Compound Interest Calculator
Project how your savings or investments grow over time.
About this tool
Compound interest snowballs: each year's interest goes on to earn interest of its own. This calculator projects the growth of your money from a starting balance, regular contributions, an annual return and a time horizon, then splits the final figure into what you paid in and what the compounding earned you.
It is the clearest way to see why starting early matters more than almost anything else, and there is an inflation setting so you can read the result in today's money. Everything is calculated locally — no personal financial data is collected.
Frequently asked questions
What is the compound interest formula?
For a lump sum, final value = P × (1 + r)ⁿ, where P is the starting balance, r the annual rate and n the number of years. With regular contributions, each payment compounds over its own remaining time — which is what this calculator works out for you.
Why does starting early matter so much?
At 7% a year money roughly doubles every decade. $10,000 invested at 25 becomes around $76,000 by 55, but only about $38,000 if you start at 35. Time, not the size of the contribution, is the dominant factor.
Does it account for tax and inflation?
Inflation is optional — switch it on and you get the real, purchasing-power value alongside the nominal one. Tax depends entirely on the account you use (ISA, 401(k), pension, general investment account), so it is not modeled.
What return rate should I assume?
Historically, a diversified global equity portfolio has returned roughly 7% a year before inflation over long periods, and cash savings far less. Model a pessimistic and an optimistic figure rather than betting on one number.
Is it really free, with no sign-up?
Yes — the tool is free, needs no account and has no usage limit. Processing happens locally in your browser, so your data never leaves your device.