Compound Interest Calculator
See how savings grow over time. Enter a starting amount, expected return, and a monthly contribution — we compound it monthly and show the future value and interest earned.
Example: with Starting amount $1,000 · Annual return 7% · Years 10 yrs · Monthly contribution $100 → Future value: $19,318.
- Total contributed$13,000
- Interest earned$6,318
Computed by the calculator below using its default values. Change any input to see your own numbers.
Projected balance
How you compare
Year-by-year growth
How your balance builds from contributions and compounding.
💹 Open a high-yield account / brokerage
Check it outWhy compounding matters
Compound interest pays you interest on your interest, so growth accelerates the longer you stay invested. Two levers dominate the outcome: time and contribution size. Starting earlier often beats contributing more later, because each early dollar compounds for more years.
How it’s calculated
Future value = P(1 + r)^n + PMT × ((1 + r)^n − 1) ÷ r, compounded monthly (r = monthly rate, n = months).
You can also choose a compounding frequency other than monthly — annually, semiannually, quarterly, daily, or continuously. Since contributions are always monthly, we convert whichever frequency you pick into an equivalent effective monthly rate: for periodic compounding with k periods a year, effective monthly rate = (1 + annual rate ÷ k)^(k ÷ 12) − 1; for continuous compounding, effective monthly rate = e^(annual rate ÷ 12) − 1. Choosing “Monthly” reproduces the exact original formula above. The contribution timing toggle controls whether each month’s deposit is added before that month’s growth (beginning of period, an annuity-due) or after (end of period, an ordinary annuity, the default) — beginning-of-period contributions get one extra month of growth each, so they compound to a slightly larger balance.
Results update as you type and are estimates, not professional advice — verify important decisions with a qualified professional.
Year-by-year growth
Common mistakes
- Assuming an optimistic, steady return every year.
- Ignoring taxes and fees on the growth.
Frequently asked questions
What return should I assume?
Be conservative. Long-run stock market averages are often cited near 7% after inflation, but returns vary widely year to year and aren't guaranteed.
Is this before or after tax?
It ignores taxes and fees. Tax-advantaged accounts keep more of the growth.
How often is it compounded?
Monthly, which matches the monthly contributions. More frequent compounding changes the result only slightly.
What is the Rule of 72?
Divide 72 by your annual return to estimate doubling time: at 8%, money doubles roughly every 9 years; at 6%, every 12. It's a mental shortcut — this calculator does the exact math.
Does monthly vs annual compounding actually matter?
Less than people think at typical rates: $10,000 at 7% for 20 years is about $40,387 compounded monthly vs $38,697 annually — a ~4% difference. Contribution amount and time in the market matter far more than compounding frequency.
What does "continuous" compounding mean?
It's the mathematical limit as compounding periods approach infinity, calculated with e^(rt) instead of a periodic formula. In practice it barely beats daily compounding — the gap between daily and continuous is negligible for real-world balances.
Should I contribute at the beginning or end of the month?
Beginning-of-period (annuity-due) contributions grow for one extra month each compared to end-of-period (ordinary annuity), so they produce a slightly higher future value. The difference compounds over many years but is usually small compared to your rate of return and time horizon.