Compound Interest Calculator
Free online interest calculator with monthly contributions. Enter your principal, rate and years to see future value, total invested and interest earned.
Compound Interest Calculator
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Interest Calculator: Compound Interest with Monthly Contributions
Use this free online interest calculator to estimate how a starting amount grows when interest compounds and you add money every month. Enter a principal, an annual interest rate, a time period and a monthly contribution, then choose a compounding frequency. You will see your future value, total invested and interest earned. Results are estimates, not guaranteed returns.
What This Interest Calculator Calculates
This calculator models compound growth on a starting amount plus a fixed monthly contribution. It returns the estimated future value, total invested and interest earned, with a doughnut chart comparing invested money with interest.
It is designed for savings and investments. For credit card debt, use our credit card payoff calculator instead. More tools are in our Finance Calculators.
How to Use the Interest Calculator
- Principal Amount: your starting balance. Type it in or use the slider (1,000 to 1,000,000).
- Interest Rate: the annual rate as a percentage. Enter 7 for 7%. Decimals such as 4.5 work.
- Time Period: the number of years (the slider runs from 1 to 50).
- Monthly Contribution: the amount you add each month (the slider runs from 0 to 5,000). Use 0 to grow a single lump sum.
- Compounding: Monthly (default), Quarterly or Annually.
- Currency: USD, EUR, GBP or INR. This changes the symbol and number formatting only. Amounts are never converted between currencies.
- Results: they appear beside the form and update as you change the inputs.
Understanding Your Results
- Future Value: the estimated balance at the end of the period, including everything you put in and the interest it earned.
- Total Invested: your principal plus every monthly contribution: principal + (monthly contribution × 12 × years).
- Interest Earned: future value minus total invested. This is growth from compounding, not money you deposited.
Total Invested + Interest Earned = Future Value. The doughnut chart shows that split. It compares two totals and is not a year-by-year growth chart. Amounts are rounded to the nearest whole currency unit.
How Compound Interest Is Calculated
Compound interest is interest earned on your principal and on interest already added, so each period's interest is calculated on a larger balance. Simple interest is calculated only on the original principal (see our simple interest calculator).
For a single starting balance, the standard formula is:
A = P × (1 + r/n)^(n×t)
A is the final amount, P the principal, r the annual rate as a decimal (5% = 0.05), n the compounding periods per year (12, 4 or 1) and t the years.
Because this calculator also adds a contribution every month, it first converts your compounding frequency into the growth that happens each month:
g = (1 + r/n)^(n/12)
With monthly compounding, g is simply 1 + r/12. The calculator then computes:
Future Value = P × g^(12t) + PMT × [(g^(12t) − 1) ÷ (g − 1)]
PMT is the monthly contribution. Each contribution is treated as made at the end of its month and grows at the equivalent monthly rate implied by the selected compounding frequency. This means quarterly and annual results are modeled with monthly growth, not with interest credited only at the end of each quarter or year. At a 0% rate, the result is simply your principal plus your contributions.
Monthly Contributions and Compound Growth
Each monthly contribution adds to total invested and to the balance that earns interest. The calculator assumes one fixed amount, 12 times a year, for the whole period, with no changes or withdrawals.
Money added earlier has more time to compound, so the timing of contributions affects the final result.
Compound Interest Example
This is an illustration. The 5% rate is a round number, not an expected or guaranteed return. To reproduce it, enter a $5,000 principal, 5% interest, 10 years and a $200 monthly contribution. Total invested is $29,000 in every case ($5,000 + $200 × 120 months).
| Compounding | Future value | Interest earned |
|---|---|---|
| Annually | $39,017 | $10,017 |
| Quarterly | $39,240 | $10,240 |
| Monthly | $39,292 | $10,292 |
Displayed amounts are rounded. The monthly result is exactly $39,291.50 before rounding. With monthly compounding, about 74% of the future value is money you put in and 26% is interest, the split the chart shows.
Frequently Asked Questions
How does this interest calculator calculate compound interest?
It converts your compounding frequency into monthly growth, compounds your principal over the total number of months, and adds the compounded value of your monthly contributions.
Can I use it without monthly contributions?
Yes. Set the monthly contribution to 0 to see growth on your principal alone.
How does the monthly contribution affect the result?
It raises total invested, and each contribution starts earning interest once added. Earlier contributions compound for more months, so they add more growth than later ones.
How does compounding frequency affect the result?
With the same annual rate, the selected compounding frequency produces slightly different results in this calculator, as the example table shows.
Does the currency selector convert currencies?
No. It changes only the symbol and number formatting. The amounts you enter are not converted.
How accurate is an interest calculator?
The arithmetic follows the formulas on this page, but the output is only as useful as the assumptions and inputs. Real accounts can differ because of rate changes, fees, taxes, how and when interest is credited, and when contributions are made.
Assumptions and Limitations
Results are estimates based on a fixed model: a constant annual rate, one equal contribution each month made at the end of the month, and compounding that is monthly, quarterly or annual. An account that credits interest only on set dates (for example quarterly) may show slightly different results from the equivalent monthly growth this calculator uses.
The calculator does not include daily compounding, taxes, fees, inflation, withdrawals, changing rates or changing contributions. Actual results may vary. This tool is for education and illustration and is not personalized financial advice.