This brief models and analyzes the CMOS cross-coupled differential-drive (CCDD) rectifier in subthreshold operation for ambient radio frequency (RF) energy harvesting. The developed model can predict accurately the output voltage against the input RF level, number of stages, transistor size, and output load. For validation, three CCDD rectifiers with 3, 4, and 7 stages, and dissimilar transistor sizes, were fabricated in 0.18-μm CMOS. Each rectifier was tested against different load conditions (10/30/100/500 k Ω and open circuit), and input RF levels below the threshold level of the transistor. The verified model has an accuracy of >90% up to 0 dBm input power, and 80% up to 3 dBm input power, across all denoted variations.