Bursting strength vs ECT: choosing box strength
Two tests dominate corrugated box specs, and buyers often confuse them. Bursting strength and the Edge Crush Test measure very different things — pick the wrong one and you can over-pay for board or watch your stack collapse in the container. Here is how to choose.
What is bursting strength?
Bursting strength, measured by the Mullen test, is the pressure required to rupture the surface of the board. A rubber diaphragm is pushed against a clamped sample until the board bursts, and the result is reported in kilopascals (kPa) or pounds per square inch (psi). Bursting strength reflects how well the board's liners resist punctures, snags and the rough handling of manual loading. For decades it was the default way to certify a shipping box, and you will still see a bursting-strength figure on many Box Certificate stamps.
What is ECT?
The Edge Crush Test (ECT) measures how much vertical force a short strip of board can withstand on its edge before it crushes, reported in kilonewtons per metre (kN/m) or pounds per inch (lb/in). This matters because a box in a stack carries the weight of everything above it down through its vertical walls and edges. ECT therefore predicts stacking strength — the real-world ability of the bottom box on a pallet to survive the load — far more directly than bursting strength does.
How are the two tests different?
In short, bursting strength is about rupture and rough handling, while ECT is about vertical compression and stacking. Bursting strength is driven largely by liner quality; ECT is driven by the combined edgewise stiffness of liners and flute. A board can score well on one and only modestly on the other, which is exactly why specifying the wrong test leads to mismatched packaging. If you remember nothing else: punctures point to bursting strength, stacking points to ECT.
When does each one matter?
Choose by your dominant failure mode. If your shipments face a lot of manual handling, hooks, tight loading or sharp contents, bursting strength is the relevant guardrail. If your boxes are palletised, stacked several high and ride a long sea route — the typical RMG export pattern — ECT is the better predictor of whether the bottom carton survives. Humidity matters too: long, humid sea transit softens board and erodes compression strength, so a higher ECT target builds in a safety margin. Many global buyers now write ECT minimums into their packaging manuals, sometimes alongside a bursting-strength floor for handling.
How do I choose the right value?
Work backwards from the load. Add up the contents weight per box, the number of boxes stacked on top of one another, the transit mode and the humidity along the route. Those numbers set the compression the bottom box must survive, with a safety factor applied for time and moisture. From there you can derive an ECT (or bursting-strength) target and select a board grade and flute to hit it. Our board grades guide explains how paper weights feed into that choice, and the free board-grade tool gives you an indicative starting point.
Let us match the spec to your shipment
You do not have to calculate this alone. Send us your product weight, carton size, stack height and whether you ship by sea or air, and we will recommend a bursting-strength or ECT target and the board that meets it — then verify the finished cartons against that spec before the run. It is the difference between a box that merely looks strong on paper and one that is proven against your real load. When you are ready, request a quotation and we will respond within 24 hours.
Box strength FAQs
What is bursting strength on a box?
Bursting strength (the Mullen test) measures the pressure needed to rupture the board surface, reported in units such as kPa or psi. It reflects how well the board resists punctures and rough handling, and it relates to the strength of the liners. It is often the basis for a Box Certificate stamp.
What is ECT on a corrugated box?
ECT (Edge Crush Test) measures how much vertical force a strip of board can take on its edge before crushing, reported in kN/m or lb/in. Because boxes carry stacked loads on their vertical edges, ECT predicts stacking strength far better than bursting strength does.
Should I specify bursting strength or ECT?
Choose by failure mode. If your risk is rough handling and punctures, bursting strength is relevant. If your risk is boxes crushing under stacked loads on a pallet or in a container, ECT is the better predictor. Many buyers now specify ECT for stacking, sometimes alongside a bursting-strength minimum.
How do I know which strength value my shipment needs?
Work from the load: total contents weight, how many boxes stack on top of each other, the transit mode and the humidity along the route. Those inputs set the compression the bottom box must survive. Share them with us and we will recommend an ECT or bursting-strength target and the board to hit it.
Worried your boxes might crush in the stack?
Send your load details and we'll recommend an ECT or bursting-strength target — quote within 24 hours.