π Compound Interest Calculator
Watch your savings grow with compounding and monthly contributions.
Future Value
Estimates for planning only, not financial advice.
How to Use This Calculator
This calculator shows what your savings become when interest compounds and you keep adding money every month. Enter your starting balance, monthly contribution, expected annual return, time horizon, and how often interest compounds, and you get the future value plus a year-by-year breakdown.
Enter your Initial Principal, the lump sum you are starting with, and your Monthly Contribution, the amount you add at the end of each month.
Enter the Annual Rate (APR) you expect, such as 7 for a stock-heavy retirement account, and the number of Years you plan to let it grow.
Pick a Compounding Frequency. Monthly matches most savings accounts; switch between options to see how frequency changes the total.
Read the results: the future value, how much you contributed versus how much interest did the work, your rule-of-72 doubling estimate, and the yearly table. Use the CSV button to save the table.
The Compound Interest Formula
The calculator simulates every month one at a time. It converts your nominal APR into an effective monthly rate based on the compounding frequency, grows the balance by that rate, then adds your contribution. This month-by-month method is exact for any frequency, which is why the yearly table always matches the final total.
Compounding Frequency and APY vs APR
APR is the advertised rate before compounding; APY is what you actually earn after compounding is counted. With monthly compounding at 7 percent APR, the APY is about 7.23 percent. With annual compounding, APY equals the APR exactly. More frequent compounding always helps, but the gaps are small: on $10,000 plus $200 a month for 10 years at 7 percent, daily compounding finishes at about $54,791, monthly at $54,714, quarterly at $54,556, and annually at $53,882. Your contribution amount and your rate move the needle far more than the frequency ever will.
Worked Example
Start with $10,000, add $200 at the end of each month, earn 7 percent APR compounded monthly, and wait 10 years. The monthly rate is 0.07 / 12 = 0.5833 percent. After 120 months the balance reaches $54,713.58. Your total contributions were $10,000 + ($200 x 120) = $34,000, so the interest earned is $54,713.58 - $34,000 = $20,713.58. The rule of 72 says money doubles in 72 / 7 = about 10.3 years at this rate. Now consider the cost of waiting: start one year later with the same plan and the same rate, and you finish at about $48,713, roughly $6,000 less. That missing $6,000 is the interest you never gave time to compound.
What This Does Not Cover
This is a planning estimate, not a prediction. Real investment returns bounce around year to year, inflation quietly eats purchasing power, and taxes can take a cut depending on the account type. A steady 7 percent every year is a smoothing assumption, useful for comparing plans, not a guarantee. For a conservative plan, run the numbers again at a lower rate and see whether the plan still works. Estimates for planning only, not financial advice.
Frequently Asked Questions
What is compound interest?
Compound interest means you earn interest not only on your original deposit but also on the interest already added to your balance. Each compounding period, your balance grows a little, and the next period earns interest on that bigger balance. Over years, this snowball effect is why starting early matters more than contributing large amounts late.
What is the difference between APY and APR?
APR (annual percentage rate) is the nominal yearly rate before compounding is counted, while APY (annual percentage yield) reflects what you actually earn after compounding. Because interest compounds, APY is always slightly higher than APR when compounding happens more than once a year. This calculator lets you enter an APR and shows the effective result for any compounding frequency.
How often should interest compound?
More frequent compounding grows your balance a little faster, but the difference is small at normal rates. On $10,000 plus $200 a month at 7 percent for 10 years, monthly compounding gives about $54,714, quarterly gives about $54,556, and daily gives about $54,791. The contribution amount and the rate matter far more than the frequency.
What is the rule of 72?
The rule of 72 is a quick mental shortcut: divide 72 by your annual rate to estimate how many years it takes money to double. At 7 percent, 72 / 7 is about 10.3 years. It works for any rate, and this calculator shows the estimate next to your results.
How much will $200 a month grow to in 10 years?
At a 7 percent APR with monthly compounding, $10,000 starting principal plus $200 a month grows to about $54,714 in 10 years. Of that, $34,000 is money you put in and about $20,714 is interest. If you wait just one year to start, you end up with roughly $6,000 less, which shows the real cost of delaying.