Compound Interest Calculator

How compound interest actually works

Compound interest is the reason a modest habit of saving can grow into a meaningful amount of money over a lifetime. The core idea is simple: every dollar of interest you earn starts earning its own interest. Over one year the effect is small. Over thirty years the effect is dramatic.

The formula that powers this calculator is the standard future value formula, applied first to your starting balance and then to a stream of monthly contributions. Your starting balance grows at the compounding frequency you chose. Each monthly deposit grows for whatever time remains until the end of the period. The tool adds those two pieces together to get the total future value.

Why time matters more than rate

People often obsess over squeezing an extra percentage point of return. Time in the market usually matters more. Two savers who each contribute $300 a month at a 7 percent return will end up worlds apart if one starts at 25 and the other starts at 40. The early years feel slow, but those dollars have the longest runway to compound.

Compounding frequency is a small lever

Interest that compounds daily earns slightly more than interest that compounds monthly, which earns slightly more than annual compounding. The gap is real but small. Focus on rate, time, and how much you can contribute before worrying about frequency.

The Rule of 72

A handy shortcut: divide 72 by your annual return to estimate how many years it takes for money to double. A 6 percent return doubles money in about 12 years. A 9 percent return doubles it in 8. This is not exact, but it is close enough for mental math.

Nominal returns vs real returns

This calculator shows nominal dollars, meaning the raw future value without accounting for inflation. Inflation quietly reduces purchasing power over time. A rough way to see real growth is to subtract your expected inflation rate (historically about 2 to 3 percent) from your expected return before running the numbers.

What can throw off your projection

• Markets are volatile. A steady 7 percent line is a planning tool, not a promise.
• Fees and taxes eat into real returns.
• Missed contributions in one year compound the same way growth does, just in reverse.

Use this as a directional estimate. The most important input is not the rate you type in. It is the habit of contributing consistently over decades.