The glomerular filtration rate (GFR), the maximum rate for tubular reabsorption (Tm), and the plasma concentration (P) are known values for a reabsorbed substance. The amount of the substance appearing in the urine per unit time, at a plasma concentration above that for saturation of reabsorption, would be:

The amount of the substance filtered per unit time (assuming that it is freely filtered) equals the plasma concentration (P) times the glomerular filtration rate (GFR). The amount of the substance filtered that appears in the urine per unit time will be the amount filtered per unit time minus the amount reabsorbed per unit time. Because the plasma concentration is above that necessary to completely saturate the reabsorptive process, the amount reabsorbed per unit time will be equal to the maximum rate of reabsorption (Tm). Therefore, the amount excreted equals P (GFR) - Tm.