Box Strength Tests Explained: Bursting Strength, BCT and ECT
Bursting strength is the pressure that ruptures the board. BCT, the box compression test, is the top load a whole box takes before it crushes, which decides stacking. ECT, the edge crush test, is how much edgewise load a short strip of board carries. For stacked shipments ask for BCT or ECT; for parcel handling, bursting strength.
What is bursting strength?
Bursting strength is the pressure a sheet of board takes before it ruptures. In the test, a sample is clamped over a rubber diaphragm and hydraulic pressure pushes the diaphragm up until the board bursts; the highest pressure reached is the result. TAPPI's method for corrugated board, T 810, describes exactly that, and India's method for board, IS 1060 (Part 7/Sec 1):2025, is identical to ISO 2759:2014, which covers board from about 350 to 5,500 kPa.
Bursting strength is the long-standing Indian box number. The 1990 edition of IS 2771 (Part 1), the corrugated fibreboard box standard, specified boxes by bursting strength and required each box to be marked with its minimum bursting strength. It is a good test of how the board resists a puncture or a sharp knock in handling, the kind a parcel takes on a sorting belt.
What it does not tell you is how well a box stands up under weight from above. A board can burst high and still buckle in a stack, because stacking strength depends on the edge strength of the board and the shape of the box, not on resisting a point of pressure.
Bursting strength tells you how the board takes a knock. It does not tell you how the box takes a stack.
What is BCT and why does it matter for stacking?
BCT, the box compression test, squeezes a whole box between two flat plates until it fails, and records the top load it carried. TAPPI's method, T 804, measures the ability of a shipping container to resist external compressive force, and notes that for quality work the empty box is usually tested. In India, compression and stacking tests on complete filled transport packages are covered by IS 7028 (Part 9):2020, identical to ISO 12048.
BCT is the number that maps most directly to stacking. If cartons sit five or six high on a pallet or deep in a container for weeks, the bottom box carries all the weight above it, and humidity and time eat into its strength. That is why buyers who stack heavily specify a BCT target rather than a bursting strength.
Note that the 1990 IS 2771 left compression strength to the purchaser to specify. So if stacking matters to you, put the requirement in your own spec: the load per box, the stack height and how long the stack stands.
What is ECT?
ECT, the edge crush test, measures how much load a short strip of board carries on its edge, with the flutes standing vertical, before it crushes. It is a board test, not a box test, reported as force per length of edge, for example in kN per metre. India's method is IS 7063 (Part 2):2018, identical to ISO 3037:2013; TAPPI's are T 811 and T 839.
ECT matters because it predicts BCT. The McKee formula, from 1963, links a box's compression strength to the board's ECT, its thickness and the box perimeter, which is why a higher ECT board or a smaller box perimeter both raise stacking strength. Treat the formula as a guide, not a guarantee: Roman Popil of Georgia Tech calls the simple version easy but not accurate, so a real BCT test settles the number.
| Test | What it measures | Indian method | When it matters most |
|---|---|---|---|
| Bursting strength | Pressure that ruptures the board | IS 1060 (Part 7/Sec 1):2025 (ISO 2759) | Parcel handling, knocks and punctures |
| ECT (edge crush) | Edgewise load a strip of board carries | IS 7063 (Part 2):2018 (ISO 3037) | Comparing board for stacking |
| BCT (box compression) | Top load a whole box carries | IS 7028 (Part 9):2020 (ISO 12048) | Pallets and container stacks |
Which test should you ask your supplier for?
Ask for the test that matches how your boxes fail. The current box standard, IS 2771 (Part 1):2022, lists bursting strength, edge crush, water absorption, flap bend and joint strength among its quality requirements, and an IPPTA industry presentation notes a steady shift in India toward specifying boxes by BCT and ECT rather than by burst alone.
- State the load per box, the stack height and the route when you ask for a quote.
- Ask which test the supplier runs and what result your board achieved.
- Approve a physical sample before the run, and check it against your real product.
Here is what we do. Board is GSM-checked and burst-tested, and you can ask for a test on your order. You approve a physical sample before any production run. For loads that punish ordinary board, we spec 5 or 7 ply by weight, stack and route.
Specify the stack, not just the board. The bottom box carries everything above it.
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- When you need a double wall box
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- What BF means in kraft paper
- Price your box on the calculator
Frequently asked questions
Is bursting strength the same as box strength?
No. Bursting strength is the pressure that ruptures the board, which reflects how it takes knocks and punctures. Box strength under a stack is measured by BCT, the box compression test, and is predicted by ECT, the edge crush test of the board.
What BCT does my box need?
It depends on the load per box, how high the cartons stack and how long the stack stands, especially in humidity. Give us the weight, the stack height and the route, and we spec the ply and board to suit.
Does IS 2771 apply to my boxes?
IS 2771 (Part 1):2022 is the Indian Standard for corrugated fibreboard boxes. It sets quality requirements such as bursting strength and edge crush. Whether you need to buy to it is your decision or your customer's; put the requirement in your spec if you do.
Do you test your boxes?
Board is GSM-checked and burst-tested, and you can ask for a test on your order. You also approve a physical sample before any production run, so you hold the actual board before the order is made.