Corrugated basics: flute, wall, and edge crush

Corrugated board is rated two ways that matter when you ship: the flute profile and the strength rating. Flute is the wavy inner layer that gives board its rigidity. Common profiles run from coarse to fine — C-flute is the general-purpose shipping standard, B-flute is thinner and better for retail-ready or die-cut cartons, and E- and F-flutes are fine profiles used for small mailer boxes and product packaging. Coarser flutes cushion better and stack taller; finer flutes print cleaner and fold tighter.

Strength is stated as Edge Crush Test (ECT) or the older Burst (Mullen) rating. ECT measures how much stacking force the board's edge can take and is the better predictor for palletized, stacked freight. A single-wall 32 ECT box handles most parcel shipments; step up to 44 ECT single-wall or double-wall board for heavier contents, taller stacks, and long-haul freight. Match the box to the weight and stacking of the load, not just its dimensions — an under-rated box that fits fine will still crush under a pallet.

Single-wall board covers the majority of ecommerce and general shipping. Double-wall — two fluted layers — is for heavy, dense, or high-stack shipments and for reusable or export cartons. When in doubt on a heavy SKU, size up the wall before you size up the box.

Dimensional weight: why box size is a cost

Carriers bill parcels on the greater of actual weight or dimensional (DIM) weight, which is calculated from the box's cubic size. An oversized box full of void fill can cost more to ship than a right-sized box that weighs the same, because the carrier prices the air. Measure your product, pick the smallest box that leaves room for adequate cushioning, and standardize on a small set of box sizes so your pack station moves fast and your DIM cost stays predictable. Buying boxes in bulk across a tight size range is usually cheaper per unit and cheaper to ship than a wide, ad-hoc size mix.

Void fill and cushioning: immobilize, don't just fill

The job of void fill is to stop the product from moving inside the box and to absorb shock, not simply to occupy empty space. Match the fill to the product: paper and kraft for general goods and interleaving, air pillows for lightweight void volume, foam and bubble for fragile or edge-sensitive items, and loose fill for irregular shapes. Fragile and heavy items need cushioning that holds them off all six walls; light, durable items often need only enough fill to keep them from shifting. Under-filling causes damage; over-filling wastes material and inflates DIM weight.