Compound Interest Calculator
A compound-interest calculator that makes investment growth tangible — set a lump sum and a monthly amount, pick a return (or load an index-fund preset), and watch the balance snowball on an interactive chart.
“Compound interest calculator” is one of the most-searched personal-finance terms, yet most of the tools are cluttered, ad-heavy and hard to read — and almost none let you quickly model a realistic index-fund return. There was a clear gap for the cleanest, most intuitive version that still does the maths properly.
I put the maths in a pure, unit-tested module that compounds month-by-month and snapshots the balance each year, so the numbers are trustworthy independent of the UI. On top sits a slick single-screen tool: sliders and inputs that update instantly, one-tap index-fund presets (S&P 500, FTSE 100, MSCI World and more) at illustrative long-run rates, and an interactive SVG growth chart that splits contributions from interest with a hover tooltip. Everything runs in the browser, scenarios are shareable via the URL, and it stays readable from two options to balances in the hundreds of millions.
- Lump sum + monthly contributions at any rate, over up to 50 years
- One-tap index-fund presets (S&P 500, FTSE 100, MSCI World, bonds, cash)
- Interactive growth chart — contributions vs interest, with hover details
- Live summary: future balance, total paid in, and interest earned
- GBP / USD, and shareable links that reopen the exact scenario
- Provably-consistent maths in a pure, unit-tested module
- Fully responsive + respects reduced-motion — and not financial advice
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