Compound Interest with Contributions Calculator
Grow a starting balance plus regular monthly deposits with compound interest — see the final value, what you put in, and interest earned.
How it works
Monthly rate r = rate ÷ 1200 · FV = principal × (1 + r)^n + contribution × [((1 + r)^n − 1) ÷ r], n = months
This combines two engines: your starting balance compounds on its own, and each monthly deposit compounds from the moment it lands (an ordinary annuity, deposited at month-end). The longer the horizon, the more the interest-on-interest dominates — in later decades the growth on prior gains can outpace the fresh contributions entirely. That crossover is the whole case for starting early: time does more work than amount.
Worked example
Starting at $10,000 and adding $500 a month at 7% for 20 years grows to about $299,600. You contribute $130,000 in total ($10,000 plus $120,000 of deposits), so roughly $169,600 is compound interest.
Frequently asked questions
Does the timing of contributions matter?
Slightly. This calculator assumes deposits at the end of each month (an ordinary annuity). Depositing at the start of each month lets each contribution earn one extra month of interest, raising the total a little.
Why does interest overtake my contributions?
Early on, your deposits dwarf the interest. But interest compounds on an ever-larger balance, so given enough years the annual growth can exceed everything you add — which is why long horizons matter so much.
What rate should I assume?
For a diversified stock portfolio, 6–8% before inflation is a common long-run planning range; savings and CDs are lower. Use a rate that matches where the money is actually invested, and remember returns are not guaranteed.
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Educational estimate, not financial advice.