How to Calculate Box Compression Strength (BCT)

Box Compression Test (BCT) measures how much weight a corrugated box can bear before collapsing. Understanding BCT calculations helps you design boxes that protect products during shipping and storage.

What is BCT?

BCT (Box Compression Test) measures the maximum load a box can withstand when force is applied to its top. It's expressed in kilograms (kg) or kilonewtons (kN).

Why BCT Matters:

Prevents product damage during shipping
Determines safe stacking height in warehouses
Helps select appropriate paper grades
Builds customer confidence

The McKee Formula

The industry-standard formula for estimating BCT:

BCT = k × ECT × √(h × Z)

Where:

k = Constant (typically 5.87 for kg/cm units)
ECT = Edge Crush Test value of corrugated board (kg/cm)
h = Caliper (thickness) of corrugated board (cm)
Z = Box perimeter = 2 × (Length + Width) in cm

Calculating ECT

ECT depends on the paper combination:

ECT = (Ring Crush of Liner 1 + Ring Crush of Liner 2 + Ring Crush of Fluting) × Take-up Factor

Typical Ring Crush Values (kg/cm)

Paper Grade	GSM	Ring Crush
Kraft	150	7.5
Kraft	180	9.0
Kraft	200	10.0
Test Liner	150	6.0
Test Liner	180	7.2
Semi-Chemical	100	5.5
Semi-Chemical	120	6.6
Semi-Chemical	150	8.0

Flute Take-up Factors

Flute Type	Take-up Factor	Caliper (mm)
A-Flute	1.56	4.8
B-Flute	1.35	2.5
C-Flute	1.45	3.6
E-Flute	1.24	1.5
F-Flute	1.22	0.8
BC-Flute	1.40	6.5
EB-Flute	1.29	4.0

Step-by-Step BCT Calculation

Example: 3-Ply Box

Box Dimensions: 40cm × 30cm × 25cm Board Specification:

Top Liner: Kraft 150 GSM (RC: 7.5)
Flute: Semi-Chem 120 GSM, B-Flute (RC: 6.6)
Bottom Liner: Test Liner 150 GSM (RC: 6.0)

Step 1: Calculate ECT

ECT = (7.5 + 6.0 + 6.6 × 1.35)
ECT = 7.5 + 6.0 + 8.91 = 22.41 kg/cm

Step 2: Get Caliper

B-Flute caliper = 0.25 cm

Step 3: Calculate Perimeter

Z = 2 × (40 + 30) = 140 cm

Step 4: Apply McKee Formula

BCT = 5.87 × 22.41 × √(0.25 × 140)
BCT = 5.87 × 22.41 × √35
BCT = 5.87 × 22.41 × 5.92
BCT = 778.8 kg

Result: This box can withstand approximately 779 kg of top load.

Safety Factor

Never use 100% of calculated BCT. Apply safety factors:

Condition	Safety Factor
Dry storage, short term	3×
Normal warehouse, 30 days	4×
Humid conditions, 90 days	5×
Export shipping, unknown conditions	6×

Safe Stacking Load for our example:

Safe Load (normal warehouse) = 779 ÷ 4 = 195 kg

Factors Affecting Actual BCT

1. Humidity

BCT drops 40-60% in humid conditions
Use moisture-resistant liners for exports

2. Storage Duration

BCT degrades over time (creep)
After 90 days, box retains only 55-60% of original BCT

3. Box Style

RSC boxes have lowest BCT
Full overlap slotted (FOL) boxes +20% BCT
Die-cut designs vary significantly

4. Printing

Heavy printing coverage can reduce BCT by 10-15%
Water-based inks affect less than solvent inks

5. Hand Holes

Each pair of hand holes reduces BCT by 15-20%

When to Use 5-Ply vs 3-Ply

Use 3-Ply When:

Product weight <10 kg
Short supply chain
Controlled storage
BCT requirement <400 kg

Use 5-Ply When:

Product weight >10 kg
Long transportation
Unknown storage conditions
BCT requirement >500 kg

Quick BCT Reference

3-Ply Boxes (B-Flute, 150 GSM Liners)

Box Size (cm)	Perimeter	BCT (kg)
20×15×10	70	550
30×20×15	100	660
40×30×25	140	780
50×40×30	180	885
60×45×35	210	956

5-Ply Boxes (BC-Flute, 180 GSM Liners)

Box Size (cm)	Perimeter	BCT (kg)
40×30×25	140	1,450
50×40×30	180	1,640
60×45×35	210	1,772
70×50×40	240	1,895

Calculate BCT Instantly

Use our free BCT Calculator to get accurate compression strength values for your boxes.

Open BCT Calculator →

Key Takeaways

BCT = k × ECT × √(h × Z) - memorize this formula
Always apply safety factor (3-6×) based on conditions
Humidity is the biggest enemy of BCT
5-ply for heavy products (>10 kg)
Test actual BCT when possible, calculations are estimates