Compound interest calculator
See what savings grow into, with contributions and interest split out.
Future value
$144,572.72
| Year | You put in | Interest earned | Balance |
|---|
How compounding does the heavy lifting
Simple interest pays only on your starting amount. Compound interest pays on the starting amount and on every piece of interest already earned, so growth accelerates the longer money stays invested. The standard formula for a single amount is written below. This calculator runs the same maths month by month, which lets it add a monthly contribution on top and show the balance at the end of every year.
A worked example from the defaults on this page: $10,000 growing at 7% for 20 years, with $200 added each month. Your own money in is $58,000 in total. The rest of the final balance is interest, and most of that interest arrives in the second decade, when the balance being compounded is much larger. That back-loaded shape is exactly why starting early usually beats contributing more later.
Compounding frequency matters, but less than people expect. Daily compounding beats annual compounding on the same stated rate, yet the gap is small next to the effect of the rate itself, the years invested, and whether contributions keep going in flat markets as well as good ones. The Rule of 72 gives a quick sense of pace: divide 72 by the annual rate to estimate the years a single amount takes to double. At 7%, that is roughly ten years.
These figures are projections, not promises. Real investments move up and down, returns are not a fixed rate, and taxes, fees and inflation all reduce what a balance is worth in spending terms.
Frequently asked questions
What is the compound interest formula?
The standard formula is A = P(1 + r/n)nt, where P is the starting amount, r the yearly rate, n how often interest compounds each year and t the number of years. Monthly contributions are added on the same schedule, each one compounding for the time it stays invested.
How much will $10,000 grow at 7% over 20 years with $200 a month added?
About $144,573. Of that, $58,000 is money you put in ($10,000 at the start plus $200 a month for 20 years) and roughly $86,573 is interest the balance earned on itself.
Does compounding frequency make a big difference?
A small one. Monthly compounding beats yearly compounding, but the gap is minor next to the rate, the number of years and how much you contribute. Starting earlier and contributing steadily move the result far more than the compounding schedule.
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Estimates only. This tool projects growth at a fixed rate; it is not investment, tax or financial advice.