Methane (CH₄) is a potent greenhouse gas that is normally produced by microbial fermentation in the rumen and released to the environment during the eructation process. Prediction of ruminal CH₄ is important for ruminant nutrition, especially for determination of ME intake. Equations have been developed to predict ruminal CH₄ production based on dietary constituents, but none have considered condensed tannins (CT). Our objective was to develop an equation to predict ruminal CH₄ inhibition by CT. We gathered CH₄ production data from 24- to 48-h in vitro fermentation of diverse forages containing different concentrations of CT over the course of 3 years. Our analysis included 113 observations. The predictor variable CT was regressed on the response variable CH₄ using PROC NLIN of SAS and the Gauss-Newton method to converge the parameters of the nonlinear regression. We used the following exponential decay model to express the relationship between CT and CH₄: Y = span x e^(-KxX) + plateau, where Y is CH₄, g/kg FOM; span is the difference between Y when CT equals zero and the plateau (Y value at infinite), g/kg FOM; K is the fractional rate of decline, 1/% DM; and X is CT concentration, % DM. The following nonlinear exponential decay regression equation was developed: CH₄ = 113.6 x e^-0.1751xCT- 2.18 (r² = 0.52; P < 0.0001). This equation predicted that CH₄ production could be reduced by 50% when CT is about 3.85% DM. We used several statistics to evaluate the adequacy of this equation, including precision and accuracy. We determined that this equation is more accurate when screening CT-containing forages for their potential ability to mitigate CH₄ production by ruminants when the CT concentration is greater than 5% DM. We concluded that despite the large degree of variability in ruminal CH₄ production, this equation can be used as a tool for predicting potential ruminal CH₄ inhibition that occurs when feeding CT-containing forages to ruminants. Future research should focus on the development of predictive equations when other potential reducers of ruminal CH₄ are used in conjunction with CT.