Compound Interest Calculator
Compound interest is the engine behind almost every long-term wealth-building strategy — it’s what turns steady, unremarkable monthly contributions into a large balance decades later, because each year’s interest starts earning its own interest. This calculator projects the future value of a lump sum plus regular monthly contributions, assuming monthly compounding, so you can see exactly how much of your ending balance came from your own money versus growth. It’s the same underlying math used across our FIRE, retirement, and millionaire-timeline tools.
How This Calculator Works
Your initial investment grows on its own using standard monthly-compounding math, while your monthly contributions are treated as a separate stream added at the end of each month and compounded forward from that point. Adding these two pieces together gives the future value shown above. Total interest earned is simply the future value minus every dollar you actually put in — it’s the purest measure of how much compounding did the work for you versus how much came from your own contributions.
Why Time Matters More Than Timing
Because each year’s growth compounds on top of the last, the final years of a long time horizon typically add far more in dollar terms than the first years — even though the contribution amount hasn’t changed. This is why starting even a few years earlier, or leaving a portfolio untouched a few years longer, tends to matter more than trying to perfectly time when you invest. It’s also why the same monthly contribution produces a dramatically different result at 10 years versus 30 years — try adjusting the years field above to see the effect for yourself.
A Few Practical Notes
This calculator assumes a constant annual return compounded monthly, which is a simplification — real markets move up and down year to year, and no investment guarantees a fixed rate. It also doesn’t account for taxes, investment fees, or inflation, all of which reduce your real-world spending power at the end of the period. Treat the result as a planning estimate rather than a guarantee, and consider running the numbers at a more conservative rate (5–6%) alongside your expected rate to see a realistic range of outcomes.
Freedom Wealth Lab Apps provides general educational tools, not personalized financial advice. Read our full Disclaimer and Affiliate Disclosure. This calculator uses standard, universal compound-interest mathematics — no country-specific tax or regulatory data.
