Compound Interest Calculator
Start with what you have, add what you can each month, and watch compounding do the rest. See your final balance, how much of it is growth, and the year your money starts out-earning your deposits.
Assumes a steady annual return compounded monthly, with contributions added at the end of each month. Real returns vary year to year; taxes and fees aren't included.
Not sure what you can invest each month? Find it in your budget →Ultimate Budget Workbook
The monthly contribution is the whole engine here — and it has to come out of a real budget. The workbook helps you find that money and keep it flowing month after month.
See the Budget Workbook →Why compounding feels slow, then sudden
Compound growth is money earning money. In year one that's pocket change, and it's tempting to conclude it isn't working. But every dollar of growth starts earning its own growth, and the curve bends harder every year you stay in. The chart above makes the pattern visible: the gap between what you contributed and what you have is compounding.
The crossover point
Watch the milestones for the year your total growth passes your total contributions. From that point on, your money is doing more of the work than you are. With steady contributions and a typical long-run return, that crossover usually lands somewhere in the second decade — which is exactly why starting early beats starting big.
Time beats amount
Doubling your years invested does far more than doubling your monthly contribution. Ten extra years at the end of the timeline — the years where the curve is steepest — often add more than every deposit you made in the first ten. If you can only change one variable, change when you start.
The rule of 72
A quick sanity check: divide 72 by your annual return to estimate how many years your money takes to double. At 8%, that's about nine years — so a 36-year timeline is roughly four doublings, or 16× your early dollars.
How does compound interest work?
Compound interest means you earn returns on your returns. Each period, growth is calculated on the whole balance — original money plus everything it has already earned. Early on the effect looks small; over decades it becomes the majority of the balance. The chart above shows exactly when growth overtakes your own contributions.
What return rate should I use?
It depends on what you're modeling. Long-run U.S. stock market averages are often cited around 7–10% before inflation, high-yield savings accounts are far lower, and no rate is guaranteed. Try a conservative and an optimistic rate and treat the range — not either single number — as your planning picture.
How often does this calculator compound?
Monthly. Your annual rate is divided by 12 and applied each month, and monthly contributions are added at the end of each month. That matches how most people actually invest — a recurring monthly deposit.
Is this compound interest calculator free?
Yes — free, no ads, no signup, and it runs entirely in your browser. Nothing you enter is sent anywhere or stored.