Compound interest calculator guide
What does this calculator help you figure out?
Compound interest grows a balance by charging interest on interest already earned, so the same rate produces more each period than the one before. This calculator starts with an initial deposit, adds a monthly contribution, applies an annual interest rate at either monthly or annual compounding, and runs the balance forward for the number of years you enter. Each period the balance is multiplied by one plus the periodic rate and the contribution is then added, which matches a deposit made at the end of each period. The result shows the estimated future value, how much of it is your own contributions, and how much is interest. Use it to compare a savings account, a fixed-rate bond, or a long-term investing plan on equal footing. It excludes account fees, income tax on interest, inflation, changes to the rate over time, and the timing differences of contributions made at the start of a period rather than the end, so the future value is a projection of the arithmetic rather than a promise.
How is the result calculated?
Periodic rate = annual rate (%) ÷ 100 ÷ periods per year (12 for monthly, 1 for annual). Periodic contribution = monthly contribution × 12 when compounding annually, otherwise the monthly contribution. For each period: balance = balance × (1 + periodic rate) + periodic contribution. Interest earned = final balance − initial deposit − all contributions.
Worked example
Worked example with the default inputs: a $10,000 deposit, $200 added each month, 6% annual interest, 10 years, monthly compounding. Periodic rate = 6 ÷ 100 ÷ 12 = 0.005 per month over 12 × 10 = 120 periods. Each month the balance is multiplied by 1.005 and $200 is added. After 120 months the balance is $50,969.84. Total contributions = $10,000 + $200 × 120 = $34,000.00, so estimated interest = $50,969.84 − $34,000.00 = $16,969.84. Switching to annual compounding, with $2,400 added once a year at 6%, gives $49,542.38 instead: fewer compounding events and later deposits earn slightly less.
Units and conversion notes
Enter the deposit and monthly contribution in the same currency. The rate is an annual percentage, so type 6 for 6%; the calculator divides it by 12 for monthly compounding. Years can be fractional and are rounded to whole periods, so 2.5 years is 30 monthly periods or 3 annual periods (2.5 × 1 rounds to 3). Daily compounding is not offered; it produces a result only slightly above monthly at the same rate.
What does the result mean?
The result is a planning figure that depends entirely on the assumptions you entered. Fees, taxes, payment timing, and rate changes are not captured unless you built them into the inputs, and small errors in a rate compound quickly over time. Verify the rates and terms against your own agreements or your accountant before making a commitment. On this page the figure rests entirely on initial deposit, monthly contribution, annual interest rate, time period and compounding, so start there if the compound interest calculator returns something you did not expect.
Common mistakes to avoid
Good to know: the advertised rate on a savings product may already be an annual percentage yield that includes compounding, in which case entering it here with monthly compounding double-counts the effect; use the nominal rate instead. Interest here is gross, and tax on savings interest can take a noticeable share. Contributions are treated as end-of-period deposits, so a plan that deposits on the first of the month will show a little more in practice.
Use the result for planning, then confirm taxes, fees, contract terms, and other business-specific assumptions. This is not accounting, tax, or investment advice.
Common questions
Frequently asked questions
How is compound interest calculated monthly?
Divide the annual rate by 12 to get the monthly rate, then multiply the balance by one plus that rate every month and add any deposit. At 6% a year the monthly rate is 0.5%, so $10,000 becomes $10,050 after one month and $10,616.78 after twelve.
How much will $10,000 grow to in 10 years at 6%?
With no further deposits and monthly compounding, $10,000 × 1.005^120 = $18,193.97. Adding $200 a month raises the balance to $50,969.84, of which $34,000 is your own money and $16,969.84 is interest.
What is the difference between monthly and annual compounding?
Monthly compounding credits interest twelve times a year, so each credit starts earning sooner. On the default inputs it produces $50,969.84 against $49,542.38 for annual compounding, a difference of about $1,427 over ten years. More frequent compounding always yields slightly more at the same nominal rate.
Does this compound interest calculator include tax and inflation?
No. The future value is a nominal, pre-tax figure. Interest on ordinary savings is usually taxable in the year it is credited, and inflation reduces what the final balance will buy. For a rough real return, subtract expected inflation from the rate before calculating.
Is APR the same as the annual interest rate here?
Not quite. The calculator wants the nominal annual rate, which it divides into periods. APY or AER already reflect compounding within the year. If a product quotes only APY, entering it here overstates growth slightly; the difference is small at low rates but grows with the rate.
