Fouling is a costly problem in heat exchanger design and operation. Over the past 20-30 years, design capabilities have advanced such that most fouling can be mitigated through effective design techniques. A design margin is added to the initial clean design in order to handle uncertainties in design and any deterioration in performance due to the reduced fouling that occurs in spite of good design practice. This article explains the advantages and disadvantages of three common methods for adding design margin to heat exchangers. It then introduces a new design margin method that combines the strengths of the previous three while avoiding their weaknesses.