Compound interest calculator
Simulate how your savings grow with compound interest: starting amount, regular contributions, nominal or effective rate, inflation, and a year-by-year table.
Enter the initial amount, the rate, and the term.
Keep in mind
Scenario A vs. scenario B
How the balance grows
Year by year
Amounts are rounded to whole numbers. The CSV includes the cents.
How it is calculated
- Formula
—- With your values
—
This is a simulation
A constant rate is assumed for the whole term, and fees and taxes are not deducted. Past or assumed returns do not guarantee future results.How to use it
- Choose what you want to find out: the final balance, the contribution needed to reach a goal, or the time it takes to get there.
- Enter the initial amount, the regular contribution, the annual rate, and the term in years and months.
- Under “More options,” set whether the rate is nominal or effective, how often it compounds, whether you contribute at the start or the end of each period, the yearly increase in contributions, and inflation.
- Review the balance, what you contributed, and the interest earned, plus the chart and the year-by-year table. Turn on “Compare with another scenario” to see the difference with another rate or term, or download the breakdown as a CSV file.
Frequently asked questions
What is compound interest?
It is interest calculated on the principal and also on the interest already earned. That is why growth speeds up over time: $1,000 at 10% a year becomes $1,100 after the first year, $1,210 after the second, and $1,331 after the third.
What is the difference between a nominal and an effective rate?
A nominal rate (such as an APR) is quoted per year but compounds several times, for example every month. An effective rate (such as an APY) already includes the effect of compounding: it is what your money really grows in one year. A 12% nominal rate compounded monthly equals a 12.68% effective annual rate.
How are the contributions calculated?
The tool simulates month by month using the monthly rate equivalent to your annual rate. Each contribution is made at the start or the end of the period you choose, and you can make it grow every year.
What is the inflation field for?
If you fill it in, we also show the final balance in today’s purchasing power: what your savings would be worth if prices rise at that pace. It does not change the nominal balance.
Are the results a promise of returns?
No. The tool assumes the rate stays constant for the whole term, and real rates change. It also does not deduct fees or taxes. Use it to compare scenarios, not to predict.
Is my data saved?
No. Everything is calculated in your browser. The share link contains only the values you copy.
The compound interest formula
Without contributions, the final balance is principal × (1 + rate)years, using the effective annual rate. With $10,000 at an 8% effective annual rate for 10 years, you end up with $10,000 × 1.0810 = $21,589.
Regular contributions
Adding a monthly contribution changes the result a lot, because every contribution earns interest too. Contributing $100 a month for 20 years at a 6% effective annual rate adds up to $24,000 in contributions and leaves a balance of about $45,300: almost half of it is interest.
Nominal rate and compounding
The more often interest compounds, the higher the effective rate. A 12% nominal annual rate yields 12% if it compounds once a year, 12.68% if it compounds monthly, and 12.75% if it compounds daily.
The rule of 72
To estimate how long it takes for money to double, divide 72 by the annual rate: at 6% it takes about 12 years, and at 9% about 8 years.
Tips
- Starting earlier matters more than contributing more: time is the factor that multiplies the most.
- Always compare effective rates (APY), not nominal ones (APR).
- Check the effect of inflation with the real value of the balance.
Updated on September 29, 2026