Are box dimensions inside or outside?

It depends on who quotes them. Box makers often give inside dimensions; carriers measure outside. Check which one a listing means.

Which dimensions do carriers use?

The outside dimensions of the closed, packed box.

How much smaller is the inside of a box?

About two wall thicknesses on each side. With 3/16 in walls, an 18 × 18 × 16 in box loses about 6.4% of its volume.

How thick is cardboard box wall?

Single-wall corrugated is commonly around ⅛–3/16 in (3–5 mm), and double-wall is thicker, but board varies.

Should I use inside dimensions for box volume?

For capacity, yes. For storage or freight space, use outside dimensions.

Illustration of a cut-away box wall showing the inside and outside measurements on either side of the board

Shipping & packaging

Inside vs Outside Box Dimensions

Box sizes are quoted inside or outside, and the two can differ by a noticeable amount. Use the wrong one and a product that should fit doesn’t, a pallet plan comes up short, or a shipping estimate is off. The rule is simple: inside for what goes in, outside for the space the box takes.

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Which measurement for which job

Which measurement for which job comparison
Job	Use
Will the product fit?	Inside
How much can the box hold?	Inside
Shipping and dimensional weight	Outside
Shelf, pallet, or truck space	Outside

How much the walls take

Each side loses the board twice — once at each end. With 3/16 in (about 0.19 in) walls, an 18 × 18 × 16 in outside box is about 17.625 × 17.625 × 15.625 in inside. That is about 4,854 in³ (2.809 ft³) instead of 5,184 in³ (3 ft³): about 6.4% less.

Single-wall corrugated board is commonly around ⅛–3/16 in (3–5 mm); double-wall is thicker. Measure your own board when the fit is close.

Why manufacturers quote inside sizes

Box makers often quote inside dimensions because that is what a packer needs. Flaps, score lines, and the way the box is folded mean the inside height in particular can differ from a simple outside-minus-two-walls estimate.

Measuring a box yourself

Open the box and measure inside, wall to wall, for length and width.
Measure the inside height from the bottom to the fold of the top flaps.
Close and tape the box, then measure the outside for shipping.
Write down which set you measured.

Inside vs Outside Dimensions FAQ

Are box dimensions inside or outside?: It depends on who quotes them. Box makers often give inside dimensions; carriers measure outside. Check which one a listing means.
Which dimensions do carriers use?: The outside dimensions of the closed, packed box.
How much smaller is the inside of a box?: About two wall thicknesses on each side. With 3/16 in walls, an 18 × 18 × 16 in box loses about 6.4% of its volume.
How thick is cardboard box wall?: Single-wall corrugated is commonly around ⅛–3/16 in (3–5 mm), and double-wall is thicker, but board varies.
Should I use inside dimensions for box volume?: For capacity, yes. For storage or freight space, use outside dimensions.