Mailer Box Sizes and Royal Mail Bands

Why does the band matter when you choose a box size?

Every parcel you post is sorted into a size band before it is priced. A box that is a few millimetres too deep can drop into the next band up. On a single order that is a small annoyance; across a month of orders it adds up.

The fix is simple. Work out the band you want first, then size the box to sit inside it with a little room to spare. That way the band decides the box, not the other way round.

Band limits change from time to time, so check the current band limits before you order. The figures in this guide are the ones we work to at the time of writing.

What are the Royal Mail band limits?

Three bands cover almost every mailer box we make. Each one has a maximum length, width and depth measured on the outside of the packed parcel.

Read the table as outside limits. The size we quote you is the inside size, so the board and any tape or label have to fit within the gap between the two.

Inside size and outside size are not the same

We make every mailer to an inside size in millimetres, written like 250 x 180 x 80 mm. That is the space your product sits in. The outside is larger, because the board has thickness and some panels fold over twice.

E-flute board is 1.5 mm thick and B-flute is 3 mm. On a tuck top mailer, the front wall and the lid are double layers, so the depth grows by more than one board thickness. A box with an 80 mm inside depth will measure a few millimetres more on the outside.

When we quote, we can state the outside size alongside the inside size. Ask for it if you are close to a band edge. The mailer box sizes page lists typical starting sizes for each style.

How do you fit a box into the Large Letter band?

Large Letter is the tightest band. The 25 mm depth limit covers the board on both faces as well as whatever sits inside, so the inside depth has to be well under 25 mm.

Flat items suit it best: art prints, greeting cards, socks, thin books and folded scarves. E-flute is the usual board here, because the thinner wall leaves more room for the product.

A side loading mailer keeps the depth low, since there is no lid folding over the top. Our side loading tab lock mailers are often sized for this band. Push the parcel through a slot gauge made from card before you commit to a full run.

Sizing for the Small Parcel band

Small Parcel takes most ecommerce orders. At 450 x 350 x 160 mm on the outside, it fits a folded jumper, two candles, a cosmetics kit or a small gift hamper.

The depth limit of 160 mm is generous, so length and width are where most boxes run into trouble. A box that is long and shallow can still go over 450 mm once you add the board. Keep about 10 mm clear of each limit on the inside size to leave room for board and tape.

Tuck top and ear lock styles both fit this band well. A starting size of 250 x 180 x 80 mm sits comfortably inside it with space to grow.

When does a box move up to Medium Parcel?

Medium Parcel starts where Small Parcel stops. Any outside measurement above 450 mm long, 350 mm wide or 160 mm deep moves the parcel up, even if the other two are small.

Larger subscription boxes, homeware, bedding and multi-item kits often land here. At this size B-flute is the usual choice, as the longer panels need more stiffness to keep their shape.

If a box only just tips into Medium Parcel, look at the depth first. Lying the product flat, or splitting a kit across two layers with a divider, can bring it back down a band.

How to size a box against a band

This is the order we follow when a customer gives us a product and a target band.

1. Measure the product, or the packed product with tissue or an insert, at its longest, widest and deepest points.
2. Add 3 to 5 mm a side for clearance. That gives the inside size of the box.
3. Choose the board. E-flute adds 1.5 mm per layer and B-flute adds 3 mm per layer.
4. Work out the outside size, allowing for double layers on the lid and front wall.
5. Compare the outside size with the band limits. If any figure is over, reduce clearance, change the board, or rotate the product.
6. Ask us to state the outside size on the written quote, and check the current band limits before you order.

Our guide on measuring a mailer box properly covers the first two steps in more detail.

Which mistakes push a parcel into the next band?

Most band problems come from small additions that nobody counted at the design stage.

• Sizing the outside of the box to the band limit and forgetting the label and tape on top.
• Adding paper void fill that bulges the lid so it no longer sits flat.
• Switching from E-flute to B-flute late in the job without rechecking the depth.
• Packing a product with a lid or handle that sticks up past its main body.
• Using a gift ribbon or sticker seal that adds height across the top.
• Measuring the product without its retail box, then packing it in the retail box.

A sample from the first run helps here. Pack it as you would on a real order and measure the parcel, not the empty box.

Does the box style change the outside size?

It does. A tuck top mailer has a rolled front wall and a lid with a tuck flap, which adds board on the depth and the front. An ear lock box has locking ears at the sides, which sit inside the walls and add little to the outside.

End open and side loading mailers have no lid over the top face, so the depth stays close to the inside size plus two board layers. That is why they suit the shallow bands.

If you are torn between styles, our range of mailer box styles shows how each one closes. Pick the closure first, then let us work out the outside size for that style.

Ordering a mailer box sized for a band

Send us the product size, the band you want to stay in and the board you prefer. If you are unsure of the board, say what the product is and we will suggest one.

Prices start from £0.25 a box, and every order gets a written quote within one working day. You get a free design proof before anything is printed, and production runs 4 to 6 working days after you approve it.

Tracked delivery to the UK mainland is free and takes 2 to 3 working days. When you are ready, ask for a written quote with your target band noted.