Equations for the calculation of the erosion wear caused by ash particles on the convective heat-exchanger tubes of steam boilers are presented. A three-dimensional test arrangement was used in the testing of the erosion wear. Erosion testing was carried out with three types of tube constructions: one three-tube row, an inline tube bank with six rows of tubes, and a staggered tube bank with six rows of tubes. 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. Measurements of mass loss of the test sleeves were taken. Multi-variable linear regression analysis was used in developing the erosion calculation equations. The parameters used in the equations were the properties of the particle and tube materials, flow velocity, horizontal and vertical location parameters in the test apparatus, and time.