How this compound interest calculator works
Enter what you're starting with, how much you'll add each month, the annual rate, how many years and how often interest compounds. The calculator converts the rate into an equivalent monthly rate, applies it to your balance each month, then adds your contribution at month's end.
The stacked chart separates the money you put in from the interest it earned, so you can watch growth overtake contributions. The year-by-year table lists what you contributed and earned each year and your balance at year end.
Add an inflation rate to see the result in today's purchasing power — the most honest way to judge a long-term goal. Turn on Compare to test a higher contribution side by side.
The compound interest formula
For a single deposit, compound growth follows the classic formula; regular contributions add the future value of an annuity:
FV = P(1 + r/n)^(n·t) + PMT × [ (1 + i)^m − 1 ] ÷ i
- P = initial deposit; PMT = monthly contribution
- r = annual rate; n = compounding periods per year; t = years
- i = equivalent monthly rate = (1 + r/n)^(n/12) − 1; m = months
- Real value = FV ÷ (1 + inflation)^t
Worked example
Start with $10,000, add $500 a month and earn 7% compounded monthly for 25 years. You contribute $160,000 in total, but the balance grows to roughly $462,000 — about $302,000 of it is interest, nearly twice what you put in.
At 2.5% inflation, that balance would buy roughly what $250,000 buys today. Still a strong result, but it's a reminder to plan in real terms.
Why compounding is so powerful
Simple interest pays you only on what you deposited. Compound interest pays you on your deposits and on the interest already earned, so growth accelerates over time. In the early years, contributions do most of the work; later, the interest earned each year can exceed what you add. That crossover point is visible in the stacked chart, and reaching it sooner is the main reason to start early.
The rule of 72 gives a quick check: divide 72 by the annual rate to estimate how many years money takes to double. At 6%, about 12 years; at 9%, about 8.
Time matters more than timing
Because growth builds on itself, years invested matter more than almost anything else. Someone who invests $300 a month from age 25 to 35 and then stops can end up with a balance comparable to someone who invests the same amount from 35 to 65, at the same return. Starting small today usually beats waiting until you can invest more.
Consistency matters too. Automatic monthly contributions remove the temptation to time the market and capture growth every month.
Does compounding frequency matter?
Compounding more often helps, but less than people expect. At 5%, yearly compounding yields 5.00% a year, monthly 5.12% and daily 5.13%. For savings accounts, compare the APY, which already includes compounding. For investments such as index funds there is no stated compounding schedule; returns are irregular, so treat the rate here as a long-run average and use monthly compounding.
Choosing a realistic rate
High-yield savings accounts and Treasury bills pay a rate set by current interest-rate conditions and can change at any time. Broad stock market index funds have historically averaged high single-digit annual returns before inflation over long periods, with large swings from year to year and no guarantee. Bonds have historically returned less than stocks with smaller swings.
A sensible approach is to run three cases: a cautious rate, a middle rate and an optimistic one. Base decisions on the cautious case, and treat anything above it as a bonus.
Inflation, taxes and fees
Inflation reduces what future dollars can buy, so a nominal balance overstates your real progress. Enter an inflation rate to see today's-dollar value, or use an after-inflation return as your rate.
Taxes and fees reduce growth too. Interest in a regular savings account is taxed every year; tax-advantaged accounts such as a 401(k), IRA or HSA let growth compound without annual tax drag. A 1% annual fund fee can consume a large share of long-term growth, so low-cost index funds make a measurable difference over decades.
Frequently asked questions
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Not financial advice. ClearCalc provides estimates for educational purposes only. Results depend on the assumptions you enter and may differ from offers from lenders or institutions. Consult a qualified professional before making financial decisions.