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Carton measurement calculator & free packaging tools
Enter a carton's length, width and height in mm, cm or inch to get its board area, volume, estimated weight and how many cartons fit a 20ft or 40ft container. The figures are indicative and help you plan a spec before you ask for a quote.
Indicative RSC blank — real sizes depend on joint, flap and trim allowances. Get an exact quote →
Guidance only — final grade confirmed against your stacking, transit and buyer spec.
Indicative — accounts for container volume and payload-weight limit (20ft ≈ 28 t, 40ft ≈ 26.5 t). Real loading depends on orientation, pallets and stack height.
RSC blank estimate with standard joint and flap allowances. Get an exact quote →
Carton size, measured the way factories quote it
Carton size is written as length × width × height (L × W × H), measured on the inside of the box in millimetres. Length is the longer side of the opening, width is the shorter side, and height runs from the base to the top flaps.
- Inside vs outside size: the outside size adds the board thickness on each side, roughly 6 mm per dimension for 3 mm B flute board and 14–16 mm for 7–8 mm AB double-wall board.
- Carton volume (CBM): multiply the outside L × W × H in metres. A 400 × 300 × 250 mm carton is 0.4 × 0.3 × 0.25 = 0.03 m³.
- Master carton size: start from the packed unit size and the count per carton, then add a few millimetres of clearance before you check pallet and container fit.
For the full method, read how to choose the right export carton and the flute types guide for board thickness. To see your size as a printed box, open Build Your Box.
Carton calculator FAQs
How do I calculate carton size?
Measure the packed product, multiply by the number of units per carton in each direction, and add a few millimetres of clearance. The result is the inside length × width × height. Add the board thickness on each side for the outside size.
How do I calculate the CBM of a carton?
Convert the outside length, width and height to metres and multiply them. A 400 × 300 × 250 mm carton is 0.03 m³. Multiply by the number of cartons for the shipment volume.
How many cartons fit in a 20ft or 40ft container?
Divide the usable container volume by the carton volume, then check the payload weight limit. The container fit tool above does both and shows which limit applies. Real loading also depends on orientation, pallets and stack height.
Got your spec? Get a real price.
Send the dimensions and quantity and we'll quote within 24 hours.