | Thickness | Flutes per foot | What it is good at |
|---|---|---|---|
| E-flute | about 1/16 in | about 90 | Mailer boxes and retail shippers that are printed and opened on camera. Folds tight, prints full color. |
| B-flute | about 1/8 in | about 47 | Shipping boxes and heavier mailers. Good crush resistance, takes one or two color print. |
| C-flute | about 5/32 in | about 39 | Plain shipping cartons that get stacked. More cushion and top-to-bottom strength than B. |
| BC double wall | about 1/4 in | B plus C | Heavy, bulky or fragile loads, and anything that travels on a pallet. |
A-flute, about 3/16 inch, and F-flute, about 1/32 inch, exist too. At short printed runs they rarely beat E, B or C for printed boxes, so they are not part of our standard range.
What ECT and burst ratings mean
Two tests rate corrugated board. The edge crush test stands a strip of board on its edge and presses down until it buckles, which is what the bottom box in a stack goes through. The burst test, the older Mullen test, pushes through the face of the board. The carrier rules, Rule 41 and Item 222, accept either one and link each grade to a weight and size limit for the box.
| Board | ECT | Burst equivalent | Box and contents up to | Length + width + depth up to |
|---|---|---|---|---|
| Single wall | 32 | 200# | 65 lb | 75 in |
| Single wall | 44 | 275# | 95 lb | 95 in |
| Double wall | 42 | 200# | 80 lb | 85 in |
| Double wall | 48 | 275# | 100 lb | 95 in |
Those are the ceilings a box can be certified to, not a target. A box loaded right up to its limit and left in a humid warehouse will crush well before the number on the stamp, so we size the board with room to spare.
How to pick one
Start with what the box has to do, then the weight.
- It is the box the customer opens. E-flute, as in our kraft mailer box. It takes full color inside and out and folds into a crisp mailer. Under about 3 pounds it rarely needs more than an insert.
- It ships on its own and carries some weight. A corrugated shipping box in B-flute single wall at 32 ECT up to about 20 pounds, 44 ECT from about 20 to 40 pounds.
- It gets stacked or travels on a pallet. C-flute for plain stacking cartons, BC double wall at 48 ECT for anything over about 40 pounds or packed several to a pallet.
- It holds glass or ceramics. The flute matters less than the insert, which is why our bottle shipper box has partitions built in. A die-cut cradle that keeps the item off the walls does more than a heavier board.
Our box size calculator applies these same weight breaks and shows the outside size the flute adds, which is what the carrier bills on.
Where people go wrong
Ordering double wall for a light product. It costs more, weighs more and adds outside size, which can push the box into a higher dimensional weight. A snug single wall box with a good insert usually protects better.
Forgetting that the flute adds to the outside. A 10 by 8 by 4 inch box inside is about 1/4 inch bigger each way on B-flute and closer to 1/2 inch on double wall. That matters for shelf space, shipper cartons and carrier rates.
Expecting photographs on brown B-flute. Kraft and coarse flutes suit bold, simple one or two color print. For photography, go E-flute with a white top.
If you want the drawing first, every style here is in the free dieline library, and the Tria Studio shows the board and outside size for any size you type in.
Sources: flute sizes from the standard US flute table (Corrugated fiberboard); ratings and limits from the Rule 41 and Item 222 box maker certificate tables (summary).
Questions people ask
Is 32 ECT the same as 200# test board?
They are two different tests that the carrier rules treat as equivalent for the same box. ECT crushes a strip of board on its edge, which is what happens at the bottom of a stack. The burst test, the 200# figure, pushes through the face, which is closer to a corner hitting the box. For boxes that get stacked, ECT is the more useful number, and it is what most boxes are rated on now.
Does thicker board always mean a stronger box?
No. C-flute is thicker than B-flute, but a 32 ECT box is rated for the same load whichever flute it is made on. Thickness buys cushioning and stacking height. ECT and the liner weights are what set the load rating.
What is the stamp on the bottom flap?
That circle is the box maker's certificate. It states the board rating and the size and weight limits the box was built for, and the carriers use it when they judge a damage claim. If your product sits near the limit, ask for the next grade up.
Can I print full color on B-flute?
You can, but the flutes can show through heavy ink coverage as faint stripes, which the trade calls washboarding. For photographs and big solids, E-flute or a litho-laminated sheet looks far better. B-flute is at its best with one or two flat colors.
Should I just use double wall to be safe?
Only if the weight or the trip calls for it. Double wall is heavier, costs more, takes more room flat, and adds to dimensional weight because the box is bigger on the outside. For most products under about 40 pounds a well-sized single wall box with the right insert does the job.