Compound interest is the reason small, regular savings can grow into much larger sums over time. When interest is added to your balance, the next round of interest is calculated on a bigger amount, so growth builds on growth, month after month. This calculator shows what that effect is worth for a starting amount, a regular monthly contribution, an annual interest rate and a number of years.
It is a planning tool, not a promise. The rate you enter is an assumption you choose, and real savings accounts and investments pay rates that change over time. The value of the calculator is comparison: change one number at a time, the monthly contribution, the rate or the number of years, and see which change matters most to the final figure.
Use it when you are starting a savings habit, comparing accounts, or checking whether a long-term goal is realistic. All of the maths happens in your browser, and nothing you enter is sent anywhere or stored.
What the four numbers mean
The starting amount is the money you already have saved or invested at the beginning. It starts earning interest from the first month, so it has the longest time to grow and usually contributes the most to the final figure in the early years.
The monthly contribution is the amount you add every month on top of the starting amount. Regular contributions matter more the longer the period runs, because each monthly payment then earns interest for the months that remain. Setting the monthly contribution to zero shows what the starting amount alone would grow to.
The annual interest rate is the percentage return you assume per year. This calculator compounds monthly, so a 6% annual rate means 0.5% added each month. Small differences in the rate make a large difference over many years, which is why it is worth testing a lower rate as well as the rate you hope for.
The number of years is how long the money stays invested. Years can include fractions, so 7.5 years means seven and a half years, or 90 months. Time is the most powerful input of all: doubling the number of years usually grows the final figure by far more than doubling the monthly contribution.
The calculator shows three results. The future value is the total balance at the end of the period. Total contributions is everything you put in: the starting amount plus every monthly contribution. Growth earned is the difference between the two, which is the interest the money earned along the way.
How the calculation works
The calculation compounds monthly. The monthly rate is the annual rate divided by 100 and then by 12, and the number of months is the number of years multiplied by 12. The future value is the starting amount grown by monthly compounding, plus the monthly contributions grown in the same way: future value equals principal times (1 + monthly rate) to the power of the number of months, plus the monthly contribution times ((1 + monthly rate) to the power of the number of months, minus 1) divided by the monthly rate.
If the annual rate is zero, there is no interest to compound, and the future value is simply the starting amount plus the monthly contribution multiplied by the number of months. The calculator rejects negative starting amounts, negative monthly contributions and negative years, and the rate cannot be negative either, because a savings rate below zero is not a meaningful planning assumption here.
With the default values, 10,000 starting amount, 500 a month, 6% a year and 10 years, the monthly rate is 0.5% and the number of months is 120. The starting amount grows to about 18,193.97, and the 120 monthly contributions grow to about 81,939.67, giving a future value of about 100,133.64.
How to use this calculator
- Enter the starting amount you already have, or zero if you are starting from nothing.
- Enter the amount you can add every month, or zero if you will not add anything.
- Enter the annual interest rate you want to assume, and the number of years the money will stay invested.
- Click “Calculate compound interest” and read the three results: future value, total contributions and growth earned.
Worked example
With the default values, a starting amount of 10,000, a monthly contribution of 500, an annual rate of 6% and 10 years, the total contributions are 70,000.00, the future value is about 100,133.64, and the growth earned is about 30,133.64. In other words, about 30% of the final balance is interest, and the rest is money that was paid in.
As a check with no interest, enter a starting amount of 1,000, a monthly contribution of zero, a rate of zero and 5 years. The future value is 1,000.00, total contributions are 1,000.00 and growth earned is 0.00, exactly as expected when nothing earns interest.
As a one-year check, enter a starting amount of 1,000, a monthly contribution of 100, a rate of 12% and 1 year. The future value is 2,395.08, total contributions are 2,200.00 and growth earned is 195.08. The monthly rate here is 1%, and the twelve monthly contributions each earn interest for the months that remain.
What this calculator does not do
This calculator does not include tax, account fees or inflation. The future value is a nominal figure in the currency you enter: if prices rise over the period, the same amount of money will buy less at the end than it does today. To allow for that, test a lower rate that represents the return after inflation.
It also assumes one fixed rate for the whole period, with no withdrawals and no missed contributions. Real returns vary from year to year, and a single bad year early on changes the outcome less than the same bad year near the end, when the balance is largest. Treat the result as a labelled estimate for planning, not as a guaranteed return or financial advice.
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Frequently Asked Questions
What is compound interest?
Compound interest is interest calculated on both the original amount and the interest already added. Each month the balance grows, and the next month’s interest is calculated on the new, larger balance, so growth accelerates over time.
How often does this calculator compound?
This calculator compounds monthly. The annual rate you enter is divided by 12 to give the monthly rate, and interest is applied every month for the full number of years.
What happens if the interest rate is 0%?
With a rate of 0%, no interest is earned. The future value is simply the starting amount plus the monthly contribution multiplied by the number of months, and growth earned is zero.
Does this calculator include tax or fees?
No. The result is before any tax on interest and before any account fees. If your account charges fees or your interest is taxed, the real balance will be lower than the figure shown.
Can I use it with no monthly contribution?
Yes. Set the monthly contribution to zero and the calculator shows what the starting amount alone grows to over the period at the rate you enter.
What if the rate changes over time?
This calculator uses one fixed rate for the whole period. To model a change, run the calculation for the first period, then use its future value as the starting amount for a second calculation at the new rate.
Is the future value guaranteed?
No. The future value is an estimate based on the rate and contributions you enter. Real savings rates and investment returns vary, so the actual outcome will differ.