The measurement of grain size in terms of the nominal sectional diameter is being used more and more in sedimentary petrography. Most methods of measurement contain significant errors and are time consuming. The use of a large diaphragm in estimating the diameter of a circle, whose area is equal to the area of the grain, greatly shortens the time. The errors involved have been found to be small enough to be ignored. The apparatus is described and parallel measurements are compared which lead to the conclusion that such a device is valuable in securing accurate grain size data in a very short time.