In the study of sedimentary particles, volume is the best definition of size. Measurements made on cross sections of grains in thin section are estimates of volume. Measurements were made of the A (long) and B (perpendicular to A) axes and the areas of maximum projection planes, random planes and thin-section planes of 60 pebbles to test the effectiveness of each in estimating the known volume of the pebbles. Regression analysis was used to compare methods. The area of the maximum projection plane is the most precise estimator of size. The thin-section plane area is slightly more precise than the A axis but not enough to justify the extra time required. There is no significant difference between use of the A or B axis.