Abstract: As is generally known, the computation value of the diameter of a tricone bit depends on its journal angle, off-set, configuration and size of the cone. Expressed in a 4-degree equation, a new functional relation between the essential geometric parameters and the diameter of bit has been derived, on the basis of which a method of checking the bit diameter is proposed. From this 4-degree equation, a 6-degree equation is worked out which may be used to compute some of the geometric parameters, given the diameter of bit. Both equations can be solved with computer, accurate enough for calculation of reasonable tolerance of the geometric parameters.