First measure across the front cover from the left side to the right side. This is the length of the book — it tells you how long the book is when it’s closed. Keep the ruler flat against the cover for an accurate reading.

Next, stand the book upright and measure from the front cover to the back cover — this is the width, sometimes called the spine thickness. It shows how thick the book is.

Lay your ruler or tape measure along the longest edge of the book — this runs from the top of the book to the bottom. This number is the height. Record the measurement in millimeters.