Corrugated Board Grades Explained: What Is Best for Postal Boxes?
Understanding board grades so you can choose the right strength postal box for your products.
Board grade basics
Corrugated board grade determines how strong and protective your postal box will be. The grade is defined by the flute type (the wavy middle layer) and the liner weight (the flat outer layers). Together, these determine burst strength, compression resistance, and cushioning ability.
Quick answer
E-flute (1.5mm) is best for lightweight items and premium printing. B-flute (3mm) suits most postal applications up to 5-8kg. C-flute (4mm) provides maximum cushioning for fragile items. For liner weight, 125gsm kraft is standard, 150gsm provides extra strength, and 200gsm suits heavy-duty applications. Most postal boxes use B-flute with 125-150gsm liners.
Flute types explained
Best for lightweight items, retail packaging, and applications where print quality matters most.
Excellent all-round performance. Good cushioning, stacking strength, and print quality.
Thickest single-wall option. Best for fragile items needing maximum shock absorption.
Two flute layers for maximum strength. Used for heavy items or extreme transit conditions.
Related: single vs double wall guide or choosing the right size.
Liner weights
Liners are the flat sheets bonded to each side of the fluted medium. Heavier liners create stronger, more puncture-resistant boxes. Standard postal boxes use 125gsm kraft liners. Premium or heavy-duty boxes use 150-200gsm liners. Test (T) liners contain more recycled content and are cheaper but slightly less strong than kraft (K) liners.
Choosing the right grade
Grade selection guide0 / 4 checked
Postal boxes in every grade
From lightweight E-flute to heavy-duty double-wall, find the right board grade for your products.
Browse postal boxesFrequently asked questions
B-flute with 125-150gsm liners suits most postal applications. E-flute for lightweight items, C-flute for fragile goods.
Grams per square metre - it measures liner weight and directly correlates with strength and puncture resistance.
Kraft (K) liners use more virgin fibre and are stronger. Test (T) liners have more recycled content and are cheaper.
Only for heavy items (over 5-8kg) or very fragile products. Single-wall B-flute handles most postal needs.
Yes, E-flute provides the smoothest surface for high-quality printing. B-flute is good. C-flute is rougher.
Part of our guide
Product Guide: Postal Boxes
Expert guides on postal boxes, from sizing and materials to branding and cost optimisation.
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