Compound Interest Calculator
See how savings and regular contributions grow over time.
Formula
About this calculator
Compound interest is often called the eighth wonder of the world because your returns start earning returns of their own. The longer your money stays invested, the more the curve bends upward — which is why starting early matters far more than starting big.
This calculator models two forces at once: your initial lump sum growing at a compound rate, and your regular contributions each earning compound interest from the moment they are added. The result is future value (FV), broken down so you can see exactly how much came from your own deposits versus how much the compounding generated for free.
Example: investing $5,000 up front plus $300 a month for 30 years at a 7% annual return grows to roughly $404,000 — of which only about $113,000 is money you deposited. The other ~$291,000 is compound growth. Shift the same plan to a 20-year horizon and the total drops to about $173,000, showing how the final decade does the heaviest lifting.
Frequently asked questions
What compounding frequency should I use?
Most index funds and savings vehicles are modeled well with monthly or annual compounding. More frequent compounding raises the result slightly; the rate and time horizon matter far more.
Why does starting early matter so much?
Because the biggest gains come in the final years, when the balance is largest. Every extra year at the start adds one more of those high-growth years at the end.
Is investment return guaranteed?
No. Market returns vary year to year and can be negative. Use a conservative long-run average and treat the result as a projection, not a promise.
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⚠️ Projections assume a constant rate of return, which real investments do not provide. This is not investment advice.