Understanding Board Grades

Corrugated containers are constructed with two main components: linerboard and medium. Linerboard is the flat facing that adheres to the medium. The medium is the wavy, fluted paper in between the liners (see below – C Flute). The following are four types of combined board. Singleface consists of one medium glued to one flat sheet of linerboard. Singlewall is made of one medium glued between two sheets of linerboard. Doublewall is three sheets of linerboard with two mediums in between and Triplewall is three mediums in between four sheets of linerboard.

Different board strengths are achieved through various linerboard and medium combinations (see common grades below). Additionally, different board thicknesses are obtained by altering the flutes, which are the wavy arches anchored in between the inside and outside liners. The most popular flute profiles are B and C for Singlewall and BC for Doublewall (see pictures below).

B Flute
C Flute
BC Flute
Comparison of B,C and BC Flutes

There are two main ways to measure the strength of corrugated fiberboard. ECT (Edge Crush Test) measures the force the board can withstand by applying pressure to the board with the flutes vertically aligned. This simulates the board’s stacking strength and performance.  Mullen testing measures the material’s Burst Strength, or ability to withstand forces at pressure points. Some corrugators prefer to grade their boards using ECT while others prefer to use Mullen grades. While there is no direct relationship, general equivalencies and industry standards have been established to cross-reference and relate these two measurement systems. See table below:

Bursting Test
Edge Crush Test (ECT)
Singlewall 12523
Singlewall 15026
Singlewall 17529
Singlewall 20032
Singlewall 25040
Singlewall 27544
Singlewall 35055
Doublewall 20042
Doublewall 27548
Doublewall 35051
Doublewall 40061
Doublewall 50071
Doublewall 60082
Triplewall 70067
Triplewall 90080
Triplewall 110090
Triplewall 1300112
Singlewall and Doublewall(psi)
Triplewall(inch oz. per inch of tear)
(lbs per inch width)

Source =Fiber Box Handbook published by the Fibre Box Association www.fibrebox.org