Corrugated box sizes explained: internal, external and grade

Corrugated board has thickness. Fold it into a box and the walls take up room on every side. The space inside is smaller than the box measured from outside, in the length, the width and the height alike.

Packers care about the inside size. That is what their product must fit. Transport and warehouse staff care about the outside size, because that is what fills a shelf, a pallet layer or a courier limit.

Dimensions are given in the order length, width, height. Length and width describe the opening. The longer side comes first. Height comes last.

Buyers should measure the product and add room for padding, then compare that figure with the inside size on your listing. Most wrong orders come from the outside size.

Tell buyers how to measure. Show one clear drawing with the three dimensions marked. One line of text saves many returns.

Add a tolerance note. Board swells in damp stores. A carton may measure a few millimetres off its printed figure. Buyers packing a tight product, such as a glass jar in a divider, should order one size up or ask for a sample before ordering a pallet.

Grade is the build of the board. A three ply box has one fluted layer between two liners, and a five ply box has two fluted layers with three liners. More layers usually carry more. Heavier goods, such as a case of glass bottles, usually call for the extra layer. Light goods in a courier parcel rarely do.

Flute is the wave in the middle layer. A flute is tall and cushions well. B flute is shallower and stiffer. It prints well. C flute sits between them. E flute is thin and suits small retail boxes.

Strength is stated as bursting strength, edge crush or box compression, depending on the mill, and the three figures cannot be compared with one another. These are test results, not promises. Ask the mill which test applies. A mill can usually send the test report for a batch.

Put the inside size first. Show the outside size beside it. Name the style, such as a regular slotted carton, a die-cut mailer or a tray, then add the ply, flute and test figure your mill gives you.

Then say how the box is sold. Cartons ship flat in bundles. The bundle count changes with size. A buyer who assumes the price is per box and receives a bundle of fifty will be shocked by the invoice, and will blame the listing.

Show a photograph beside the specifications. A flat bundle, an opened carton and a close-up of the cut edge, where the flute shows, tell a buyer more than any adjective. Name each image file after the style and size, so the catalogue manager can find it later.

The commonest error is ordering by outside size. The product then will not go in. The second is assuming five ply is always stronger, when a heavy wall with the wrong flute can still crush under a stacked load.

A third is mixed units. A listing in millimetres meets a buyer who measures in inches, and the carton comes back with a note that says it is the wrong size.

Rewrite the listing of your best-selling carton with the fields above. Ask a customer to order from it. Watch where they hesitate.