Compound Interest Calculator
See how savings grow with compound interest and monthly contributions.
Worked example
You start with $5,000, add $200 a month and earn 6% a year for 20 years.
- Final balance: about $108,959
- Money you put in: $53,000
- Growth from interest: about $55,959
Why starting early matters
Saving $300 a month at a 7% annual return, starting from zero:
- After 20 years: about $156,278 (you put in $72,000)
- After 30 years: about $365,991 (you put in $108,000)
Ten extra years add $36,000 of contributions but more than double the result, because interest earns interest.
How it is calculated
The balance is compounded monthly. Each month: balance = balance × (1 + annual rate ÷ 12) + contribution. The result is an illustration, not a forecast, because real returns vary and are never guaranteed.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is earned only on the original amount. Compound interest is earned on the original amount plus the interest already added, so growth speeds up over time.
What is the Rule of 72?
Divide 72 by the annual return to estimate how many years it takes to double your money. At 7%, that is about 10 years. For example, $10,000 at 7% grows to about $20,097 in 10 years in this calculator.
How often is interest compounded here?
Monthly. Daily or yearly compounding gives slightly different results, but monthly is a good estimate for most savings and investments.
Does this account for inflation or taxes?
No. Subtract your expected inflation from the return to see growth in today's money, and remember that taxes and fees can lower your real return.
Results are illustrations for general information, not financial advice or a guarantee of returns.