A flute: thickness, flutes per foot and strength

A flute is the tallest common flute, about 3/16 in thick with roughly 33 flutes per foot. Its height gives the most stacking strength and cushioning per ply, at the cost of crush resistance and a coarse print surface.

Key numbers

Construction	Single wall: one medium, two liners
Nominal caliper	0.1875 in · 4.8 mm
Flutes per foot	33 ± 3 (108 per metre)
OD minus ID, length and width	0.375 in
OD minus ID, depth	0.563 in
McKee strength vs C flute, same ECT	110%

Strength ratio = √(caliper ÷ C-flute caliper), from the McKee formula with ECT and perimeter held equal.

Trade-offs

Thickest single wall, so the best McKee estimate at a given ECT
Tall flutes crush more easily under flat pressure, which can cost some of that strength on a corrugator or in a clamp truck
Less common in US box plants than B or C; check availability before you specify it

Typical uses

Fragile goods that need cushioning from the board itself
Inner plies of triple wall bulk bins (AAC)

Estimated box compression at 10 sizes

Simplified McKee BCT, lbf, empty box, by inside size and ECT
Inside size, in	23 ECT	29 ECT	44 ECT	55 ECT
8 × 6 × 4	309	390	592	740
10 × 8 × 6	351	442	671	839
12 × 12 × 12	405	511	775	969
14 × 10 × 8	405	511	775	969
16 × 12 × 10	437	552	837	1,046
18 × 12 × 12	453	571	866	1,083
18 × 18 × 18	496	625	949	1,186
20 × 16 × 14	496	625	949	1,186
24 × 18 × 18	536	676	1,025	1,281
24 × 24 × 24	573	722	1,096	1,370

BCT ≈ 5.87 × ECT × √(caliper × perimeter), perimeter = 2(L + W) of the inside size. An estimate; confirm with ASTM D642. Grades shown are ECT values Packgent computes for this wall type, not a list of what mills sell.

Sources

Last reviewed 2026-10-27. See all sources and how Packgent computes.