Regular saving is where compound interest really shines. Add a starting balance plus monthly, quarterly or annual contributions, choose whether you deposit at the beginning or the end of each period, and see exactly how much comes from your deposits and how much from interest.
Compound Interest Calculator with Regular Contributions
Add monthly, quarterly or annual deposits and choose whether they are made at the start or end of each period.
Results are estimates for educational purposes and are not financial advice. Actual returns depend on your provider's terms, fees and taxes.
Formulas used
The result has two parts: the growth of the starting balance and the future value of the series of contributions (an annuity).
Lump sum: P × (1 + r/n)n × t
End of period (ordinary annuity): C × [ (1 + i)N − 1 ] ÷ i
Beginning of period (annuity due): C × [ (1 + i)N − 1 ] ÷ i × (1 + i)
- P
- Initial principal.
- C
- Amount of each contribution.
- m
- Contributions per year: 12 monthly, 4 quarterly, 1 annually.
- N
- Number of contributions = m × t (whole periods only).
- i
- Interest rate per contribution period:
(1 + r/n)n/m − 1.
Contribution timing. End of period means each deposit is made on the last day of the month (or quarter, or year), so it has not earned interest yet when the period closes — typical for salary-day saving. Beginning of period means each deposit is made on the first day and earns a full period of interest, so the result is higher by a factor of (1 + i).
When the contribution and compounding frequencies differ (say, quarterly deposits into a monthly-compounding account), the calculator converts the rate to its exact equivalent per contribution period, so the deposits grow at the same effective rate as the balance.
How to use
- Enter the starting balance (0 is fine).
- Enter the contribution amount and choose monthly, quarterly or annually.
- Choose whether contributions are made at the end or beginning of each period.
- Enter the annual rate, compounding frequency and number of years.
- Press Calculate. The table shows contributions and interest for each year.
Worked example
You start with $10,000 and add $200 every month for 10 years at 5%, compounded monthly.
- Lump sum: 10,000 × (1 + 0.05/12)120 = $16,470.09.
- Monthly rate i = 0.05 ÷ 12 = 0.0041667; N = 120.
- Contributions (end of month): 200 × [(1.0041667)120 − 1] ÷ 0.0041667 = $31,056.46.
- Total: 16,470.09 + 31,056.46 = $47,526.55.
- You deposited 10,000 + 200 × 120 = $34,000, so interest earned is $13,526.55.
If you deposit at the beginning of each month instead, the total is $47,655.95 — $129.40 more, because every deposit earns one extra month of interest.
Practical uses
- Monthly savings plans and recurring deposits.
- Regular investing in a retirement or education account.
- Comparing the benefit of saving on the 1st versus the 31st of the month.
- Annual top-ups to a tax-advantaged account.
Assumptions and limitations
- The rate and the contribution amount stay the same throughout.
- Only whole contribution periods receive a deposit. For 10.5 years of annual contributions, 10 deposits are made and the balance then grows for the final half year.
- Between compounding dates, deposits are assumed to grow at the equivalent rate. Some banks pay no interest on money deposited part-way through a compounding period, so their figure may be slightly lower.
- Taxes, fees and inflation are not included.
Frequently asked questions
What is the difference between an ordinary annuity and an annuity due?
An ordinary annuity has payments at the end of each period; an annuity due has them at the start. An annuity due is worth more by one period of interest.
Does depositing weekly help?
Slightly. Depositing the same yearly total earlier means more time earning interest. The effect is small compared with the amount you save and the rate you earn.
How do I reach a target amount?
Adjust the contribution until the final amount matches your goal, or use the Principal Calculator for a single deposit.
Can I use this for a recurring deposit (RD)?
Yes. Choose monthly contributions and the compounding frequency stated by your bank (many use quarterly). Note that some banks round the maturity value differently.