Many closely related diatom species are differentiated by simple morphometric characters such as length, width, and striae density. In addition, simple shape descriptors, such as curvature of the valve, can also be distinguishing morphometric characters. The curvature calculation described here is an intermediate technique between simple, direct measurements (i.e., length, width) and more complex descriptors of shape. The goal of this work is to develop a simple technique to calculate diatom curvature. Curvature is calculated from the length of a chord and the length of its sagitta (the perpendicular segment between the chord to the midpoint of the arc). These parameters correspond to diatom valve length and valve deflection from the chord and are incorporated into the equation: κ; = 2h(l² h²)^-1(where κ; = curvature, l = half of the chord length, and h = sagitta length). This equation can be utilized to estimate the curvature of the valve margin, valve margin asymmetry, and other morphological structures including apices and central area shape, and complex valve shapes.