This calculator shows how savings grow with compound interest and regular monthly deposits, year by year. It also splits your final balance into what you put in and what interest added.
How it’s calculated
With no monthly deposits, the balance follows the standard compound interest formula:
A = P × (1 + r ÷ n)^(n × t)
A is the final balance, P the initial deposit, r the annual rate as a decimal, n the number of
times interest compounds each year, and t the number of years.
To include monthly deposits, the calculator works month by month. It converts your annual rate to the matching monthly rate, applies it to the balance, and then adds your deposit:
Monthly rate = (1 + r ÷ n)^(n ÷ 12) − 1New balance = balance × (1 + monthly rate) + deposit
Worked example
$1,000 at 5%, compounded annually for 10 years, with no monthly deposits:
- Add the rate to 1: 1 + 0.05 = 1.05.
- Raise it to the number of years: 1.05^10 = 1.62889.
- Multiply by the deposit: $1,000 × 1.62889 = $1,628.89.
The balance after 10 years is $1,628.89, of which $628.89 is interest.
With monthly deposits
Now take $10,000, adding $200 a month at 7% compounded monthly for 20 years. You put in $58,000 in total: the $10,000, plus $200 × 240 months.
The balance after 20 years is $144,572.72, of which $86,572.72 is interest. Interest ends up larger than everything you deposited, which is what compounding does over long periods.
How compounding frequency changes the result
These figures come from a University of Hawaiʻi example: $10,000 at 5% for 40 years, with no further deposits.
Common questions
What is compound interest?
It's when you earn interest on the money you've saved and on the interest you've already earned. The US Consumer Financial Protection Bureau gives this example. $1,000 at 5%, compounded once a year, becomes $1,050 after one year and $1,102.50 after two. The second year earns more because the interest now earns interest too.
How much does the compounding frequency matter?
Less than the rate, the time or your deposits. Going from annual to monthly compounding makes a visible difference over decades, but going from monthly to daily adds very little. In the table above, 40 years of daily compounding beats monthly by about $300 on a $10,000 deposit.
When does the calculator add my monthly contribution?
At the end of each month, after that month's interest. A deposit made at the start of the month would earn one extra month of interest, so your real balance could be slightly higher. Your bank's own timing and rules are what actually apply.
What rate should I use?
Use the annual rate on your account or a rate you're comfortable planning around. A savings account usually states its rate. Investments don't have a fixed rate, so try a few to see a range. The calculator applies the same rate every year, and real returns vary and aren't guaranteed.
Does this include taxes, fees or inflation?
No. Each of them reduces what you actually keep, and inflation also reduces what your money can buy. This is why the result is an estimate for learning and planning, not a promise of what you'll end up with. For decisions about your own money, consider a qualified financial professional.
Can I use this for a loan?
Not directly. Loans compound the same way, but your payments reduce the balance rather than adding to it, and lenders have their own rules. This calculator is built for savings and investments.