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Dike propagation is an intrinsically multiphase problem, where deformation and fluid flow are intricately coupled in a fracture process. Here we perform the first fully coupled simulations of dike propagation in two dimensions, accounting for depressurization of a circular magma chamber, dynamic fluid flow, fracture formation, and elastic deformation. Despite the complexity of the governing equations, we observe that the lengthening is well explained by a simple model a(t)=c1log(1+t/c2), where a is the dike length, t is time, and c1 and c2 are constants. We compare the model to seismic data from eight dikes in Iceland and Ethiopia, and, in spite of the assumption of plane strain, we find good agreement between the data and the model. In addition, we derive an approximate model for the depressurization of the chamber with the dike length. These models may help forecast the growth of lateral dikes and magma chamber depressurization. Plain Language Summary: Volcanic dike intrusions, propagating magma‐filled fractures, precede most eruptions. Dike propagation has been studied for decades through simplified analytical and numerical models. To date, no study has fully addressed how the fluid magma, host rock, and the magma chamber all interact at the same time and drive the dike forward. We present such simulations for a two‐dimensional configuration and deduce that a simple formula can explain how the dike lengthens with time. We suggest that this simple formula may be used to forecast dike growth. Key Points: Fully coupled simulations of dike growth and magma chamber depressurization are performedA simple model for dike length with time is identified and compared to seismic observationsA simple model of chamber pressure versus dike length is derived and compared to simulations [ABSTRACT FROM AUTHOR]