The capacity of plants to minimize uncontrolled water loss is essential for survival in adverse and changing climatic conditions. In order to assess and compare the effectiveness of apoplastic barriers to water, the permeability of the barrier must first be quantified. Studies have accomplished this directly by quantifying tritium flux or indirectly by measuring the influx/efflux of water surrogates such as dyes, chlorophyll, and herbicides. Other studies have relied on comparative methods such as survival rates after drought. These methods rely on radioactive material, correlations, or qualitative comparisons. However, a quantitative method is necessary that directly measures water efflux and that allows easy comparisons within and between experiments, plant parts, plant species, and especially research laboratories. Here we outline in detail a gravimetric protocol first described by Schönherr and Lendzian (1981) that can be set up in less than half a day and completed in one to ten days depending on the plant barrier. This approach has been used in numerous studies on leaf and fruit cuticles and recently also on petals from cosmos (Cosmos bipinnatus; Buschhaus et al., 2015).