Systems and techniques are described for estimating optimal gate sizes in a circuit design using numerical delay models of cells and cell types in a technology library. Gate sizes are optimized in the circuit design in a reverse-levelized processing order. Gates that are at the same level in the reverse-levelized processing order, and whose inputs are electrically connected to the same driver output are optimized together. A closed-form expression is used to determine the optimized size for each gate in a set of gates that are optimized together. Some embodiments perform multiple optimization iterations, wherein in each optimization iteration all of the gates in the circuit design are processed in the reverse-levelized processing order. The iterative optimization process terminates when one or more termination conditions are met.