Compound Interest Calculator
Work out how much an investment will be worth when the interest you earn starts earning interest of its own.
Last updated: August 5, 2026
How compound interest works
Compound interest is interest on interest. At the end of each compounding period, the interest you have earned is added to your balance. From then on, interest is calculated on the new, larger balance rather than on the original amount alone.
That is why money left invested for a long time grows far beyond what simple interest would produce. The effect is modest in the first years and becomes dramatic later, which is why starting early matters more than the exact rate you earn.
The formula
Future value is calculated with the compound interest formula:
- A — future value, what the investment is worth at the end
- P — initial principal amount
- r — annual interest rate as a decimal (5% becomes 0.05)
- n — number of compounding periods per year
- t — number of years the money is invested
Worked example
You invest $1,000 at 5% per year, compounded monthly, for 10 years.
A = 1,000 × (1 + 0.05 / 12) ^ (12 × 10) ≈ $1,647.01
Interest earned: $647.01, a total return of about 64.7%.
The calculator gives the same result instantly, and lets you compare how changing the rate, term or compounding frequency affects the outcome.
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Frequently asked questions
Is the result guaranteed?
No. This is a maths tool, not a promise of returns. Interest rates change, and investments can lose money. Use the figure as an estimate of growth at a constant rate, not as a forecast.
What is compounding frequency?
It is how often interest is added to your balance each year. The same annual rate produces slightly more growth when interest is added monthly or daily instead of yearly.
Can I use it for monthly contributions?
Not directly. This calculator assumes one lump sum with no further deposits. For regular monthly saving, use the Savings Goal calculator, which adds contributions over time.
Does the rate include inflation?
No, the result is in today's money before inflation is considered. To think about real purchasing power, subtract your assumed inflation rate from the interest rate.
This calculator gives an estimate only and is not financial advice. Actual investment returns vary. Consider consulting a financial professional for decisions about your money.