Compound Interest Calculator
Enter a starting amount, a monthly contribution, a return and a time span to see what it grows into. An average-down calculator sits right below.
Bar = year-end balance. Light = what you put in, dark = growth.
| Year | Contributed | Growth | Balance |
|---|
Average-down calculator
Next steps
What compounding means
Interest earns interest. At the same rate, the longer you stay the more of your balance is growth rather than deposits. At 7% a year, a lump sum grows about 3.9× in 20 years and about 7.6× in 30.
Rule of 72
Divide 72 by the yearly return to estimate the years to double: 12 years at 6%, 9 at 8%, about 7 at 10%. The calculator above shows the exact figure.
How averaging down is calculated
New average = (old average × old shares + new price × new shares) ÷ total shares. Holding 100 at 80 and buying 100 more at 60 gives an average of 70, and the rise needed to break even drops from 33.3% to 16.7%.
- Enter a target average to see how many more shares it takes and what it costs.
- The same formula works for stocks, crypto and ETFs. Fractional amounts are fine.
- Fees and taxes are not included.
FAQ
Monthly vs yearly compounding?
Monthly adds returns to the balance every month, yearly once a year. At the same yearly rate, monthly ends slightly higher.
When are contributions added?
At the end of each month for monthly compounding, and at the end of each year for yearly.
How is the new average price calculated?
(Old average × old shares + new price × new shares) divided by total shares.
Are taxes and fees included?
No. Figures are before tax and fees.