Measure before you open the range
Measure the finished parcel, not the goods alone.
These are the boxes aimed at the Royal Mail small parcel limit. The page for Royal Mail small parcel boxes is where Packaging Boxes shows that line.
The published limit, if there is one
The outside maximum on the changes page is 450 x 350 x 160 mm. The box has to finish under that, tape included.
A larger single wall box may be stronger or roomier and still be a different, dearer format.
Write the size down
Reorder a size you have checked against the finished parcel.
On Packaging Boxes, put Royal Mail small parcel boxes on the order with the lines it belongs to, so you are not paying a second delivery for the piece you forgot. Free UK delivery starts over £50 ex VAT.
Open Royal Mail small parcel boxes when you already know this line. The bundle count is on the page. Packaging Boxes will still dispatch an order placed by 3pm on the same day.