The goods set the size
A box is the right size when the goods sit inside it without a large empty space and without the walls pressing on them. Measure the length, width and height of what you are packing, including anything that cannot be folded. Then look for a box close to that size.
Packaging Boxes groups single wall, double wall, crash lock, postal boxes, book wraps and tubes. The first choice is the shape. A flat book is a wrap. A poster is a tube. A stacked product is a carton.
Then the board
Single wall boxes are described as strong, affordable and versatile. Double wall boxes are the stronger board for heavy, valuable or fragile goods, and that range is manufactured in the UK. A bigger single wall box is not a stronger box. If the contents are heavy or will be stacked, use double wall.
Crash lock boxes are a style as well as a size. The base locks open and a peel-and-seal strip closes the pack. The category gives the board as 150K/T and lists six sizes it describes as fitting Royal Mail small parcels.
Then the carrier
If the box is going as a Royal Mail small parcel, the outside of the box is what counts. Our changes page gives the current maximum as 450 mm long, 350 mm wide and 160 mm deep. A box that is right for the goods but over those figures is a different, dearer parcel.
Tubes have their own roll rule on that page: 450 mm long and 80 mm in diameter if they are still to count as a small parcel. A square box maximum does not describe a tube.
Pack quantity
Once the size is right, the product pages are that box in different quantities. Order the quantity you will actually send. A trade business reordering the same size can use TradePro, from 5% off up to 10% over £1,000 a month.
Orders placed by 3pm are dispatched the same day. UK delivery is free on orders over £50 ex VAT.