Construction beam sizes can be a little misleading, as the width and depth of the beam is usually slightly smaller than the size stated. For instance, a 10 by 10 cm (4 by 4 inch) beam in the lumber section at the local DIY shop is, in reality, around 8.75 by 8.75 cm (3 1/2 by 3 1/2 inches). This tends to be the norm for most lumber. Beams made of other materials like steel generally have no discrepancy. If needing a replacement wooden beam, it's best to round up the measurements of existing lumber to the nearest 2.5 cm (one inch) increment before going to purchase lumber of the same size.
Measure the width of the beam by running a tape measure from one edge to the other. Place the metal lip at the end of the tape measure over one edge of the beam, and rest the tape diagonally across the width of the beam -- that is, at 90-degrees to the edge of the beam. Look at the reading on the tape where it meets the other edge of the beam. Remember to round up this measurement if you are looking to purchase a new beam of the same size. Therefore, a beam of 13.75 cm (5 1/2 inches) width would be 15 cm (6 inches) in width.
Measure the depth of the beam following the same steps -- note that the depth of the beam is the edge of the beam at 90-degrees to its width edge.
Measure the length of the beam -- that is, from one end of the beam to the other. Place your three measurements together. The width measurement goes first, then the depth measurement, then the length. For example, a 10 by 15 cm (4 by 6 inch), 3.6 m (12 foot) beam means a beam with a 10 cm (4 inch) width, 15 cm (6 inch) depth, and 3.6 m or 360 cm (12 foot) length.
The width edge of the beam is always smaller than the depth of the beam. However, some beams (such as a 10 by 10 cm or 15 by 15 cm beam) can have the same width and depth measurements.