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Compound Interest Calculator with Regular Contributions
Start with a lump sum, add money every month or every year, and see how the balance grows when interest is added back and earns interest itself. A yearly table shows each step.
How the balance is built
Each month the calculator adds interest to the running balance, then adds the contribution when it falls due. With monthly compounding the monthly rate is simply the annual rate ÷ 12. For other frequencies the monthly rate is (1 + r ÷ n)n ÷ 12 − 1, which grows to the same amount over a full year.
For level end-of-month deposits this matches the future value of an annuity: FV = PMT × ((1 + i)N − 1) ÷ i, plus the starting balance grown as P × (1 + i)N, where i is the monthly rate and N the number of months.
Start or end of the period
A deposit at the start of a period earns interest for that whole period, so it ends up slightly larger than the same deposit at the end. On monthly deposits the difference is roughly one month of interest on each payment, which is small but adds up over decades. Pick the option that matches how your money actually moves, for example a salary-day transfer.
Reading the table
The yearly table splits growth into money you added and interest earned. In the early years most of the growth comes from your deposits. Later the interest column can overtake the contributions column, which is the point of compounding: a larger balance produces larger interest. If the interest column looks flat, check the rate; if it looks too steep, check that you chose the right compounding and contribution frequency.
Assumptions and limits
- The rate is constant. Real savings rates and investment returns change from year to year.
- Deposits stay the same; they do not rise with pay or inflation.
- Fees, tax and withdrawals are not modelled, and inflation is ignored, so the result is in nominal money.
For a version that includes fees and inflation see the investment calculator. The output is an illustration of arithmetic, not a forecast.
Spotting unrealistic inputs
Check the compounding frequency your account actually uses, because daily, monthly and yearly compounding give different results from the same headline rate. Be careful with contributions: a deposit made at the start of each period earns one more period of interest than one made at the end. If the final figure looks too good to be true, check that you entered the rate as a percentage per year and not per month. Compare accounts using the effective annual rate, which converts any compounding schedule into one comparable yearly figure.
Worked example
Someone opens an account with 2,000 and adds 150 at the end of every month for 20 years at 5% a year.
Inputs
| Starting balance | 2000 |
|---|---|
| Regular contribution | 150 |
| Contribution frequency | Monthly |
| Contributions are made at the | End of each period |
| Annual interest rate | 5 % |
| Interest compounded | Monthly |
| Years to grow | 20 |
Result
After 20 years at 5% compounded 12 time(s) a year.
| Total you paid in (including the starting balance) | 38,000.00 |
|---|---|
| Interest earned | 29,080.33 |
| Interest as a share of the final balance | 43.4% |
Year-by-year growth
| Year | Contributed | Interest earned | Balance at year end |
|---|---|---|---|
| Year 1 | 1,800.00 | 144.15 | 3,944.15 |
| Year 2 | 1,800.00 | 243.62 | 5,987.77 |
| Year 3 | 1,800.00 | 348.17 | 8,135.94 |
| Year 4 | 1,800.00 | 458.08 | 10,394.02 |
| Year 5 | 1,800.00 | 573.61 | 12,767.63 |
| Year 6 | 1,800.00 | 695.04 | 15,262.67 |
| Year 7 | 1,800.00 | 822.70 | 17,885.37 |
| Year 8 | 1,800.00 | 956.88 | 20,642.25 |
| Year 9 | 1,800.00 | 1,097.92 | 23,540.17 |
| Year 10 | 1,800.00 | 1,246.19 | 26,586.36 |
| Year 11 | 1,800.00 | 1,402.04 | 29,788.40 |
| Year 12 | 1,800.00 | 1,565.86 | 33,154.26 |
- Interest is applied month by month at the rate equivalent to your chosen compounding frequency, so yearly compounding gives the exact yearly result when no deposits are made.
Financial disclaimer: Results are estimates for planning and education. They are not financial, tax or legal advice. Fees, taxes, rates and rules vary by lender, product and country, so confirm figures with your provider or a qualified adviser before making decisions.
Frequently asked questions
What is the compound interest formula with regular deposits?
The balance equals the starting amount times (1 + i) to the power N, plus the deposit times ((1 + i) to the power N minus 1) divided by i, where i is the rate per period and N the number of periods.
Does it matter whether I deposit monthly or yearly?
Yes. The same total deposited in monthly instalments gets to earn interest sooner than one yearly lump, so monthly deposits usually finish slightly higher.
Why is my balance lower than I expected?
Check the rate, the years and the contribution frequency. A yearly contribution entered as monthly, or the reverse, changes the result a great deal.
Can I use this for a loan?
Not directly. Loans reduce through repayments. Use the loan calculator for that.
Is the result guaranteed?
No. It is a projection from the numbers you typed. Actual returns vary and some products carry fees or risk.
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References and sources
- Investor.gov (U.S. SEC): compound interest calculator background and saving basics. Link
- Standard time-value-of-money texts: future value of an ordinary annuity.