Some common I beam, also known as W beam, sizes are W 27 X 128, W 24 X 162, W 21 x 147, W 18 x 119, W 12 x 136 and W 6 x 25, says EngineeringToolbox.com. The two numbers in the size specification describe the depth or height of the beam in inches and the pounds per every feet of length. Other variations among dimensions in I-beam sizes include the width and stem thickness, measured in inches.
Along with the variation in height, I beams with same height specification are constructed differently so that other variables may have significantly different values, as seen in the chart in EngineeringToolbox.com. For example, beams with a height of 27 may vary in pound per foot values from 178 to 84, in width from 14.09 to 9,96 inches, in web thickness from 0.725 to 0.46 inches, and in sectional area from 52.3 to 24.8 square inches.
Aside from this, there are I beams with 12 different height variations, and some of these beams have up to 19 variations within each specific height measurement, as evident in EngineeringToolbox.com. For example, I beams that are marked 12 inches high can range in pound per foot values from 136 to 14. The I beam that is 4 inches high only has one version.