Edge crush test (ECT)
ISO 3037 / TAPPI T811. The compression test for assembled corrugated board — not the individual liners or medium. Why ECT predicts box compression strength and what typical values look like.
Edge crush test (ECT) measures the edgewise compression strength of a sample of corrugated board — the two liners bonded to the corrugated medium — as a complete assembled unit. It is the standard method for characterising the load-bearing capacity of corrugated board and the primary input to box compression strength calculations.
The governing standards are ISO 3037:2013 and TAPPI T811. These are board-level tests, not paper-level tests. A critical distinction: RCT and CMT test the flat paper components; ECT tests the finished sandwich.
How the test works
A sample of corrugated board is cut to standard dimensions — typically 25 mm × 100 mm for single-wall (the 25 mm dimension is perpendicular to the flutes, allowing each flute arch to contribute to the load). The sample edges are trimmed or waxed to prevent edge fibrillation from affecting the result. The sample is placed between parallel compression platens with the board edge (the 25 mm face showing the corrugated profile in cross-section) facing up and down, and compressed at a constant speed until peak load is reached.
ECT is reported in kN/m — kilonewtons per metre of board width. This normalises for sample width, allowing comparison across different sample sizes and board structures.
Why ECT predicts box compression strength
The McKee formula (and its variants) relates ECT to box compression test (BCT) — the actual force a filled, closed box can sustain:
BCT ≈ K × ECT × √(board caliper × box perimeter)
Where K is an empirical constant (~0.67 for regular slotted containers). The formula shows that ECT is the primary determinant of BCT. Doubling ECT roughly doubles the box compression strength (at constant geometry). Caliper matters too, but less dominantly.
This is why ECT has become the dominant corrugated board specification in technically sophisticated markets: it directly measures what predicts the in-service performance. Burst strength does not appear in the McKee formula.
The component grades above — fluting medium and testliner — are the building blocks of a typical recycled corrugated box. Their individual RCT/CMT values feed directly into the board's ECT.
From components to ECT: the prediction
The ECT of single-wall corrugated board can be estimated from component compression values:
ECT ≈ (SCT_outer_liner + SCT_inner_liner + C × SCT_medium) × board_basis_factor
Where C is a take-up factor (~1.4 for C-flute, ~1.3 for B-flute) and SCT values are in kN/m, accounting for the fluting medium area contribution after corrugation.
Real-world ECT results from this estimate because:
- Glue bond quality affects how effectively liner and medium share load
- Moisture content changes paper stiffness significantly (ECT at 90% RH can be 50–60% of conditioned values)
- Edge trimming quality of the test sample matters substantially
Typical ECT values for single-wall board
Standard conditioning: 23 °C / 50% RH per ISO 187.
| Board specification | Flute | Typical ECT (kN/m) | |---|---|---| | 125T/115M/125T (all recycled) | C | 4.5–5.5 | | 150T/115M/150T (all recycled) | C | 5.5–6.5 | | 125K/115M/125K (virgin liner) | C | 6.0–7.5 | | 175K/115M/175K (heavy virgin) | C | 8.0–10.0 | | 125T/115M/125T (all recycled) | B | 4.0–5.0 |
(T = testliner; K = kraft liner; M = medium; numbers = nominal g/m²)
ECT vs burst: the long-running specification debate
Burst strength (Mullen test) was the dominant corrugated board specification for most of the 20th century, enshrined in carrier regulations like the Uniform Freight Classification (US) and FEFCO standards (Europe). Burst correlates loosely with overall material quality — a board that fails burst usually has other problems — but it does not predict stacking strength well.
ECT displacement of burst as the primary specification has been ongoing since the 1990s in North America and Europe. In many trading relationships, both are still specified: burst as the historical anchor, ECT as the engineering metric. In technically sophisticated supply chains (automotive, electronics, e-commerce fulfilment), ECT is often specified alone.
WPI editorial note: The Maltenfort formula — an older variant of the McKee approach — uses the same ECT × caliper structure but different empirical constants. Different references will quote different K values. The key point is that ECT belongs in every corrugated box specification; burst alone is not sufficient if you care about actual stacking performance.