A measure of the shape of a drainage basin, normally expressed as the ratio between two dimensions of the basin being considered. One such measure is the circularity index (or ratio), C, expressed as C = A_b/A_c, where A_b is the area of the basin and A_c is the area of a circle with the same length of perimeter as the basin. Another index is the form factor, F, expressed as F = A/L², where A is the area of the basin and L is its length. Such indices may help in forecasting the flood potential of a basin.