Savings
Compound Interest Calculator
See exactly how compound interest will grow your money over time. Enter your starting balance, monthly contributions, expected return, and time horizon — we'll show you the final balance and how much of it is interest.
Savings
Compound Interest Calculator
Result
Albert Einstein supposedly called compound interest the eighth wonder of the world. Whether he actually said it or not, the math is genuinely remarkable: money grows on money grows on money, and given enough time, even modest contributions become large sums. This calculator lets you see your specific numbers.
The math behind the result
You enter four values:
- Starting balance. What you have today that you can leave invested.
- Monthly contribution. What you'll add each month going forward.
- Annual return rate. Your expected average annual return. For broad stock market index funds, 7% is a common long-term assumption (10% historical, minus roughly 3% inflation). For high-yield savings, current rates are 4-5%. For bonds, around 4-5%.
- Time horizon. How many years you'll leave the money invested.
The calculator compounds the return monthly (matching how most investment accounts actually report performance), adds your contributions, and rolls the balance forward. The result shows your final balance, total contributions, and total interest earned.
What your result actually tells you
Three numbers matter:
- Final balance. What the account is worth at the end of your time horizon.
- Total contributions. The principal you put in yourself.
- Total interest. The growth from compounding — money you didn't have to earn.
The third number is the whole point. If you contribute $200 per month for 30 years at 7%, you'll put in $72,000 of your own money — but the account will be worth about $244,000. The difference, $172,000, is interest. The longer you let it run, the more of the final balance comes from interest rather than your own contributions.
The Rule of 72
A quick mental shortcut: divide 72 by your expected annual return rate to estimate how many years it takes for money to double.
At 4% (a high-yield savings rate), money doubles every 18 years. At 7% (a typical stock market assumption), every ~10 years. At 10%, every 7.2 years. The Rule of 72 isn't precise, but it's accurate enough to be useful when comparing rates in your head.
Why starting early beats contributing more
The single most important variable in compound interest isn't the rate — it's time. Consider two savers:
Contributes $200/month for 10 years ($24,000 total), then stops contributing. Leaves the money invested at 7% until age 65.
Final balance at 65: approximately $340,000
Contributes $200/month for 30 years ($72,000 total) at 7%, until age 65.
Final balance at 65: approximately $245,000
Saver A contributed one-third as much money and ended up with significantly more. The difference is ten extra years of compounding on the early contributions. This is the single strongest argument for opening a retirement account in your twenties, even if you can only spare $50 per month.
Compounding frequency matters less than you think
You might see "compounds daily" or "compounds monthly" advertised on savings accounts. The difference between daily and monthly compounding on the same APR is real but small — on a $10,000 deposit at 4%, you'd earn about $1 more per year with daily compounding. Don't choose accounts based on this; choose them based on the APY (which already accounts for compounding frequency) and on whether the institution is FDIC-insured.
Variables outside the calculation
The estimate assumes a fixed average return, which never happens in real markets. In a year like 2008, an S&P 500 index fund lost about 37%; in 2019, it gained 31%. The 7% long-term average comes out of decades of these swings averaged together. Your account balance will rise and fall around the projection — that's normal and not a calculator error.
The calculator also doesn't account for taxes (which can reduce returns by 1-2% per year in a taxable brokerage account), inflation (which erodes purchasing power by 2-3% per year on average), or fees (which can quietly drain 0.5-1% annually if you're in expensive mutual funds). For tax-advantaged accounts like a 401(k), Roth IRA, or HSA, the tax drag is much smaller, which is why those accounts beat taxable accounts for long-term growth.
Questions readers ask
What is the Rule of 72?
Divide 72 by your expected annual return rate to estimate how many years it takes for money to double. At 7% return, money doubles every ~10 years. At 4%, every 18 years. The rule isn't perfectly precise but it's accurate enough to be useful as a mental shortcut when comparing investment options.
How often does interest compound on a savings account?
Most savings accounts compound daily and pay monthly. The difference between daily and monthly compounding on the same APR is small (about $1 per year on a $10,000 deposit at 4%). What matters more is the APY (Annual Percentage Yield), which already accounts for the compounding frequency. Compare APYs directly when shopping for savings accounts.
Is it better to start saving early or save more later?
Starting early almost always wins. Someone who invests $200/month from age 25-35 and then stops will likely end up with more money at retirement than someone who invests $200/month from 35-65, despite contributing one-third as much. The reason is that the early contributions get 10+ extra years of compounding, and those late years are when compound growth accelerates most dramatically.
What's a realistic return rate to use in this calculator?
It depends on what the money is invested in. For broad stock market index funds, 7% is a common assumption for long-term planning (representing the historical average return of about 10% minus 3% inflation). High-yield savings accounts currently yield 4-5%. Bonds typically return 4-5%. For a diversified portfolio of stocks and bonds, 5-7% is a reasonable planning range.
Does compound interest work on debt too?
Yes — and it works against you. Credit card balances compound monthly at APRs of 18-29%, which is why minimum payments barely make a dent. The same math that grows your retirement account also grows your debt. Pay off high-interest debt before building investments beyond a starter emergency fund.
How do I account for inflation in compound interest projections?
The simplest approach is to use a 'real' return rate instead of a nominal one. Subtract roughly 3% (the long-term average inflation rate) from your expected return. If you think your investments will return 7% nominally, plug in 4% instead, and the calculator's output will be in today's dollars — comparable to your current cost of living.
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Important note
WalletCalcs provides educational estimates only. Results are not financial, tax, lending, legal, or investment advice. Always confirm important decisions with the appropriate professional or provider.