How to use the compound interest calculator
- Enter your starting amount and, if you'll keep saving, a regular deposit with how often you'll make it.
- Enter the annual interest rate and choose how often interest is compounded. Check your account or product details; monthly and daily are common for savings.
- Set the time. You'll see the final balance, how much of it is interest, a year-by-year table and the working.
Compound interest formula
A = P × (1 + r ÷ n)n × t
P is the starting amount, r the annual rate as a decimal, n how many times a year interest is added, and t the number of years. Each time interest is added, the next round of interest is worked out on the bigger balance, which is why growth speeds up over time.
With a regular deposit (PMT) at the end of each period, the deposits grow to:
Deposits' value = PMT × ((1 + i)k − 1) ÷ i
Here i is the interest rate per deposit period and k the number of deposits. If you deposit at the start of each period, multiply by (1 + i), because each deposit earns interest for one extra period.
Worked example
You put in 10,000 and add 200 a month for 10 years at 7% a year, compounded monthly.
The starting amount grows to 10,000 × (1 + 0.07 ÷ 12)120 = 20,096.61. The 120 deposits of 200 grow to 34,616.96. Together that's 54,713.58 (each figure is rounded to the cent), of which you deposited 34,000 and 20,713.58 is interest.
How much does compounding frequency matter?
10,000 at 7% a year for 10 years, with no extra deposits:
| Interest added | Balance after 10 years |
|---|---|
| Simple interest (never compounded) | 17,000.00 |
| Yearly | 19,671.51 |
| Monthly | 20,096.61 |
| Daily | 20,136.18 |
Moving from simple to compound interest makes a big difference. Moving from monthly to daily compounding makes very little. The rate and the time you leave the money invested matter far more.
The rule of 72
For a quick estimate of how long money takes to double, divide 72 by the annual rate. At 6% that's about 12 years; at 9%, about 8 years. It's an approximation that works best for rates between about 4% and 12%.
Frequently asked questions
What's the difference between APR and APY?
APR (or the nominal rate) is the yearly rate before compounding. APY, also called AER or the effective annual rate, includes the effect of compounding. At 7% compounded monthly, the APY is 7.23%. Use APY to compare accounts that compound at different frequencies.
What's the difference between simple and compound interest?
Simple interest is only ever paid on the original amount. Compound interest is also paid on interest you've already earned, so the balance grows faster each year.
Can I use this for investments like index funds?
You can use it to explore what an average yearly return might add up to, but investment returns go up and down and are never guaranteed. Treat the result as an illustration, not a forecast.
Does this include tax and inflation?
No. Interest is often taxable, and inflation reduces what your money will buy in the future. Rules differ by country and account type, so check what applies to you.
Formulas checked 29 October 2026. This calculator is for planning and education, not financial, tax or legal advice.