The basic principle behind the Clegg Impact Soil Tester is to obtain a measurement of the deceleration of a free falling mass (Hammer) from a set height onto a surface under test. The impact of the hammer produces an electrical pulse, which is converted and displayed on the Control Unit in units of gravities "G" or tens of gravities "CIT". Reference ASTM test methods D5874 and F1702.

The standard test protocol developed by Dr. Clegg is to drop the hammer four consecutive times on the same location with the highest value result in the series taken as the Peak Clegg Impact Test result. Since that time, other test protocols have been used based on the materials under test and the application. These other protocols vary the Hammer drop height or use the average result in place of the peak. The Clegg offers the convenience of rapidly scanning compaction variation over large areas. In research studies, 250 tests were performed with the Clegg in a half-day.

Site location via GPS and moisture measurements are optional features available with the Control Unit.