Subthreshold leakage current is becoming an increasingly significant portion of the power dissipation in microprocessors due to technology and voltage scaling. Techniques to estimate leakage at the full chip level are indispensable for power budget allocation. In addition, simple and practical approaches and rules of thumb are needed to allow leakage to become part of the vocabulary of all designers. This paper focuses on the impact of circuit topology on leakage, which is often abstracted through what is referred to as the stacking factor. The stacking factor, which captures the leakage reduction in series connected devices, is a first order term in leakage estimation equations and has significant impact on estimation results. The authors present two analysis methods, a mathematical and an empirical, to identify the stacking factor for leakage prediction. Understanding the stacking factor, as well as obtaining an accurate estimate of its value, is critical in reducing prediction uncertainty. As leakage prediction becomes a bigger factor in roadmap decisions, reducing leakage prediction uncertainty will be key in accurate determination of product specifications.