How to choose a mailer box size
Measure the packed product, add padding, then check the result against your carrier limits before you order.
The right mailer size is the packed product plus a little padding, rounded to the nearest standard size. Too tight and the box tears or the product scuffs. Too loose and the item slides, and you pay to post empty air.
Step 1: measure the packed product, not the product
If your item goes into a bag, a sleeve or tissue first, measure it after that step. Use a ruler or calipers and write down length, width and height in that order. Measure the largest point on each side, including any lid or cap.
Step 2: add padding
Add about 3 mm (0.12 in) on each side for items that sit snug and do not break. For glass, ceramics or anything that scratches, add 6 to 10 mm and plan for a board insert or paper fill. Always add to the inside size. Outside dimensions are bigger because the board has thickness.
Step 3: choose inside or outside dimensions
Mailer boxes are specified by their inside dimensions. The outside is about 3 to 4 mm larger on each axis for E-flute. When you check carrier limits, use the outside size.
Step 4: check the carrier
Postage bands depend on size and weight. In the UK, Royal Mail Large Letter limits are 353 x 250 x 25 mm and 750 g. In the US, USPS and the large carriers charge by weight and, above a size threshold, by dimensional weight. Limits change, so confirm them with your carrier. Our postage band guide has the full explanation.
Step 5: order a sample
Plain sample mailers cost little and show you the real board, the fold and how your product sits. Pack it, shake it and post it to yourself. If it survives, order the print run.
Common sizing mistakes
- Measuring the product naked when it ships in a pouch.
- Forgetting that the lid needs room to close over the contents.
- Picking a depth that pushes the parcel into the next postage band for a few millimeters.
- Using outside dimensions when ordering to inside size.
Frequently asked questions
Can I order a custom size?
Yes. Custom sizes need a new cutting die, which adds a one-time cost and a few days. Standard sizes avoid both.
How many sizes do I need?
Most brands do well with two or three. A small, a medium and a large cover most orders and keep stock simple.