Compound Interest Calculator
Starting amount, monthly contribution, return rate, years — watch the balance and the growth curve it produces.
Compound interest is growth on top of growth: each month's returns get added to the balance, and next month's returns are earned on that bigger balance. Over years the effect snowballs, which is why starting early matters more than starting big. This calculator makes that concrete — enter a starting amount, what you'll add monthly, an expected annual return and a number of years, and see the final balance split into what you contributed versus what compounding earned.
The milestone table is where the story lives: in the early years, contributions dominate; somewhere in the middle, growth catches up; and in the late years, the interest column dwarfs everything you put in. Try the default — $10,000 plus $250/month at 7% — and watch the crossover happen. A 7% return is the classic long-run stock-market assumption after inflation; use 4–5% for a conservative mix or your savings account's rate for cash. The same math works in reverse for debt, which is why the loan calculator's total-interest number gets so large — compounding is working against you there. Figuring out what you can contribute each month? Start from your pay with the salary calculator.
How to use the Compound Interest Calculator
- Enter your starting amount — zero works if you're starting from scratch.
- Enter what you plan to contribute each month.
- Set an expected annual return (7% is the common long-run stock-market assumption) and the number of years.
- Read the final balance and the contributed-versus-growth split, and scan the milestone table to see when compounding takes over.
The rule of 72: doubling in your head
Here's a shortcut worth memorising. To estimate how many years it takes for money to double at a given rate, divide 72 by the rate. At 6% a year, money doubles in about 72 ÷ 6 = 12 years; at 8%, about 9 years. It isn't exact, but it's close enough to do without a calculator and it makes the value of a slightly higher rate obvious.
The rule works because of compounding — you earn interest on your interest, so growth accelerates over time rather than moving in a straight line. That's why the later years in the calculator above add so much more than the early ones, and why starting early beats saving more. The compound interest guide shows the widening gap year by year.
Frequently asked questions
How does the calculator compound?
Monthly: each month the balance grows by one-twelfth of the annual rate, then your contribution is added. Monthly compounding with end-of-month contributions matches how most brokerage and savings scenarios are modeled.
What annual return should I assume?
For long-term stock investments, 7% is the standard inflation-adjusted historical assumption (about 10% before inflation). Use 4–5% for a balanced portfolio, and your actual APY for savings accounts or CDs. The honest move is running the calculator at a couple of rates and looking at the range.
Why do small monthly contributions matter so much?
Because every contribution starts its own compounding clock. $250/month at 7% is $60,000 contributed over 20 years — but roughly $130,000 in balance. The earlier dollars do the heaviest lifting, which is the whole argument for starting now rather than starting bigger later.
Does this account for inflation or taxes?
No — results are in nominal dollars, before taxes. A common shortcut: use an inflation-adjusted return (like 7% instead of 10%) and the result reads roughly in today's purchasing power. Taxes depend on the account type (401k, IRA, taxable), which is beyond a single calculator.
Is the growth guaranteed?
No. The calculator assumes a constant return; real markets swing widely year to year and can be negative. Over multi-decade horizons the average has historically smoothed out, but the path is bumpy — treat results as a planning estimate, not a promise.