VariFlex is a program package for estimating rate constants of several types of gas phase reactions. Its focus is the application of transition state theory with an accurate treatment of the flexibility of the interfragment modes for variable definitions of the reaction coordinate. Although aimed at calculating rates at this high level of sophistication for barrierless reactions, the package also allows convenient estimates of the rates using various alternative approaches such as phase space theory (PST), tight transition state theory, [i.e., with rigid rotor harmonic oscillator estimates (RRHO)], inverse Laplace transformations, and generic standard hydrocarbon estimates.