Here is a plan showing the measurements, cuts, and creases for this enclosure. The formulas for the measurements correspond to the measurements of the book, L representing the length, W the width, and T the thickness. It is advisable to compute all the measurements needed, before doing any cutting, in order to have a "set" of measurements from which to work. For the sake of simplicity, we will say our book measures:

* 30 cm (300 mm) wide (W), by
* 40 cm (400 mm) long (L), by
* 10 cm (100 mm) thick (T).

So, all the individual measurements we need to calculate are:

W + 3 mm = (300 mm + 3 mm) = 303 mm or 30.3 cm
W + 6 mm = (300 mm + 6 mm) = 306 mm or 30.6 cm
L + 3 mm = (400 mm + 3 mm) = 403 mm or 40.3 cm
L + 9 mm = (400 mm + 9 mm) = 409 mm or 40.9 cm
T + 3 mm = (100 mm + 3 mm) = 103 mm or 10.3 cm
T + 6 mm = (100 mm + 6 mm) = 106 mm or 10.6 cm

The formulas for the overall dimensions of the piece of board needed are:

Length: [L + 2T + 20 mm]
[L (400 mm) + 2T (2 x 100 mm = 200 mm) + 20 mm]
[400 mm + 200 mm + 20 mm] = 620 mm or 62.0 cm

Width: (2W + 3T + 30 mm)
[2W (2 x 300 mm = 600 mm) + 3T (3 x 100 mm = 300 mm) + 30 mm]
[600 mm + 300 mm + 30 mm] = 930 mm or 93.0 cm