Archimedes' principle provides an inexpensive, simple and effective way to determine the average cross-sectional area of odd-shaped or flexible specimens. Based on the principle, the volume of a specimen is just the difference between its weight in water and in air, which can be directly measured, divided by the density of water. The average cross sectional area is obtained by dividing the volume by the specimen length. The method is verified by comparing with predictions made with finite element analysis.