The short answer
A box size means inside dimensions, in the order length, width, depth. Length is the longer side of the opening, width the shorter, depth the drop from the opening to the bottom.
The outside is an estimate, not arithmetic. Adding two board thicknesses gets you close, but the scoring allowance and the joint decide the rest, and every panel carries a published tolerance.
A box arrives and it is wrong by a quarter inch. The product does not drop in, or it rattles, and somebody has to decide whose quarter inch it was.
Most of those arguments trace to one of three things. The order of the three numbers, whether they described the inside or the outside, or a tolerance nobody had quoted.
Published convention settles all three, and a technical report settles the last one with figures in it.
This reference covers which dimension is which, why the outside does not follow from the inside, and what tolerance a correctly made box may carry.
Inside Dimensions Are the Spec
Start with the default, because it is nearly universal. The trade specifies box sizes by their inside dimensions, written as length by width by depth.
The reason is the job the box does. The inside decides whether a product fits, so that is the measurement the converter builds to.
Order forms follow suit. Unless something says otherwise, dimensions on a corrugated quote are internal, which is the assumption a converter will work from.
Outside dimensions still matter, just for different people. Palletizing and distribution run on the outside, which is where cube, trailer fit and freight charges get decided.
Two downstream numbers depend on it directly. Dimensional weight prices the shipment off the outside cube, and a case has to fit the standard pallet sizes the network already runs.
So a specification that does not say which it means is ambiguous in a way that costs money. Writing ID or OD on the line removes the whole class of argument.
Which Side Is the Length
The three numbers have fixed definitions, and they are about the opening rather than the footprint. Length is the longest dimension of the opening.
Width follows from that. It is the shorter dimension of the same opening, so length and width come together from looking at one face.
Depth is the third, measured from the opening to the bottom. Some specifications call it height, which is the same number under another name.
The opening is the part people get wrong. A box can open on its top or on its end, and the opening is the face the product goes through, not whichever face happens to be largest.
Which is why a tall narrow carton and a flat wide one can carry the same three numbers in a different order. Reading the sequence as given, rather than sorting it yourself, is the habit that avoids it.
Why Outside Is Not Inside Plus Two Calipers
The obvious arithmetic is to add board on both sides. That is a sound first estimate: outer dimensions run larger than inner ones by roughly twice the board caliper per side.
The numbers are small and they add up. Singlewall C-flute runs about 5/32 inch thick, so each outside dimension lands about 5/16 inch over the inside, and double or triple wall widens the gap further.
That estimate has a limit, though, and a box design tool states it plainly: outside dimensions cannot always be calculated by adding two calipers to every inside dimension.
Both claims are right, and the reason sits in the folds. The industry’s own technical report notes that score width varies with flute size, basis weight and score pattern, and that the designer adjusts the blank to allow for scoring.
So the outside depends on a scoring allowance somebody chose for your board. Estimate it with two calipers, then ask the converter for the actual outside before you plan a pallet pattern around it.
Each panel drifts on its own allowance, and the sheet they add up to carries a separate one.
The Tolerance, and the Scope It Comes With
A correctly made box is not exact, and the trade has published how inexact it may be. The figures live in a technical report on slotted container tolerances.
| What is measured | Allowed variation | Why it matters to you |
|---|---|---|
| Individual panel, scoreline to scoreline | ± 1/16 in (1.5 mm) | Each face can drift this much on its own |
| Overall flat sheet | ± 1/8 in (3 mm) | The blank as a whole, before folding |
| Slot depth | ± 1/8 in from the flap score centerline | Sets how the flaps land |
| Slot centering | within 1/16 in of the aligning scores | Off-center slots skew the closure |
| Warp | 1/4 in over 12 in, up to 1 in over 48 in | A flat sheet can still refuse to run |
Those numbers are the useful part of the report, and the scope is the part to read next. They apply to top and end opening slotted containers in B or C flute singlewall, one family among the corrugated box styles in service.
The board has to be in range too, certified between 150 and 275 psi burst or 26 to 44 pounds per inch edge crush. Panel dimensions have to sit between 4 and 25 inches, and the report assumes automatic machinery erects, fills and closes the containers.
Step outside any of that and the report says so. Thicker or heavier board, or dimensions beyond those limits, may exceed the stated tolerances, which does not make the box defective.
Which is the practical reading. Quote the tolerance by all means, and check first whether your box is the kind of box the report covers.
The Joint and the Flap Gap
Two more allowances get mistaken for faults, and both are in the same report. The first is the manufacturer’s joint, where the converter fastens the blank into a tube.
The joint may carry a gap. Measured at the flap scorelines it should be no less than 1/8 inch and no greater than 1/2 inch. At the flap ends the gap should stay at or above 1/16 inch.
The joint may also taper. Skew on a single box should not exceed 3/16 inch, and the flap score alignment between the two panels should stay within the same 3/16 inch.
That alignment figure is worth more than it looks. Converter practice notes what happens when a machine loses sheet control. The scores stop aligning and the box is not square, even though the joint gap can still look perfectly straight.
So a straight joint is not proof of a square box. Checking the scores across the joint catches a problem that a glance at the glue line misses.
The second allowance is the flaps. Major flaps should not overlap, and the gap between them may run up to 1/4 inch. That is why a correct slotted box often does not meet neatly in the middle.
Common Questions
Are box dimensions inside or outside measurements?
Inside, unless the specification says otherwise. The inside decides whether the product fits, so that is the measurement the converter builds to. Outside dimensions govern cube, pallet fit and freight charges, so write ID or OD on the line and remove the ambiguity.
Which dimension is the length?
The longest dimension of the opening, with the width being the shorter dimension of that same opening and the depth running from the opening to the bottom. The opening is the face the product goes through, which is not always the largest face.
Does adding board thickness give the outside size?
Close enough to plan with, not close enough to commit to. Adding twice the board caliper per side is the right estimate, but score width changes with flute, paper weight and score pattern, and the designer adjusts the blank for it. Ask the converter for the actual outside before fixing a pallet pattern.
How much can a box be out of size before it is defective?
Published tolerances allow ± 1/16 inch on an individual panel and ± 1/8 inch across the whole flat sheet, with slots held to 1/8 inch on depth and 1/16 inch on centering. Those figures cover B or C flute singlewall slotted containers with panels between 4 and 25 inches, and heavier board may exceed them legitimately.
Should the flaps meet in the middle?
Not necessarily. On a slotted container the major flaps should not overlap, and a gap of up to 1/4 inch between them sits within tolerance. A visible gap is usually the design rather than a short-cut.
How to weigh this page. The convention and the definitions here are near-universal in the corrugated trade, and the tolerance figures come from a published technical report whose own scope is stated above. Your supplier may work to tighter limits than the report allows, or to a different standard entirely outside North America, and a buyer specification can override both. For anything load-bearing, get the tolerance you are being held to in writing on the drawing rather than inferring it from a general figure.
Key takeaways
- Box sizes mean inside dimensions in the order length, width, depth, because the inside is what the product has to fit.
- Length is the longest dimension of the opening, width the shorter dimension of that opening, and depth the distance from the opening to the bottom.
- Outside dimensions run roughly twice the board caliper larger per side, about 5/16 inch for singlewall C-flute, but the scoring allowance means that stays an estimate.
- Published tolerances allow ± 1/16 inch per panel and ± 1/8 inch across the flat sheet, with slot depth at 1/8 inch and slot centering at 1/16 inch.
- Those tolerances are written for B or C flute singlewall slotted containers with panels between 4 and 25 inches, and heavier board may legitimately exceed them.
- A straight manufacturer’s joint does not prove a square box; checking that the scores align across the joint catches what the glue line hides.