How to Measure a Mailer Box

Why does the order of the three numbers matter?

Box sizes are always written as length x width x depth, for example 250 x 180 x 80 mm. The order is not a matter of taste. Each figure tells us which panel is which once the box is folded.

Length is the longer side of the opening, measured left to right as you face the box. Width is the shorter side of the opening, front to back. Depth is the height from the floor of the box to the underside of the lid.

Swap two of them and you get a box that holds the same space but opens the wrong way. A shallow tray becomes a tall tower, and the lid ends up on the narrow side.

If in doubt, write a label beside each figure in your message, such as L, W and D. It takes seconds and removes any guesswork on our side.

Inside size or outside size: which do we need?

We make every mailer box to its inside size. That is the space your product actually sits in, and it is the figure we put on the quote and the proof.

The outside size is larger by the thickness of the board, and on some panels by two layers of it. E-flute adds 1.5 mm for each layer and B-flute adds 3 mm. The outside size matters when you are posting close to a size band limit.

If you only know the outside size of a box you already use, tell us that clearly. We can work back to the inside size for the board you choose.

How do you measure the product?

Lay the product on a flat table and measure it as it will be packed. Use a rigid ruler or a tape measure that reads in millimetres.

1. Pack the product exactly as it will ship: in its retail carton, wrapped in tissue, or with its lid and pump fitted.
2. Measure the longest side at its widest point. Write it down as the length.
3. Turn the ruler across and measure the next longest side. That is the width.
4. Measure the height from the table to the highest point, including any handle or cap. That is the depth.
5. If several items share one box, arrange them as they will sit and measure the whole group.
6. Add 3 to 5 mm a side to each figure for clearance. The result is the inside size to send us.

Round up to the nearest whole millimetre. Rounding down is how a lid ends up refusing to close.

How much clearance should a mailer box have?

Clearance is the gap between the product and the wall. We work to 3 to 5 mm a side, which means adding 6 to 10 mm to each measurement in total.

Use the smaller figure for firm, square items such as boxed cosmetics or a stack of books. Use the larger figure for soft goods like folded knitwear, which spring out a little once packed.

Tissue wrap, a thank-you card or a sachet all take space. Add them to the measured product before you add clearance, not after.

Too much clearance is a problem too. A product with 15 mm of free space on one side will slide on every corner, so either reduce the box or fill the gap with an insert.

How do you measure an existing box you already use?

If you have a box that fits well and want the same size printed, measure that box rather than the product. Open it fully and measure inside, from wall to wall.

Take the length and width across the floor of the box, not at the top edge, as walls can lean outward. Measure the depth from the floor to the fold line where the lid starts.

Measure two or three boxes from the same batch if you can. Board flexes, and an average gives a truer figure than one box. Send us the inside size and say it came from an existing box.

Does the box style change how you measure?

The three numbers stay the same across styles, but where the opening sits changes which side is the length.

Small items follow the same rules. Our folding small mailing boxes have a typical starting size of 160 x 110 x 50 mm inside, made to your own size from there.

For an end open mailer, think of the open end as the front, with the product sliding in along the length.

Which measuring mistakes cost the most?

Most wrong sizes come from one of a few small slips.

• Measuring the product without its retail carton, then packing it in the carton.
• Giving the outside size of an old box when we asked for the inside.
• Writing the depth first, so the box is built tall instead of shallow.
• Forgetting the pump, cap or handle that stands above the main body.
• Measuring in inches or centimetres and converting with a rounding error.
• Leaving no room for an insert, tissue or paper void fill.

Send a photo of the product next to the ruler with your sizes. It lets us spot most of these before the proof.

How do the measurements fit the postal bands?

Once you have an inside size, compare the outside size with the band you post in. Royal Mail Large Letter allows 353 x 250 x 25 mm, Small Parcel 450 x 350 x 160 mm and Medium Parcel 610 x 460 x 460 mm.

Those are outside limits, so leave room for board, tape and label. Check the current band limits before you order, as they can change.

Our guide to box sizes and Royal Mail bands shows how to work out the outside size for each board.

What if you are measuring for an insert?

Measure the product first, then build the box around the insert. The insert takes up space on every side and usually raises the product off the floor.

Leave about 15 mm between the product and each wall for the insert to fill, and add the insert height to the depth. If you are unsure, send the product sizes and we will size the box and insert together.

Our guide to inserts for fragile goods explains the common insert shapes.

Sending your sizes for a quote

Write the inside size as length x width x depth in millimetres, name the style and board, and say how the box will be printed. Prices start from £0.25 a box, set out on a written quote within one working day.

A free design proof shows the flat net with every panel dimension marked, so you can check the size before anything is printed. Production takes 4 to 6 working days after approval, and free tracked UK mainland delivery takes 2 to 3 working days.

Fold the first box from the delivery, pack a real order and check the fit. Ready to start? Send your box measurements for a quote.