Internal vs external carton dimensions (and how to calculate them)
Internal dimensions are the inside length, width and height of a carton; external dimensions are the outside size after the board thickness of every wall and flap is added. Specs go wrong when nobody says which of the two a number means: a buyer emails "600 × 400 × 400", the plant builds it, and the cartons either rattle or refuse to close.
Why are internal and external dimensions different?
Because corrugated board has thickness, and every wall of the box adds it. A B-flute wall is roughly 3 mm thick; an AB double-wall is closer to 8 mm. On a regular slotted container, length and width each grow by about twice the board thickness from inside to outside — one wall on each side. Height grows by more, typically around four times the board thickness, because the top and bottom each close with two layers of flaps. So a B-flute RSC with a 400 × 300 × 200 mm interior measures roughly 406 × 306 × 212 mm outside. Which flute you choose changes those allowances, which is why our guide to corrugated flute types belongs next to every dimension discussion.
How do you calculate external carton dimensions from internal?
For a regular slotted carton (RSC), add board thickness to the inside size: external length = internal length + 2 × board thickness, external width = internal width + 2 × board thickness, and external height = internal height + about 4 × board thickness. To go from external to internal, subtract the same amounts. The table shows typical allowances by flute; exact figures depend on liner grade and the plant's scoring.
| Board | Board thickness | Add to length and width | Add to height |
|---|---|---|---|
| E flute | ~1.5 mm | ~3 mm | ~6 mm |
| B flute | ~3 mm | ~6 mm | ~12 mm |
| A flute | ~5 mm | ~10 mm | ~20 mm |
| AB double wall | ~7–8 mm | ~14–16 mm | ~28–32 mm |
To size a master carton, start from the packed unit. Internal length is the number of units along the length times the unit length, plus a few millimetres of clearance; repeat for width and height. Then add the allowance for your board to get the external size for pallet and container planning. Our free carton calculators run this conversion for each flute.
Which dimension should be on your spec?
Both — but labelled, and used for different jobs. Internal dimensions are the manufacturing convention: they are what guarantees the product actually fits, so they are the numbers a box plant works from. External dimensions are the logistics numbers: they decide how many cartons form a layer on a standard pallet and how many pallets or loose-loaded cartons fill an intermodal shipping container. Quote internal to the factory; calculate external for the warehouse and the freight forwarder. Convention also fixes the order: length is the longer side of the opening, width the shorter, and height (or depth) runs perpendicular to the opening — always L × W × H.
Where do carton specs actually go wrong?
After a quarter-century of reading tech packs, we see the same five failure modes on repeat:
- Unlabelled numbers. Three dimensions with no "ID" or "OD" marking — the single most common cause of a misfit carton.
- Board changed after sizing. Upgrading single-wall to double-wall adds millimetres that quietly break a tight pallet pattern.
- Dimensions in the wrong order. Swap width and height and the box arrives rotated — flaps in the wrong plane, print sideways.
- No tolerance stated. Corrugated converting normally holds around ±3 mm; a spec that needs better must say so before quoting.
- Sizing from a sample measured outside. Copying a competitor's carton with a tape measure captures its external size, then ordering it as internal produces a box one board-thickness too big in every direction.
How do you measure a carton correctly?
To capture internal dimensions, open the flaps and measure inside face to inside face at the score lines — not at the bulging centre of a panel. If you only have a sealed box, measure outside and subtract the allowances for its board grade. Better still, skip the arithmetic: our free carton sizing calculators convert between internal and external dimensions for each flute and estimate the board area while they are at it.
What does a spec that never goes wrong look like?
Five lines cover it. State the internal dimensions in millimetres, labelled L × W × H. Name the construction — for most export work that is the regular slotted carton, but say so explicitly. Name the board: flute plus liner grades or GSM. State the tolerance you can accept. And add the pallet or container target, so the plant can sanity-check the external maths against your unit load before a single sheet is cut. Every construction we make, from RSC to telescoping, carries its own allowances — the full range is on our corrugated boxes hub — and a five-line spec lets us confirm fit on day one. Send yours through the request-for-quotation form and we will return the external dimensions, pallet plan check and price within 24 hours.
Not sure if your numbers are inside or outside?
Send the spec as you have it — we will confirm internal and external dimensions, flag any pallet-fit issues and quote within 24 hours.