Compound Interest Calculator
$10,000 invested at 7% compounded annually grows to $19,671.51 in 10 years, even without new deposits. A compound interest calculator estimates how money may grow from a starting balance, a fixed annual rate, a time period, a compounding schedule, and optional recurring contributions. Use it to compare savings goals, long-term investing habits, education funds, or other financial plans where steady growth matters. Change the rate, years, contribution amount, or compounding frequency to see how each input changes projected future value, total invested, total contributions, and total interest earned. The result is a planning estimate, not a promise, but it gives you a fast way to test what time and consistency can do.
Quick answer
Compound interest adds each period's interest to the balance, so later interest is earned on both the original principal and earlier interest.
What this tells you
- •Compound interest adds each period's interest to the balance, so later interest is earned on both the original principal and earlier interest.
- •Rate, time, and contribution size usually have a larger effect on the ending balance than small differences between monthly and daily compounding.
- •The contribution field is per compounding period, so a $100 contribution with monthly compounding means $100 each month.
- •Total invested equals the starting principal plus every recurring contribution you make during the projection.
- •Total interest equals future value minus total invested, which helps separate account growth from money you added yourself.
How to Use
- 11. Enter the starting principal, which is the amount already in the account or the amount you plan to invest today.
- 22. Enter the annual interest rate as a percent, such as 6 or 6.5. Do not convert it to a decimal first.
- 33. Enter the investment period in years. Decimals work if you want to model part of a year.
- 44. Choose the compounding frequency that matches the account, product illustration, or assumption you want to test.
- 55. If you plan to add money regularly, enter the contribution amount for each compounding period. Leave it at 0 if you want a no-deposit projection.
- 66. Calculate the result, then review future value, total invested, total contributions, and total interest. Try a second scenario with a different rate or time horizon to see which variable changes the outcome most.
How It Works
Formula
FV = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) - 1) ÷ (r/n)]The calculator uses the standard compound interest formula plus a recurring contribution term. `P` is the starting principal, `r` is the annual rate as a decimal, `n` is the number of compounding periods per year, `t` is the number of years, and `PMT` is the amount contributed each period. The first part, `P(1 + r/n)^(nt)`, grows the opening balance. The second part grows a stream of equal end-of-period deposits. For example, $10,000 at 7% compounded annually for 10 years becomes $10,000 × 1.07^10 = $19,671.51. If the periodic rate is 0, the calculator skips the division step and simply adds the contributions because no interest is being earned.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Leave $10,000 invested for 10 years
There are 10 annual periods, so the balance becomes $10,000 × 1.07^10 = $19,671.51. Because no extra deposits were made, all $9,671.51 of growth is interest. This example shows how much time alone can do when the rate stays steady.
Start with $5,000 and add $200 per month for 5 years
The starting $5,000 grows to $7,449.23 across 60 monthly periods at 8%. The $200 monthly deposits add $12,000 of contributions and grow to $14,695.37. Together they produce $22,144.60, which means $5,144.60 of the ending balance comes from interest rather than deposits.
Invest $15,000 and add $250 each quarter for 20 years
A 5% annual rate compounded quarterly means 80 periods at 1.25% per period. The original $15,000 grows to $40,522.27, and the quarterly deposits grow to $34,029.70. The account reaches $74,551.97, with $39,551.97 in interest on $35,000 invested.
Check a daily compounding estimate over 3 years
Using 365 compounding periods per year creates 1,095 periods across 3 years. The balance becomes $2,000 × (1 + 0.045/365)^1095 = $2,289.05 after rounding. This shows that more frequent compounding helps, but the difference stays modest unless the balance, rate, or time period is larger.
Model regular deposits with a 0% return
With a 0% rate, the starting principal stays at $12,000 and the calculator adds 96 monthly deposits of $150. Those deposits total $14,400, so the final balance is $26,400.00 and total interest is $0.00. This is a useful check when you want to confirm the contribution logic before testing a positive return.
Project a larger long-term monthly plan
There are 180 monthly periods in this projection. The original $25,000 grows to $61,352.34, and the $300 monthly deposits grow to $87,245.61. That produces $148,597.95 in total value, with $69,597.95 of growth on $79,000 invested.
Common mistakes
- Entering 0.08 instead of 8 for an 8% rate. The field expects a percent, not a decimal.
- Typing a monthly deposit while compounding is set to annually. The contribution amount should match the selected compounding period.
- Assuming deposits are added at the beginning of each period. This calculation treats recurring contributions as end-of-period deposits.
- Comparing the estimate with a real statement that includes fees, taxes, varying rates, or irregular deposit dates.
- Focusing only on compounding frequency and ignoring time horizon, contribution size, and annual return, which usually have the bigger effect.
Limitations
This estimate assumes a fixed annual rate, a fixed compounding schedule, and equal contributions made at the end of each period. It rounds the total number of compounding periods to the nearest whole period because real accounts do not credit fractional monthly or quarterly periods. Daily compounding uses 365 periods per year. The tool does not model taxes, fees, inflation, contribution limits, skipped deposits, changing returns, or different deposit timing inside the period.
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