How to measure a corrugated box: internal or external dimensions

When you order custom boxes, the manufacturer needs to know how much space your product will take up inside the box: the internal length, width and height, all in one unit. Box manufacturers worldwide follow FEFCO practice here, so the size on a box specification always refers to that usable space inside.

New boxes that come out smaller than the old ones, with products that no longer fit, usually trace back to the most common mistake first-time buyers make. They measure the outside of their existing box, the easiest surface to reach, and send those figures as internal dimensions. The conventions in this guide are the ones manufacturers work with. Use them to label each side and to choose an allowance for your product, and any manufacturer will read your single set of figures the same way.

Identify length, width and height from the opening

Start from the box opening, the top opening you pack through. The longer side of the opening is the length and the shorter side is the width. Height runs from the opening down to the bottom of the box.

Before you send the figures, check three things:

• Each figure is labeled with its side: length, width or height.
• All three figures are internal dimensions.
• The whole set uses one unit, millimeters or centimeters, chosen before you start measuring and written next to the numbers.

With every figure labeled, the order you write them in matters less. FEFCO lists length × width × height, with the width no greater than the length, while many manufacturers in Thailand write width × length × height. Either order works. Units are a different story. A set that mixes an inch value with centimeter values and arrives without units is a frequent reason for wrong-size boxes, even when every measurement was taken correctly.

Internal dimensions for the manufacturer, external for the warehouse

For an order, send the internal dimensions and the flute type you need. Board thickness and folding allowances are calculated when the manufacturer designs the cutting die, so the figures you send only need to describe the space for the product. External dimensions are the ones your warehouse and your carrier work with.

Every box has both sets, and the gap between them is the thickness of the corrugated board in its walls.

Board thickness sets the gap between ID and OD

How far apart the two sets of dimensions sit depends directly on the flute type, and with every flute the gap is too large to ignore.

To work out external dimensions from internal ones:

• Width and length: add 2 × the board thickness, one wall on each side.
• Height of a regular slotted box: add up to 4 × the board thickness, because the top and bottom flaps each fold over in two layers.

Take a BC flute box, with 7 mm board, and internal dimensions of 60 × 40 × 40 cm. Its outside measures about 61.4 × 41.4 × 42.8 cm, or roughly 1.4 cm more in width and length and almost 3 cm more in height. Your warehouse needs those outside figures to plan how many boxes fit on a pallet and in each layer, and carriers use them to set the dimensional weight they charge. A gap of this size can decide whether a full row of boxes sits on the pallet or overhangs the edge.

Each flute suits different work, and Corrugated board structure: how E, B, C, BE and BC flutes differ explains which to choose.

Measuring for a new box or a reorder

You have the product and no box yet

Measure the width, length and height of the product at its widest point on each side. If several items go in one box, arrange them in the layout you will actually pack and measure the whole group. Then add the allowance described in the next section to arrive at the internal dimensions to order.

For example, products arranged for packing measure 35 × 25 × 20 cm. With an allowance of 0.5 cm on each side, the internal dimensions to order are 36 × 26 × 21 cm.

You are reordering a box that already works

If your current box does the job, measure it from the inside. Choose one in good condition, because a box that has swollen or lost its shape in use gives misleading figures. Run the tape from the inside face of one wall to the inside face of the opposite wall for each of the three sides. You get the internal dimensions directly, with no need to unfold the box. Send these inside figures, and leave any outside measurements off the order.

Match the allowance to the product and the journey

The right allowance depends mainly on the product and on how the box will travel, so no single value fits every job.

• Rigid, self-supporting products, such as cans, bottles or products packed tightly and evenly: 3–5 mm on each side. That leaves enough room for easy packing and removal, and the product shares the load with the box.
• Fragile products wrapped in cushioning: an allowance equal to the actual cushioning thickness, usually 1–2 cm on each side.

Leaving extra room "to be safe" works against the box. Once the product can shift in transit, it stops supporting the walls, which then take the stacking load alone, and a loose box collapses sooner than a well-fitted one. The allowance also has to suit how the boxes will be used: the stacking pattern, the number of layers and how long they stay in the warehouse, along with the size and weight of the product.

Common mistakes before ordering

1. Measuring the outside of an old box and reporting it as internal. The new batch comes out smaller by twice the board thickness, and with 5-ply boxes that gap can stop the product fitting.
2. Sending three numbers without saying which side each one belongs to. 40 × 60 × 40 and 60 × 40 × 40 are different boxes, with a different flute direction and structure.
3. Measuring the product exactly, with zero allowance. The product goes in too tight, packing slows down, or the product pushes the walls out before the box leaves the plant.
4. Measuring an old box that has swollen or deformed. Use and humidity change a box, so its figures drift away from its actual specification.

An order a manufacturer can work from straight away lists the internal dimensions of all three sides, each labeled as width, length or height, with the unit written beside them. Describe the product as well, and explain how the boxes will be shipped and handled. With that information, the manufacturer works out the flute thickness allowances and folding calculations at the design stage and chooses a box structure that suits the job. If you have yet to settle on a box style, Corrugated box types: four groups and how to choose compares the options, and Custom corrugated boxes shows every box style Auto Boxes produces.

Frequently asked questions

Should I give internal or external dimensions when ordering boxes?
Always give internal dimensions. These are the values the manufacturer uses directly to design and calculate the box. If the box has to fit a fixed space, such as a set area on a pallet or a shelf of fixed size, give the maximum external dimensions as well, and the manufacturer will work back to suitable internal dimensions.

How do I measure round or irregular products?
Measure at the widest, longest and tallest points, as if the product were enclosed in an imaginary rectangular box, then add the usual allowance. For cylindrical products such as jars and bottles, the diameter at the widest point serves as both the width and the length.

Does an assembled box have to be unfolded before measuring?
A box in good condition can be measured from inside wall to inside wall without unfolding it. Unfolding helps when the box has swollen or deformed and the inside cannot be measured accurately. In that case, measure between fold-line centers. This is the same method the manufacturer uses on the flat sheet before forming, following FEFCO practice.