Compound interest calculator
Growth of a deposit with regular contributions.
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How it's calculated
Each month the balance earns one twelfth of the annual rate, and the contribution is added. After n months the deposit has grown by (1 + r)^n, and the contributions form an annuity worth payment × ((1 + r)^n − 1) ÷ r. Both are what the future-value function computes. Interest earned is the final balance less everything you put in.
How long does money take to double?
Divide 72 by the annual rate: at 7% about 10 years, at 10% about 7 years. The rule of 72 is a close approximation for rates under about 15%.
Does compounding frequency matter much?
Less than people expect. $10,000 at 7% for 10 years is $19,672 compounded yearly, $20,097 monthly and $20,137 daily. The rate and the time matter far more.
What rate should I assume?
Historically about 7% a year after inflation for a broad stock index, 4–5% for a high-yield savings account or bonds in recent years. Use a lower figure to be safe; the tool shows how sensitive the result is.
Results are estimates for informational purposes only and are not financial, medical or legal advice.