Compound Interest Calculator
Initial deposit + monthly contributions · Future value in one click · Free, no sign-up
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RESULTS
Future value--
Total contributed--
Total interest earned--
Fill in the fields and hit Calculate. Assumes a constant annual return compounded monthly — real returns fluctuate, so treat this as a reference figure.
How compound interest works
Compounding means each period's earnings get folded back into your balance, so next period both the original money and the earnings are working for you.
Future value = deposit × (1 + monthly rate)^months + monthly contribution × [((1 + monthly rate)^months - 1) / monthly rate], where monthly rate = annual return / 12.
An example: start with $10,000, add $1,000 a month, assume an 8% annual return, and after 10 years you land a bit over $200,000 — of which only about $130,000 is money you actually put in. The rest is compounding. Stretch the horizon and that gap widens fast, which is the whole mathematical argument for long-term investing.
FAQ
How is compound interest calculated?
Future value = deposit × (1 + monthly rate)^months + monthly contribution × [((1 + monthly rate)^months − 1) / monthly rate], with monthly rate = annual return / 12. Each period's earnings are added back and go on earning.
What is the difference between compound and simple interest?
Simple interest pays only on the original principal; compound interest adds earnings back into the balance so they earn too. The longer the horizon, the wider the gap.
Can I use this to project stock or crypto returns?
For a rough estimate, yes — but real stock and crypto returns swing wildly year to year rather than compounding steadily. This is a theoretical figure assuming a constant return, not a forecast.
How much difference do monthly contributions make?
A lot over long horizons. Steady small contributions plus time often beat a single larger lump sum with nothing added later — the arithmetic behind dollar-cost averaging.