55 ECT board: what it holds

55 ECT board carries 55 lb per inch of edge in an edge crush test. Built into a 12 × 12 × 12 in C flute box, the simplified McKee formula puts its compression strength near 884 lbf.

Estimated box compression by flute

Simplified McKee BCT, lbf, empty box
Inside size, in	B flute	C flute	E flute
8 × 6 × 4	604	675	427
10 × 8 × 6	685	766	484
12 × 12 × 12	791	884	559
14 × 10 × 8	791	884	559
16 × 12 × 10	854	955	604
18 × 12 × 12	884	989	625
18 × 18 × 18	969	1,083	685
20 × 16 × 14	969	1,083	685
24 × 18 × 18	1,046	1,170	740
24 × 24 × 24	1,118	1,250	791

Working load after storage

McKee gives the strength of a new, dry box. In storage it loses strength. Using Packgent's default storage losses (an assumption, see method), this is the most the bottom C flute box can carry at each age:

Allowed load on the bottom box, lbf (C flute, 55 ECT)
Inside size, in	0 days	10 days	30 days	90 days	180 days
8 × 6	675	425	405	371	338
10 × 8	766	482	459	421	383
12 × 12	884	557	530	486	442
14 × 10	884	557	530	486	442
16 × 12	955	602	573	525	478
18 × 12	989	623	593	544	494

Neighbouring grades

Grade	ECT change	BCT change, same flute and size
48 ECT	-7 lb/in	-13%

In McKee, BCT scales directly with ECT, so the percentage change in strength equals the change in ECT.

Freight rule limits

NMFC Item 222 ties each grade to a maximum gross weight and size for LTL shipping. Packgent has not published those limits yet because they could not be checked against the current tariff. See the Item 222 page for status.

Sources

TAPPI T 811: Edgewise compressive strength of corrugated fibreboard (short column test)
McKee, R.C., Gander, J.W. & Wachuta, J.R. (1963). "Compression strength formula for corrugated boxes." Paperboard Packaging 48(8): 149–159
Flute calipers: Foster (1997), Wiley Encyclopedia of Packaging Technology

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