23 ECT board: what it holds

23 ECT board carries 23 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 370 lbf.

Estimated box compression by flute

Simplified McKee BCT, lbf, empty box
Inside size, in	B flute	C flute	E flute
8 × 6 × 4	253	282	179
10 × 8 × 6	286	320	203
12 × 12 × 12	331	370	234
14 × 10 × 8	331	370	234
16 × 12 × 10	357	399	253
18 × 12 × 12	370	413	261
18 × 18 × 18	405	453	286
20 × 16 × 14	405	453	286
24 × 18 × 18	437	489	309
24 × 24 × 24	468	523	331

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, 23 ECT)
Inside size, in	0 days	10 days	30 days	90 days	180 days
8 × 6	282	178	169	155	141
10 × 8	320	202	192	176	160
12 × 12	370	233	222	203	185
14 × 10	370	233	222	203	185
16 × 12	399	252	240	220	200
18 × 12	413	260	248	227	207

Neighbouring grades

Grade	ECT change	BCT change, same flute and size
26 ECT	+3 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.