Abstract A sample of noncircular tree cross-sections was examined to determine basal area and basal area increment. Basal area estimates were calculated with a circular model from single measurements of diameter and from averages of two diameter measurements. Basal area increment estimates were made with several geometric models from single measurements of radial increment and averages of two measurements of radial increment. Commonly used tree cross-sectional area estimates investigated were biased, usually overestimating basal area. Several generalized geometric models of area increment were investigated, but none tested were uniformly supported by the data with accuracy related to the specific increment measurements selected. However, results indicated that either a single increment measurement taken on the minor axis or the shortest increment from the major axis yielded accurate estimates of basal area increment for several of the models tested. For. Sci. 34(3):621-633.