Compound interest is simple as a formula and surprisingly hard to feel. The relationship between how much you save, the rate you earn, and the number of years you stay invested rarely matches intuition. The Compound Interest Calculator is a Shiny app written in R I built to make that relationship visible. Enter a savings rate, an interest assumption, and a time horizon, and it charts how the capital develops year by year.

It goes past a single number. The app compares scenarios, marks milestones such as the year annual interest starts to exceed what you contribute, and breaks the result down across several charts and a detailed table. The idea came from a video about why the first €100,000 is the hardest part of building wealth. I wanted something I could actually experiment with rather than just watch.

You can open the hosted version directly, or run it locally as a container. The full description of every parameter and chart is on the blog.

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