Before a box is folded, it starts as a flat sheet called a blank. Knowing the cardboard sheet size you need helps you order board, estimate cost and check whether a design fits your machine or a standard sheet. This guide explains the blank size formula for the most common box style, with our free Cardboard Sheet Calculator.
The regular slotted carton (RSC)
An RSC has four side panels in a row, a glue (manufacturer’s) flap, and top and bottom flaps each half the box width.
The blank size formula
| Dimension | Formula |
|---|---|
| Blank length | 2L + 2W + glue flap |
| Blank width | H + W (top flap W/2 + bottom flap W/2) |
L, W and H are inner dimensions. The glue flap is typically 3 to 4 cm.
Allowances for board thickness
Folding thick board uses some material at each crease. Many box makers add a small allowance per panel, for example 2 to 3 mm for 3-ply and 5 to 7 mm for 5-ply. Ask your supplier for their standard allowances.
Worked example
Box inner size: 40 × 30 × 25 cm, glue flap 4 cm.
- Blank length = 2 × 40 + 2 × 30 + 4 = 144 cm
- Blank width = 25 + 30 = 55 cm
- Blank area = 1.44 × 0.55 = 0.792 m²
Blanks per sheet
If board sheets are 160 × 120 cm, how many 144 × 55 cm blanks fit?
- Along 160 cm: 1 blank (144 cm)
- Along 120 cm: 2 blanks (110 cm)
So 2 blanks per sheet, using 1.584 m² of a 1.92 m² sheet, about 82.5% usage. Rotating or changing sheet size may improve yield.
Large boxes
If the blank is longer than the maximum sheet or machine size, boxes are made with two pieces joined at two glue flaps.
From area to weight
Multiply blank area by board GSM to get box weight; see how much a cardboard box weighs and Box Weight Calculator.
For paper grades and thickness, read GSM and cardboard thickness. For other box styles, see the Corrugated Box Calculator.
Frequently asked questions
How do I calculate the blank size for an RSC box?
Length 2L + 2W + glue flap; width H + W.
What is a glue flap?
The extra strip that joins the box’s side panels, usually 3 to 4 cm.
Do I use inner or outer dimensions?
Usually inner dimensions, with allowances for board thickness.