Oct 13, 2026Help Center
How to Measure a Box: Inside Dimensions, Outside Dimensions and the L x W x D Rule
Inside vs outside dimensions, the L x W x D convention and how to measure for a perfect-fitting custom box. A two-minute guide for packaging buyers.
Why box measurements go wrong so often
Ask ten people for the size of a box and you may get ten different answers, because boxes can be measured from the inside or the outside, and the three dimensions can be listed in any order. When you order custom packaging, a small mix-up in measurement becomes a box that does not fit. This guide fixes that in two minutes.
The standard convention: L x W x D
Packaging dimensions are always written as Length x Width x Depth (or Height). Length is the longest side of the opening, Width is the shorter side of the opening, and Depth (or Height) is the distance between the opening and the base.
Think of a shoebox: the opening faces up. The long side of the opening is the length, the short side is the width, and the distance down to the bottom is the depth. The same logic applies whether you are ordering custom shoe boxes or a display shipper.
Inside dimensions vs outside dimensions
This is where most mistakes happen. The same box has two different sets of measurements.
- Inside dimensions: the space your product actually occupies. Use these when you want the box to fit your product.
- Outside dimensions: the size of the finished box as it sits on a shelf, including the board thickness. Use these for shelf planning, shipping calculations and retail display.
A rigid box made from 2 mm greyboard has outside dimensions roughly 4 mm larger than its inside dimensions. Always tell your supplier which one you are quoting.
Measuring a box you already have
Use a rigid ruler or caliper, not a soft tape. Open the box and measure the inside space from wall to wall: length first, then width, then depth. Note the board thickness as well - it tells your supplier what structure you are working with. For corrugated mailer boxes, measure the inner length, width and depth at the flutes, not the outer walls.
Measuring for a new box
When there is no box yet, measure the product itself: its length, width and height at the widest points, including any packaging, sleeves or labels. Then add tolerance for easy insertion - a little breathing room makes the unboxing feel premium rather than tight.
A note on tolerance
Paper board and corrugated board have small production tolerances, and boxes need to slide together without force. If a fit feels critical - like a lid on a base - say so in your specification and ask for a physical sample before mass production. This applies to RSC slotted cartons and retail-ready structures alike.
The takeaway
Write dimensions as L x W x D, say whether they are inside or outside, and measure the product at its widest points. Send those three things to your supplier and you remove the most common cause of a bad first order.
Want a second pair of eyes? Send us your product dimensions and we will come back with a suggested box style and dieline - free of charge.