Compound Interest Calculator
A free, browser-based calculator that projects how your investments grow over time.
A compound interest calculator built entirely in the browser - no server, no account, no tracking. Enter your investment parameters, hit calculate, and instantly see your projected balance grow year by year. Your data never leaves your browser.
What it does
Five inputs - expected annual return, initial deposit, monthly investment, starting age, and retirement age - drive a real-time projection of your portfolio. Results appear as both an interactive Chart.js line chart (portfolio balance vs. total contributions) and a full year-by-year table.
Highlights
- Instant + client-side - calculations run in the browser with inline validation on every field; no page reloads.
- Sensible guardrails - annual return capped to a realistic range, monetary values must be positive, ages validated.
- Visualised growth - animated chart with hover tooltips showing the gap between what you put in and what compounding adds.
- Portable - ships as a self-contained HTML export with iframe-embed and WordPress integration paths.
Why I built it
This was the first thing I built with Ai agents, an experiment in building a useful, self-contained financial tool with the aesthetics of cyberpunk theme. I use my agents to build out a slide deck as well looks terrible I wouldn’t want to try and ship something at that standard. Still shows how much humans need to be in the loop for taste.