A mathematical model for describing diameter-volume relationships was derived theoretically. The model is based on the assumptions that the diameter-height relationships are expressed by the height curve deriyed from the theory of column buckling, and that the stem form is expressed by the Kunze's stem-curve equation. The model is a kind of one-variable volume equation, and its mathematical expression is
V = bD^<8/3>
where V : volume
D : diameter
b : constant.
The value of D's exponent, 8/3, was exalmined by comparing with 6 actual yield tables and 249 volume tables. The value(8/3) is reasonable, and the model may have a practical use.