Compound Interest Calculator

The power of compound interest

Compound interest is interest earning interest. Each period your balance grows, and the next period's growth is calculated on that larger balance — so the curve bends upward over time instead of rising in a straight line. Einstein supposedly called it the eighth wonder of the world; whether or not he did, the math is what quietly builds wealth from steady saving.

What the inputs do

• Starting amount — your opening balance, which compounds the longest.
• Monthly contribution — money you add every month. Regular contributions are usually the biggest driver of the final number.
• Annual interest rate — your expected yearly return. Stock-market averages are often quoted around 7% after inflation, but real returns vary and aren't guaranteed.
• Compounding frequency — how often interest is added. More frequent compounding helps a little, but far less than rate or time.

Why time matters more than amount

Because growth compounds, the years closest to the end do the heaviest lifting. Someone who invests for 30 years can end up well ahead of someone who invests twice as much per month for only 15 — the early money simply has more time to multiply. The lesson: start early, even with small amounts. Delaying a few years can cost more than it seems.

A note on inflation and risk

This tool shows nominal growth at a fixed rate. Real investments fluctuate year to year, and inflation erodes purchasing power — a dollar in 20 years buys less than today. To estimate "real" growth, use a rate that already subtracts inflation (e.g. 7% market return minus 3% inflation ≈ 4%).

FAQ

Are contributions added before or after interest?

This calculator grows the balance first, then adds the month's contribution (an "end of period" deposit). That's the conservative, common convention; depositing at the start of each month would yield slightly more.

Does more frequent compounding make a big difference?

Surprisingly little. Going from annual to daily compounding at the same rate adds a small fraction — time invested and the rate itself matter far more.

Is this calculator free, and does my data stay private?

Yes on both counts. It's free with no sign-up, and every calculation runs in your browser — your starting amount, contributions, and rate are never sent to a server or saved anywhere.

Is the result financial advice?

No. This is an educational estimate using a fixed rate you choose. Actual investment returns fluctuate and aren't guaranteed, so treat the figure as a projection to plan around, not a promise, and consult a professional for personal decisions.

How do I account for inflation?

Enter a "real" rate that already subtracts expected inflation — for example, use about 4% instead of a 7% nominal market return if you assume 3% inflation. The result then approximates your future balance in today's purchasing power.

Can I set the monthly contribution to zero?

Yes. Leave the monthly contribution at zero to see how a single lump sum grows on its own, which is useful for projecting a one-time deposit or comparing it against the effect of adding regular savings.