PurposeThe purpose of this paper is to define and quantify the term robustness within the context of a Private Finance Initiative (PFI) project environment from the perspective of the Granting Authority. The paper is conceptual, based on conceptual generalisations.Design/methodology/approachThe paper considers the theory of systems thinking within PFI procurement. This is further integrated with the concept of robustness and resilience used extensively in the ecology discipline. Combining the two, this paper presents a mathematical approach of quantifying robustness in PFI projects.FindingsAn analytical model is used to support the mathematical analysis to quantify and define robustness.Research limitations/implicationsThe quantifying of robustness is based on the principal assumptions presented in the paper. The principal assumptions provide an ideal situation which is necessary to pursue and develop the proposed approach to quantify robustness. Changes to the assumptions may affect the generalisability of the approach.Originality/valueThe paper provides a greater definition to robustness within PFI projects and the possibility of quantifying robustness to better monitor and manage the characteristic.