mailer box sizes

Mailer box sizes are the internal length, width and height measurements of a folding mailer box, and choosing the right one decides how well your product is protected and what your postage costs. This guide to mailer box sizes explains how the measurements work, which sizes suit common products and when a custom size is worth ordering. Custom Postal Boxes makes mailer boxes to order in the UK, so any size in this guide can be matched or adjusted.

How mailer box sizes are measured

Mailer boxes are measured inside, in a set order: length first, then width, then height. Length is the longest side of the opening, width is the shorter side of the opening, and height is the depth from the base to the top of the walls. This matters because two boxes with the same three numbers in a different order are different boxes. When you compare mailer box sizes from any supplier, check that the figures are internal, because board thickness means the outside of the box is always a few millimetres larger.

If you are browsing rather than measuring, our postal boxes by size category groups our postal range by format, and our custom mailer boxes UK page shows the mailer styles those sizes apply to. Sellers who know the product but not the box usually start from the product measurements and work outward, as the next section explains.

Measuring your product in three steps

First, arrange the items exactly as they will sit in the box, including any wrap, insert or leaflet. Second, measure the group at its longest, widest and tallest points. Third, add a small allowance, around 5 mm to 10 mm in each direction, so the product slides in easily and the lid closes without forcing. That final packed size is the internal size you should order. Guessing from the product alone, without its wrap and insert, is the most common reason a first order comes up too small.

Internal and external sizes are not the same

Corrugated board has thickness, so the external size of a mailer box is bigger than the internal size you order. Postage is charged on the external packed size, so a box that sits just inside a Royal Mail band on paper can slip into the next band in practice. Leave sensible headroom below any limit that matters to your postage costs.

Common mailer box sizes and what fits in them

Small mailers, around 150 mm by 100 mm by 50 mm up to 200 mm by 150 mm by 75 mm, suit cosmetics, jewellery, candles in tins and small accessories. Medium mailers, around 250 mm by 200 mm by 100 mm up to 330 mm by 230 mm by 110 mm, cover clothing, books, beauty sets and subscription products, and they are the sizes most online sellers use daily. Slim mailers with a height of 25 mm or less serve the letterbox market: our letterbox postal boxes and A5 postal boxes are built around that shallow format.

Royal Mail bands shape many of these choices. A large letter can measure at most 353 mm by 250 mm by 25 mm and weigh up to 750 g, while a small parcel allows 450 mm by 350 mm by 160 mm and up to 2 kg. The Royal Mail size and weight guide gives the full set of limits. A mailer that keeps a flat product inside large letter size can save money on every single order, which is why thickness is the dimension to watch most closely.

When a custom size is the right answer

Standard sizes work when the product is a standard shape. Odd shapes, fragile items, kits with several pieces and products that sit between two stock sizes all benefit from a box made to measure. A custom size removes wasted space, cuts the fill you need and presents the product neatly instead of lost in a corner. Pricing depends on size, material and quantity, and low minimum quantities make a trial run practical, so ask for a quote through our contact page with your packed measurements.

FAQs about mailer box sizes

What order are mailer box sizes written in?

Sizes are written as length by width by height, using internal measurements. Length is the longest side of the opening, width is the shorter side, and height is the depth of the box. Always confirm the figures are internal before you compare prices or place an order.

What size mailer box do I need for clothing?

Most folded garments suit a medium mailer around 300 mm by 230 mm by 100 mm, but the right answer comes from measuring your garments as packed. Bulky knitwear needs more height, while a single flat garment may fit a much slimmer box and travel as a large letter.

How much extra room should I allow inside the box?

Around 5 mm to 10 mm in each direction beyond the packed product is a sensible allowance. It lets the product slide in, makes room for wrap or an insert and ensures the lid closes cleanly without bulging, which also keeps the external size honest for postage.

What happens if my box is slightly too big for a Royal Mail band?

Postage is charged on the finished parcel, so if the external size or the total weight passes the band limit, the parcel moves into the next band and costs more. Measure the packed box, not the empty one, and leave headroom below the limits that matter to you.