How to Calculate Compound Interest With Monthly Deposits (Formula, Examples, and Calculator)
Saving money regularly can be one of the simplest ways to build wealth over time. Whether you are saving for retirement, a house deposit, education, or an emergency fund, understanding how to calculate compound interest with monthly deposits can help you estimate how much your savings may grow.
Unlike a one-time investment, monthly deposits allow you to contribute a fixed amount consistently while earning interest on your growing balance. As your savings accumulate, you may earn returns on both your original contributions and the interest earned in previous periods. This is the power of compound interest.
For example, if you deposit $200 every month into a savings account that earns a hypothetical 6% annual interest rate, your balance can grow significantly over 10, 20, or 30 years. Your final balance will depend on your deposit amount, interest rate, compounding frequency, and investment period.
In this guide, you will learn the compound interest formula for monthly deposits, how to calculate future savings step by step, how monthly contributions affect long-term growth, and which mistakes to avoid. You can also use a compound interest calculator to estimate your potential savings without performing the calculations manually.
What Is Compound Interest With Monthly Deposits?
Compound interest is interest earned on your original savings and on the interest accumulated over time. When you make monthly deposits, each contribution becomes part of your balance and can earn interest during subsequent periods.
This creates two sources of growth:
- Regular contributions: The money you add to your savings each month.
- Compounded interest: The interest earned on your existing balance and previously accumulated interest.
Suppose you start with $1,000 and deposit another $100 every month. If your account earns interest, your balance can grow through both your contributions and the returns generated by your savings.
The earlier you begin saving, the more time your money has to compound. Monthly contributions also help you build your balance gradually without requiring a large initial investment.
However, not every monthly deposit earns interest for the same amount of time. A deposit made at the beginning of your savings plan may earn interest for years, while a deposit made near the end has much less time to grow.
Compound Interest Formula With Monthly Deposits
To calculate compound interest with monthly deposits, you need a formula that accounts for both the growth of your existing balance and the future value of your recurring contributions.
When you start with an initial deposit and add money at the end of every month, the future value can be calculated using the following formula:
FV = P(1 + r/12)n + PMT × [((1 + r/12)n − 1) / (r/12)]
Where:
- FV = Future value of your savings
- P = Initial deposit or starting balance
- r = Annual interest rate expressed as a decimal
- PMT = Monthly deposit amount
- 12 = Number of monthly compounding periods per year
- n = Total number of monthly periods, calculated as years × 12
This formula assumes a fixed annual nominal interest rate, monthly compounding, and deposits made at the end of each month. It also assumes there are no withdrawals, fees, or taxes.
Formula for Monthly Deposits Without an Initial Balance
If you begin with no savings and contribute a fixed amount every month, the formula becomes:
FV = PMT × [((1 + r/12)n − 1) / (r/12)]
This version is useful when you want to calculate the future value of regular monthly savings without an initial lump-sum deposit.
For example, if you save $150 per month for 10 years at a hypothetical 5% annual interest rate, the formula estimates how much those monthly contributions could grow to under the stated assumptions.
What If the Interest Rate Is Zero?
If the interest rate is 0%, there is no interest to compound. Your future balance is simply the initial deposit plus all monthly contributions:
FV = P + (PMT × n)
For example, starting with $500 and depositing $100 per month for 24 months would produce a balance of $2,900 before any fees or withdrawals.
How to Calculate Compound Interest With Monthly Deposits Step by Step
You can calculate the future value of your savings by following these steps.
Step 1: Determine Your Initial Deposit
Identify how much money you already have in your savings account or investment.
If you are starting from scratch, use zero as your initial deposit.
Example: Initial deposit = $1,000
Step 2: Choose Your Monthly Deposit
Decide how much money you can contribute consistently each month.
Example: Monthly deposit = $200
A realistic monthly contribution is generally more useful than an ambitious amount that you cannot maintain. You can increase your deposits later as your income or savings capacity changes.
Step 3: Enter the Annual Interest Rate
Determine the annual interest rate you want to use for your calculation.
Example: Annual interest rate = 6%
Convert the percentage into a decimal:
6 ÷ 100 = 0.06
If the rate is an estimate rather than a guaranteed return, treat the final balance as a projection rather than a promise.
Step 4: Convert the Annual Rate to a Monthly Rate
For a nominal annual interest rate compounded monthly, divide the annual rate by 12.
Monthly interest rate = 0.06 ÷ 12 = 0.005
This is equivalent to 0.5% per month.
Note that an account quoting an annual percentage yield (APY) already reflects the effect of compounding. Do not automatically divide an APY by 12; check the account's rate definition before using a formula.
Step 5: Calculate the Number of Monthly Deposits
Multiply the number of years by 12.
For a 10-year savings plan:
n = 10 × 12 = 120 months
This gives you the total number of monthly periods in the calculation.
Step 6: Apply the Compound Interest Formula
Use the initial balance, monthly deposit, monthly interest rate, and total number of months in the formula.
For an initial balance of $1,000, monthly deposits of $200, a 6% nominal annual rate, and a 10-year period:
FV = $1,000(1.005)120 + $200 × [((1.005)120 − 1) / 0.005]
The estimated future value is approximately $33,900.
Your total contributions would be:
- Initial deposit: $1,000
- Monthly contributions: $200 × 120 = $24,000
- Total money contributed: $25,000
- Estimated investment growth: approximately $8,900
The difference between your total contributions and your final balance represents the estimated interest earned under these assumptions.
Actual results may vary if the interest rate changes, deposits are made on different dates, or fees and taxes apply.