Current emphasis on translational application of genetic models of lung disease has renewed interest in the measurement of the gravimetric filtration coefficient (Kf) as a means to assess vascular permeability changes in isolated perfused lungs. The Kf is the product of the hydraulic conductivity and the filtration surface area, and is a sensitive measure of vascular fluid permeability when the pulmonary vessels are fully recruited and perfused. We have observed a remarkable consistency of the normalized baseline Kf values between species with widely varying body weights from mice to sheep. Uniformity of Kf values can be attributed to the thin alveolar capillary barrier required for gas exchange and the conserved matching of lung vascular surface area to the oxygen requirements of the body mass. An allometric correlation between the total lung Kf,t and body weight in several species (r²=1.00) had a slope which was similar to those reported for alveolar and pulmonary capillary surface areas and pulmonary diffusion coefficients determined by morphometric methods in these species. A consistent Kf is dependent upon accurately separating the filtration and vascular volume components of lung weight gain, but Kf is a consistent and repeatable index of lung vascular permeability.