Box Grades & Flute Types
Everything you need to know about corrugated box construction — flute types, wall construction, ECT ratings, and how to choose the right grade.
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What Makes a Box Strong?
Corrugated cardboard gets its strength from its unique layered construction. The wavy inner layer (called the “flute”) is sandwiched between flat outer layers (called “liners”). This combination creates an incredibly strong structure that is lightweight yet resistant to crushing and stacking forces.
The strength of a box depends on three main factors: the flute type (size and frequency of the waves), the wall construction (how many layers), and the ECT rating (edge crush test, measuring stacking strength).
Corrugated Flute Types
| Flute | Height | Flutes/ft | Description | Best For |
|---|---|---|---|---|
| A-Flute | ~3/16" | ~33 | Excellent cushioning and stacking strength. The original flute type, ideal for fragile items. | Fragile goods, stacking |
| B-Flute | ~1/8" | ~47 | Good crush resistance and printing surface. The most common flute for die-cut boxes and retail packaging. | Die-cuts, retail, canned goods |
| C-Flute | ~5/32" | ~39 | The most widely used flute size. Good balance of cushioning, stacking strength, and crush resistance. | General shipping (most common) |
| E-Flute | ~1/16" | ~90 | Thin flute with excellent printing surface. Used for retail displays, pizza boxes, and consumer packaging. | Retail display, pizza boxes |
| F-Flute | ~1/32" | ~125 | Ultra-thin micro-flute. Superb printing surface, minimal material use. Ideal for premium packaging. | Premium retail, cosmetics |
Wall Construction Types
Single Wall
23–44 ECT
One fluted layer between two liners
Flutes: A, B, C, or E flute
Use: Lightweight to medium-weight items, general shipping, retail boxes
Double Wall
42–82 ECT
Two fluted layers with three liners
Flutes: Typically BC or AC combination
Use: Heavy items, stacking, industrial shipping, gaylord boxes
Triple Wall
67–112 ECT
Three fluted layers with four liners
Flutes: Typically ACA or BAB combination
Use: Very heavy items, palletized shipping, replacement for wood crates
ECT Rating Reference Chart
ECT (Edge Crush Test) measures how much force a box can withstand before crushing. Higher ECT = stronger stacking resistance.
| ECT Rating | Max Weight | Equivalent | Wall Type |
|---|---|---|---|
| 23 ECT | Up to 20 lbs | 125# Test | Single Wall |
| 26 ECT | Up to 30 lbs | 150# Test | Single Wall |
| 29 ECT | Up to 40 lbs | 175# Test | Single Wall |
| 32 ECT | Up to 65 lbs | 200# Test | Single Wall |
| 40 ECT | Up to 80 lbs | 275# Test | Single Wall |
| 44 ECT | Up to 95 lbs | 350# Test | Single Wall |
| 48 ECT | Up to 80 lbs | 200# DW | Double Wall |
| 51 ECT | Up to 100 lbs | 275# DW | Double Wall |
| 61 ECT | Up to 120 lbs | 350# DW | Double Wall |
| 71 ECT | Up to 140 lbs | 500# DW | Double Wall |
| 82 ECT | Up to 160 lbs | 600# DW | Double Wall |
Not Sure Which Grade You Need?
Our team can help you select the right box grade for your specific application.