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.
Projected balance
Year-by-year growth
How your balance builds from contributions and compounding.
How you compare
💹 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).
Results update as you type and are estimates, not professional advice — verify important decisions with a qualified professional.
Worked example
$1,000 plus $100/month at 7% for 10 years grows to about $19,300 (~$6,300 of it interest).
Common mistakes
- Assuming an optimistic, steady return every year.
- Ignoring taxes and fees on the growth.
Where it is used
- Projecting a savings or investment balance.
- Seeing the impact of starting earlier.
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.