How Compound Interest Grows Your Money
By Tools Star Hub Editorial Team · Published
Compound interest grows a balance by applying the rate to the current balance, not only to the original amount. This calculator walks the balance month by month, then adds any monthly contribution after that month’s growth. The result is an estimate from the numbers you type, not a forecast of an investment account.
A simple interest figure multiplies the starting amount by the annual rate and by the years. Compounding can land a bit higher because later periods include earlier growth. Contributions change the balance again, and they are not the same thing as interest.
How this compound interest estimate works
- Enter the starting amount, annual rate, and time. The starting amount cannot be negative. Time can be years or months. The calculator treats months as years divided by 12, and it will not run a period longer than 100 years. The annual rate is a percent, and it must be 100 or less.
- Pick how often the rate is applied. The choices are annually, semiannually, quarterly, monthly, and daily. Convert the entered percent to a decimal before using it. For example, 5% becomes 0.05. Monthly growth multiplies the balance by 1 plus that rate divided by 12, every month. Daily growth multiplies the balance by (1 plus the rate divided by 365) raised to the power of 365 divided by 12, once per month. Quarterly growth multiplies the balance by 1 plus the rate divided by 4 when the month count is a multiple of 3. Semiannual growth multiplies the balance by 1 plus the rate divided by 2 when the month count is a multiple of 6. Annual growth multiplies the balance by 1 plus the rate when the month count is a multiple of 12.
- Add a monthly contribution only if you want one. Leave the contribution blank and it is treated as zero. If you enter one, it is added after that month’s growth, and it is counted separately from interest. The contribution is monthly even when compounding is annual or quarterly.
- Read interest apart from money you put in. Interest earned is the ending balance minus the starting amount minus the contributions. The page also shows a simple interest comparison: starting amount times the annual rate times the years, with contributions added on top and not compounded.
Example
$1,000 at 5% for one year, compounded monthly, with no contribution
Each month the balance is multiplied by 1 plus 0.05 divided by 12. After 12 of those steps the balance is about $1,051.16. Simple interest on the same inputs is $50, so the simple total is $1,050. The gap is small over one year. A longer time or a higher rate widens it. This is the calculator’s monthly method, rounded to cents at the end.
The same start, plus $100 at the end of each month
The $100 is added after the growth step, so it does not earn growth in the month it is deposited. Over 12 months you have added $1,200 of your own money. The ending balance is about $2,279.05. Interest is about $79.05, which is the ending balance minus $1,000 minus $1,200. The simple interest comparison on the same inputs is $2,250, because the $50 of simple interest is added to the start and the contributions are not compounded.
What changes the estimate
Frequency is not the same as the contribution schedule
You can compound annually and still add money every month. Those are separate inputs. Annual compounding applies the rate once each 12 months. The contribution, if any, still arrives every month.
A zero rate does not grow the balance
If the annual rate is 0, the balance stays put except for contributions you add. Simple interest is also 0 in that case.
Common mix-ups
People sometimes treat the contribution as interest, or they expect daily compounding to post a separate interest line every day. This tool folds daily growth into each month. It also does not withdraw money, charge a fee, or model a rate that changes during the term.
Limits
The starting amount, contribution, rate, and time have upper limits so a huge input does not pretend to produce a real balance. The estimate assumes the rate stays constant and that each contribution arrives in full. A bank, fund, or lender can use a different day count, a different posting date, or a rate that changes.
Which calculator to use
Use the compound interest calculator to estimate how a balance grows from a starting amount, a rate, a time, a compounding choice, and an optional monthly contribution. Use the savings goal calculator to work backward from a target savings goal and estimate how much to set aside.
FAQ
- What is compound interest?
- It is growth applied to the current balance, so earlier growth can be included in later periods. This calculator does that month by month from the rate and frequency you enter.
- How does compound interest work in this calculator?
- Each month it applies the frequency rule to the balance, then adds the monthly contribution if you entered one. Interest is the ending balance minus the start minus those contributions.
- Does a monthly contribution earn interest in the month it is added?
- No. The contribution is added after that month’s growth step. It can grow in later months.
- Is this a prediction of what an account will earn?
- No. It is an estimate from a constant rate and the timing rules above. Actual account terms can differ.
To run the same month by month estimate, open the Compound Interest Calculator. It stays in your browser and does not require an account.