What is the compound interest formula?

A = P(1 + r/n)^(nt), where P is principal, r annual rate, n compounding periods per year, t years.

What is the Rule of 72?

Divide 72 by your annual interest rate to estimate how many years it takes to double your money.

Compound Interest Calculator

See how savings or investments grow with compounding — with optional monthly contributions and a year-by-year table.

Currency

Starting amount

Annual rate (%)

Years

Compounding

Monthly contribution (optional)

—Final Balance

—Total Contributed

—Interest Earned

Year-by-Year Growth

Year	Contributed	Interest	Balance

How Compound Interest Works

Compounding means earning interest on your interest. The classic formula is A = P(1 + r/n)^nt — P is your starting amount, r the annual rate, n how many times per year interest compounds, and t the years. Regular contributions accelerate the effect dramatically: the table above shows interest overtaking your deposits over time.

The Rule of 72

A quick mental shortcut: divide 72 by your annual rate to estimate the years to double your money. At 7% → about 10 years; at 12% → about 6 years. Try it in the calculator and watch the balance column.

Frequently Asked Questions

Does compounding frequency matter much?

Less than people think: $10,000 at 7% for 10 years gives $19,672 yearly vs $20,097 daily. Rate and time matter far more than frequency.

Is this the same as bank savings interest?

Yes — banks typically quote an annual rate compounded daily or monthly. Use the advertised rate and matching frequency.

Does this account for inflation or taxes?

No — results are nominal. For a rough inflation-adjusted view, subtract the expected inflation rate from your return rate.