Edge Crush Test Calculator: ECT & Box Strength

This calculator estimates the Edge Crush Test (ECT) value of corrugated cardboard based on the combined caliper (thickness) of its liners and medium, using the standard McKee formula. You enter the caliper of each component in thousandths of an inch (mils) or millimeters, and the calculator returns the predicted ECT in pounds per inch (lb/in) and kilonewtons per meter (kN/m). The result is a reliable starting point for box stacking strength, but actual performance also depends on flute profile, board grade, and manufacturing conditions.

Formula

The calculator uses the McKee formula, which is the industry-standard approximation for predicting the edge crush test of corrugated board:

ECT (lb/in) = 1.7 × (combined caliper in mils)

Where combined caliper is the total thickness of the board, calculated as:

Combined caliper = (2 × liner caliper) + (flute factor × medium caliper)

All calipers are in thousandths of an inch (mils). The flute factor (also called take-up factor) accounts for the extra length of medium needed to form the flute; typical values are 1.4 for A-flute, 1.5 for B-flute, and 1.5 for C-flute (though actual values vary by manufacturer). The constant 1.7 has units of (lb/in) per mil and comes from empirical testing of many board combinations.

Worked Example

Using the default values from the calculator:

• Liner caliper = 12 mils
• Medium caliper = 9 mils
• Flute factor = 1.5

First, compute the combined caliper:

Combined caliper = (2 × 12) + (1.5 × 9) = 24 + 13.5 = 37.5 mils

Then apply the McKee constant:

ECT = 1.7 × 37.5 = 63.75 lb/in

The calculator rounds to 63.8 lb/in. To convert to kN/m, multiply by 0.175 (since 1 lb/in ≈ 0.175 kN/m):

63.75 × 0.175 = 11.16 kN/m, which the calculator shows as 11.16.

What Is Edge Crush Test (ECT)?

The Edge Crush Test measures the compressive strength of corrugated board when a force is applied perpendicular to the flutes (i.e., along the edge of the board). A sample of board is placed between two platens, and the force required to crush it is recorded. The result is expressed in pounds per inch (lb/in) or kilonewtons per meter (kN/m). ECT is a key indicator of a box’s stacking strength, especially for warehouse storage where boxes are stacked several high. Unlike the older Mullen burst test, which measures resistance to puncturing, ECT focuses on compression—the most common failure mode in shipping.

How to Use the Calculator

To get a meaningful estimate, you need the caliper (thickness) of the liner and medium. These are often printed on a corrugated board spec sheet or can be measured with a micrometer. If you only have the total board thickness, you can still use the calculator by entering a typical liner/medium split—for example, 50% liner and 50% medium—but the result will be less accurate. The flute factor depends on the flute profile:

• A-flute: 1.4 to 1.5
• B-flute: 1.4 to 1.5
• C-flute: 1.5 to 1.6
• E-flute: 1.2 to 1.3

If you don’t know the flute factor, 1.5 is a reasonable default for common B and C flutes.

Common Mistakes and Limitations

The McKee formula is an approximation. It assumes a linear relationship between caliper and strength, but actual ECT also depends on the fiber type, board grade, and manufacturing process. Common mistakes include:

• Using total board thickness instead of combined caliper—this overestimates ECT.
• Forgetting to multiply the liner caliper by 2 (there are two liners).
• Using inconsistent units (mils vs. mm). The formula is calibrated for mils; if you measure in mm, convert to mils by multiplying by 39.37.
• Ignoring the flute factor—using the medium caliper as-is underestimates the medium’s contribution.

The formula also does not account for the effect of scoring, creasing, or printing, which can reduce real-world strength. For critical applications, always perform an actual ECT test per TAPPI T 811 or ISO 3037.

ECT vs. Box Compression Test (BCT)

ECT is often confused with Box Compression Test (BCT), which measures the maximum load a complete box can withstand before collapsing. BCT depends on the box dimensions, the flute direction, and the ECT of the board. A common rule of thumb is that BCT (in pounds) ≈ ECT × perimeter of the box (in inches) × 0.5, but this is very rough. The McKee formula can also be used to estimate BCT directly if you know the box perimeter and caliper. This calculator focuses on ECT, which is the board property, not the box strength.

Why ECT Matters for Shipping

In the late 20th century, the shipping industry shifted from Mullen burst ratings to ECT ratings because ECT better predicts stacking performance. A box with a high ECT can withstand more vertical compression, which is crucial when boxes are stacked on pallets. For example, a box with an ECT of 32 lb/in is typically used for light loads, while 44 lb/in or higher is needed for heavy items. However, ECT alone doesn’t guarantee performance—humidity, duration of load, and handling all affect real-world strength. Always consult your box supplier for the exact ECT rating of the board you plan to use.

Frequently Asked Questions

What is a good ECT number?

There is no universal “good� number—it depends on the box size and the weight it must support. For small boxes (under 12 inches in any dimension), an ECT of 32 lb/in is often sufficient. For larger boxes or heavier contents, you may need 44, 55, or even 65 lb/in. Your box supplier can recommend a specific ECT based on your stacking requirements.

Can I measure ECT myself?

Yes, but it requires a compression testing machine that applies a uniform force to a precisely cut sample. The sample must be 2 inches long and 1 inch wide, with the flutes running parallel to the 2-inch side. The test is standardized under TAPPI T 811. If you don’t have the equipment, your box manufacturer can provide ECT data.

How do I convert ECT from lb/in to kN/m?

Multiply the lb/in value by 0.175. For example, 44 lb/in × 0.175 = 7.7 kN/m. This conversion is built into the calculator.

Does ECT change with humidity?

Yes. Corrugated board loses strength as it absorbs moisture. At 90% relative humidity, ECT can drop by as much as 50% compared to dry conditions. Always consider the storage environment when designing packaging.

What is the difference between ECT and Mullen?

Mullen measures the burst strength (resistance to puncture) in pounds per square inch, while ECT measures edgewise compression strength. A box can have a high Mullen but low ECT, or vice versa. Modern shipping standards often use ECT because it correlates better with stacking performance.

Why does the calculator ask for flute factor?

The flute factor accounts for the extra material in the medium that forms the corrugations. Without it, the calculation would underestimate the medium’s contribution to the combined caliper. Using a correct flute factor improves accuracy.

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in liner_caliper | Liner caliper (mils) | mils | 12 | 0
in medium_caliper | Medium caliper (mils) | mils | 9 | 0
in flute_factor | Flute factor (take-up) | - | 1.5 | 1
out ect_lb | ECT (lb/in) | lb/in | (liner_caliper * 2 + medium_caliper * flute_factor) * 1.7 | 1
out ect_kN | ECT (kN/m) | kN/m | ect_lb * 0.175 | 2