Equations for the calculation of decreases in the tube diameter or wall thickness of a heat exchanger caused by ash particle wear are presented. A three-dimensional test arrangement was used in the testing of the erosion wear. Erosion testing was conducted with three types of tube constructions: a three-tube row, an in-line tube bank with six tube rows and a staggered tube bank with six tube rows. Three flow velocities and two particle concentrations were used in the tests, which were carried out at room temperature. Three particle materials were used: quartz, coal ash and peat ash particles. Multi-variable linear regression analysis was used in developing the erosion calculation equations as the mass loss of the test sleeves. The equations for calculating the losses in test sleeve diameter and wall thickness were performed using the mass loss calculation equations. The parameters used in the equations were the particle material properties, tube material properties, flow velocity, horizontal and vertical location parameters in the test apparatus and time.