Lapping and polishing machines usually do not have deterministic model to pre-determine removal rate and total material to be removed. The removal process is mainly affected by relative motion between the lap and the substrates, by load applied, and by mechano-chemical characteristics of the substrate material, as well as the abrasive and lap materials. Therefore, frequent measurements of the removal is necessary. This paper, written for optical technicians, includes formulas to calculate material removal from mass loss and removal rate from mass loss during operation. Establishing the removal rate helps by reducing the frequency of intermediate thickness measurements. The paper also includes the calculation of Preston coefficient, which is a measure of lapping process efficiency.