To help field men select the proper nozzle and pump liner size, a pocket calculator has been designed which will predict the performance of the hydraulic system. Although the use of the drilling fluid to control and condition the hole is well known, the present use of high jet velocities has brought purely hydraulic problems to much wider attention. To help the field man, the design of a simple pocket calculator to solve some of these problems on the drilling rig was undertaken. Calculation of the output of the slush pumps was straightforward; but experience has shown that a direct reading flowmeter would be much more satisfactory. The energy losses in the down string were calculated by the Hazen-Williams equations with the Hazen-Williams roughness coefficient adjusted to allow for the restrictions in various types of tool joints. Losses in the drill bit nozzle were found to vary as the 1.96 power of the flow and, inversely, as the 2.19 power of the exit area. A presently accepted production nozzle exhibited an efficiency of 70 per cent. Losses in the return annulus are difficult to account for accurately; and, in this case, a rough estimate was used. This is practicable since these losses are a minor part of the total. In view of these assumptions, the overall accuracy of the calculator is of interest. Based on a comparison of calculated values and actual mud pressure gauge readings, the average error was +4 per cent; and 70 per cent of the examples were in the ±15 per cent error bracket. This accuracy is sufficient to give useful answers to the field man. Introduction The importance of the drilling mud as a medium to control formation pressure and the stability of the walls of the hole is well appreciated. However, the present widespread use of high rates of flow has focused attention on the drilling mud as a conveyor of energy to the drilling bit. In an effort to use this energy efficiently, field men have been clamoring for information that would enable them to select a bit nozzle properly matched to their hydraulic systems. It became apparent that some quick method of estimating the performance of the hydraulic system was necessary.