Most financial projections tell a tidy, deterministic story: save this much at that rate and you end up with a specific sum in twenty years. Markets do not work that way. Compound Simulation is a Shiny app written in R that takes the probabilistic view instead, running a Monte Carlo simulation over many possible paths to show the spread of outcomes rather than one smooth curve.

You give it the usual inputs: initial capital, monthly contributions, an expected return and a volatility, and a time horizon. It returns a fan chart of trajectories with uncertainty bands, the distribution of end values, and the probability of reaching a target you define. Two extras keep it honest. A stress test halves returns and doubles volatility, and a savings-elasticity view shows whether putting a little more aside each month moves the outcome more than chasing a higher return.

It is the probabilistic companion to the deterministic Compound Interest Calculator . The hosted app is linked below, and the full explanation of the model and its parameters is detailed in the blog post.

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