Cardboard Box Template Calculator (Flat Blank Size and Scores)

To make a box from a flat sheet of cardboard you need the blank size and where to fold it. This calculator lays out the standard slotted box (the regular slotted container, FEFCO style 0201: four side panels in a row, a glue tab at one end, and top and bottom flaps that meet in the middle). Enter the inside length, width and height you need and the thickness of your cardboard, and it gives the blank size, every fold line, the flap depth and how much cardboard it takes.

Cardboard box template calculator

How it works

Panels: each panel = inside dimension + one board thickness, so the box still measures what you entered inside after folding
Blank length = glue tab + 2 x length panel + 2 x width panel
Flaps = half the width panel, so the long flaps meet in the centre of the box
Blank height = height panel + 2 x flap depth

Slots are cut from both long edges in to the flap fold line at each of the four panel folds, so the flaps fold independently. The one-thickness allowance per panel is an approximation that works for hand-made boxes; commercial box makers use tables per flute type.

Worked example

A box 12 x 9 x 6 in inside, from 1/8 in single-wall cardboard, with a 1-1/4 in glue tab:

Units: Inches
Inside length: 12 in
Inside width: 9 in
Inside height: 6 in
Cardboard thickness: 0.125 in
Glue tab width: 1.25 in

The calculator gives:

Flat blank size: 43-3/4" x 15-1/4"
Panels along the length: glue tab 1-1/4", 12-1/8", 9-1/8", 12-1/8", 9-1/8"
Fold lines from the tab end: 1-1/4", 13-3/8", 22-1/2", 34-5/8"
Flap depth (top and bottom): 4-9/16": half the width panel, so the long flaps meet in the middle
Fold lines across the blank: 4-9/16" and 10-11/16" from one long edge
Slots: cut from each long edge to the fold line at the 4 panel folds, about one board thickness wide
Cardboard needed: 4.63 sq ft

The blank is 43-3/4 x 15-1/4 in. Along the length, fold at 1-1/4, 13-3/8, 22-1/2 and 34-5/8 in from the tab end; across it, fold at 4-9/16 and 10-11/16 in from one long edge. Cut the four slots down to those flap folds, glue the tab inside the last panel, and the flaps close to meet in the middle. It takes about 4.6 sq ft of cardboard.

Notes and limits

Score the fold lines before folding: run a blunt edge (a butter knife, a bone folder or the back of a craft knife) along a straightedge. Fold away from the score.
Cut the slots slightly wider than the board is thick, or the flaps will bind when they fold.
Fold along the flutes where you can: cardboard folds cleanly with the corrugations and cracks across them.
For a stronger box, glue the tab with PVA or hot glue on the inside and tape the bottom seam.
Measure the cardboard thickness with a ruler on a cut edge. Recycled boxes vary from about 1/8 in to 1/4 in for double wall.

Frequently asked questions

How do you calculate the size of cardboard needed for a box?

Length of the flat blank = 2 x length + 2 x width + a glue tab; height of the blank = box height + box width (two flaps of half the width each). Add a board thickness to each panel so the inside size is right.

How big are the flaps on a cardboard box?

On a standard slotted box each flap is half the box width, so the two long flaps meet exactly in the middle when closed.

How wide should the glue tab be?

About 1 to 1-1/2 in (25 to 40 mm) for a hand-made box. Wider is easier to glue; narrower uses less board.

How do I join a cardboard box without tape?

Glue the tab inside the last panel and glue the flaps, or use one of the interlocking and slot methods in our guide to cardboard joinery techniques.

Sources

Last reviewed: 16 October 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.