Corrugated Flute Types Chart: A, B, C, E, F and Double Wall Compared
C flute (about 5/32 in thick, 39 to 42 flutes per foot) is the standard shipping box board; B flute (1/8 in, 47 to 50 per foot) is the crush-resistant board behind literature and die-cut mailers; E and F flute are the thin, print-friendly boards for retail boxes; BC double wall pairs B and C for 100 pound loads. The chart gives thickness in inches and millimetres, flutes per foot and the stocked boxes on each board.
Flute chart: thickness, flutes per foot and use
Corrugated board is a fluted medium glued between flat liners. The flute profile sets the board thickness, how it prints, how it folds, and how it behaves under load. Larger flutes (A, C) put more air and more paper height between the liners, which gives cushioning and vertical stacking strength; smaller flutes (B, E, F) pack more flutes into each foot, which gives crush resistance across the face and a smooth print surface. Double wall pairs two flutes, nearly always B and C, and triple wall adds a third.
| Flute | Flutes per linear foot | Board thickness | Board thickness (metric) | Where it is used |
|---|---|---|---|---|
| A | 33 to 36 | 3/16 in | 4.5 to 4.8 mm | Thickest single flute; best cushioning and vertical compression; rare in stock boxes today |
| B | 47 to 50 | 1/8 in | 2.5 to 3.2 mm | Dense flutes resist puncture and crush; flat print surface; die-cut mailers, literature mailers, cans and heavy retail packs |
| C | 39 to 42 | 5/32 in | 3.6 to 4.0 mm | The default shipping box flute; balance of stacking strength and cushioning; about 80 percent of RSC boxes |
| E | 90 to 94 | 1/16 in | 1.1 to 1.6 mm | Micro flute; fine print, thin walls; folding cartons, mailer boxes, retail and cosmetics packaging |
| F | 125 to 128 | 1/32 in | 0.75 to 0.8 mm | Finest flute; near-paperboard feel with corrugated rigidity; premium retail and small mailer boxes |
| BC (double wall) | B + C | 1/4 to 9/32 in | 6.5 to 7.0 mm | Two flutes, three liners; 42 to 82 ECT grades; heavy, fragile and LTL shipments |
| Triple wall | three flutes | 1/2 to 5/8 in | 12 to 15 mm | Three flutes, four liners; 67 to 100 ECT; gaylords, bulk bins, machinery |
The lettering follows the order in which the profiles were introduced, not their size, which is why B is thinner than C and E is thinner than both. A flute is now uncommon in stock shipping boxes because C flute delivers most of its stacking strength at less thickness and cost.
What Packrift stocks by board
| Board | Stocked products | Typical grades | Examples |
|---|---|---|---|
| C flute single wall (RSC shipping boxes, cube boxes, long boxes) | 2,114 | 32 ECT and 44 ECT | 4x4x4 ECT-32 Kraft Cube Corrugated Shipping Boxes 25-Pack; 6x4x4 ECT-32 Kraft Corrugated Shipping Boxes 25-Pack |
| B flute single wall (literature mailers, die-cut mailers) | 249 | 32 ECT | 7 x 3 5/8 x 2 1/8 in ECT-32-B Kraft Corrugated Literature Mailers 50-Pack; 12.12x9.25x4 White Literature Mailers 20/Bundle |
| BC double wall | 197 | 48 ECT, 51 ECT, 71 ECT | 8x8x8 ECT-48 Kraft Double Wall Corrugated Boxes 15-Pack; 6x6x6 ECT-48 Kraft Heavy-Duty Corrugated Double Wall Shipping Boxes 15-Pack |
| Triple wall | 11 | 90 ECT | 24x36 ECT-90 Triple Wall Corrugated Sheets 5-Pack; 36x36 ECT-90 Triple Wall Corrugated Sheets 5-Pack |
Grade and flute are separate choices. A 32 ECT rating can be made on B or C flute; the rating tells you the edge-crush strength, the flute tells you how the box feels and folds. The weight capacity by ECT page covers the ratings, and the ECT to Mullen chart covers the two test scales.
How to tell flutes apart on a box you already have
- Measure the edge. Press a ruler against a cut edge. About 1/8 in is B, about 5/32 in is C, 1/4 in or more with two visible layers of flutes is BC double wall.
- Count the flutes. Mark one inch on the edge and count the peaks: 3 is C, 4 is B, 7 to 8 is E, 10 or more is F.
- Look at the score lines. B and E flute fold on crisp lines; C flute shows a wider, softer crease.
- Read the certificate. The box maker's stamp on the bottom flap states single, double or triple wall and the grade, though not always the flute letter.
Choosing a flute
| If the shipment is | Choose | Why |
|---|---|---|
| A standard parcel under 65 lb | C flute, 32 ECT | Meets carrier minimums, best cost per box, good cushioning for the thickness |
| A book, catalog, frame or flat electronics in a mailer | B flute literature or die-cut mailer | Crush resistance across a flat face, tighter fold lines, clean print |
| Retail or subscription unboxing | E flute mailer box | Thin walls keep the outer size small and the print sharp; not for heavy contents |
| Dense, sharp or fragile contents, or LTL freight | BC double wall, 48 ECT or higher | Two flutes absorb impact and carry stacks; puncture resistance from the extra liner |
| Bulk bins, gaylords, machinery | Triple wall | 300 lb gross ratings and forklift-tolerant walls |
| Long items (48 in and over) | C flute 44 ECT or BC double wall | Long panels bow; the higher grade keeps the box square |
Flute direction matters as much as flute size. Flutes run vertically in a properly made RSC so the fluted columns carry the stack; a box cut with flutes running horizontally, which happens with some die-cut mailers, loses much of its compression strength and should not sit at the bottom of a pallet. The box styles chart shows which styles are cut which way, and the single wall vs double wall guide compares the two most common boards head to head.
Print, fold and cost effects
Fine flutes print better because ink sits on a flatter surface with fewer washboard marks; E and F flute take fine text and halftones, C flute takes solid blocks and large type, B flute sits between. Fine flutes also fold to tighter tolerances, which is why die-cut mailer boxes use E and B. On cost, C flute is the least expensive board per square foot at a given ECT, B flute costs slightly more because it uses more medium per foot, and E and F cost more per square foot but save on shipping volume because the finished box is thinner. Double wall roughly doubles the board cost of single wall at the same footprint but raises the gross-weight ceiling from 65 to 100 pounds or more.
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Frequently asked questions
What is the most common corrugated flute?
C flute. It is used for the large majority of regular slotted shipping boxes because it balances stacking strength, cushioning and cost at about 5/32 in thick.
Which is thicker, B flute or C flute?
C flute, at about 5/32 in (3.6 to 4.0 mm), is thicker than B flute at about 1/8 in (2.5 to 3.2 mm). B flute has more flutes per foot and resists crush across the face; C flute stacks better.
What is E flute used for?
Retail and mailer boxes where print quality and a small outside size matter: cosmetics, subscription boxes, electronics accessories. At about 1/16 in it is too thin for heavy or sharp contents.
Is double wall always BC flute?
Nearly always in stock boxes. Some specialty double wall uses EB or AC combinations, but BC is the standard and is what the 48 ECT and 51 ECT stock boxes are made from.
Does flute type change the ECT rating?
No. ECT is a property of the finished board and is tested directly. The same 32 ECT rating can be reached on B or C flute; the flute changes thickness, cushioning and print, not the certified rating.
Related references
- Corrugated box weight capacity by ECT
- Corrugated box styles chart
- ECT to Mullen conversion chart
- Single wall vs double wall boxes