Mailer Box Size Guide: Measuring, Royal Mail Sizes and Standard Box Sizes
How to measure for mailer boxes, which sizes fit Royal Mail’s cheaper bands, and standard sizes for clothing, beauty, gifts and more.
The right size of box protects your product, keeps postage down and makes your parcel feel considered. This mailer box size guide covers how to measure, how to stay inside Royal Mail’s size bands and which standard sizes suit common products.
How to measure for a mailer box
- Measure the product at its widest points: length, width and height, including any inner packaging.
- Add clearance: 2–5 mm per side for a snug fit, more if you add tissue paper, a card or an insert.
- Use inside dimensions: boxes are specified by the space inside. The outside adds a few millimetres for the board.
- Weigh it: heavier products may need double wall mailer boxes or B-flute corrugated mailer boxes, which add to the outside size.
Royal Mail size bands
| Format | Maximum size | Maximum weight | Good box options |
|---|---|---|---|
| Large Letter | 353 × 250 × 25 mm | 750 g | Small cardboard mailer boxes, book mailer boxes, paper padded mailers |
| Small Parcel | 450 × 350 × 160 mm | 2 kg | Postal boxes, eCommerce mailer boxes |
Letterbox-friendly small cardboard mailer boxes and book mailer boxes are some of the easiest ways to cut postage on jewellery, socks, prints and accessories. Plastic-free paper padded mailers work for soft items. For everything bigger, postal boxes designed to sit just under the Small Parcel limit give you the most room for the price. Always check the current limits on Royal Mail’s website, as formats and prices change.
Standard mailer box sizes by product
| Product | Typical inside size | Recommended box |
|---|---|---|
| Jewellery, minis, socks | 150 × 100 × 50 mm | Small cardboard mailer boxes |
| Folded T-shirts | 250 × 200 × 50–80 mm | Kraft or white mailer boxes |
| Hoodies and sweatshirts | 300 × 250 × 80–100 mm | eCommerce mailer boxes |
| Caps | Sized to the crown and peak | Hat mailer boxes |
| Candles and gift sets | 200 × 200 × 100 mm | Square mailer boxes with inserts |
| Books and planners | Wrap-around fit | Book mailer boxes |
Clothing brands usually start with kraft mailer boxes or white mailer boxes for tees and ecommerce mailer boxes for hoodies, and protect caps with hat mailer boxes. Candles and gift sets suit square mailer boxes paired with custom inserts.
Our standard sizes
Products with online prices list four standard sizes you can order straight away: Small (152 × 102 × 51 mm), Medium (254 × 203 × 76 mm), Large (305 × 229 × 102 mm) and X-Large (381 × 305 × 152 mm). Any other size can be quoted. Pick the closest standard size to start, then move to a custom size once you know your bestsellers.
Common sizing mistakes
- Measuring the outside of the box instead of the inside.
- Forgetting board thickness when designing for a postal limit.
- Leaving so much space that products slide around and need void fill.
- Ordering a box for every product instead of two or three sizes that cover the range.
Not sure? Tick “Not sure, suggest a size” on any custom mailer boxes product and we’ll recommend dimensions in your free mock-up. You can also read about the different types of mailer boxes before you decide.
One size or several?
Most brands don’t need a box for every product. Lay out your bestsellers and group them by size: small accessories, folded clothing and bulky items. Two or three box sizes usually cover the whole range. Fewer sizes means simpler stock, larger runs per size and a lower price per box.
If one product dominates your sales, consider product mailer boxes made exactly for it. A close fit protects better than any amount of void fill and keeps dimensional weight down with couriers.
Depth matters more than you think
Depth is the dimension most often got wrong. Too shallow and the lid bulges or won’t close; too deep and products slide around. Measure the product at its tallest point, then add the thickness of any tissue, card or insert. For letterbox parcels, depth is the limiting factor: Royal Mail’s Large Letter allows only 25 mm in total, including the board.
Strength and weight
Bigger boxes need stronger board. Products over a couple of kilograms, glass and ceramics are safer in B-flute or double wall mailer boxes. Light items such as clothing and accessories are well protected by E-flute cardboard mailer boxes, which keep weight and cost down.
Frequently asked questions
Are mailer box sizes inside or outside?
Inside. The outside is a few millimetres larger because of the board thickness.
What is the best mailer box size for a T-shirt?
Around 250 × 200 × 50–80 mm inside fits a neatly folded adult tee with tissue paper.
What size box fits through a letterbox?
Royal Mail’s Large Letter format allows up to 353 × 250 × 25 mm and 750 g, measured on the outside of the box.
Can I order a custom size?
Yes. Every mailer can be made to your exact inside dimensions. Request a quote with your measurements.
What size mailer box do I need for a hoodie?
Around 300 × 250 × 80–100 mm inside fits a folded adult hoodie. Heavyweight hoodies may need the deeper end of that range.
How many box sizes should a small business have?
Usually two or three. Group your products by size and choose the fewest boxes that fit them all well.
