What Compound Interest Does to a Balance

Compound interest is interest earned on the original amount and on interest that was added earlier. A balance that only earns interest on the first deposit grows in a straight line. A balance that compounds curves upward, because each new period starts from a larger base.

The three inputs that change the result

• The amount you start with.
• The amount you add on a schedule, such as every month.
• The annual rate and the number of years you leave it invested.

The compound interest calculator applies those inputs and shows the path, not only the final number. Changing the monthly contribution usually moves the result more than people expect, because each contribution then has years of its own to compound.

What the chart is not

The result is a math projection at the rate you type. It does not predict markets, fees, taxes, or inflation. Use it to compare two plans — for example, adding a monthly amount versus leaving a lump sum alone — and then check the assumptions with whoever holds the account.

For a count that has nothing to do with money, the age calculator measures time from a date of birth instead of growth from a rate.

A worked example

Start with 10,000, add 1,000 a month, and leave it for 10 years at 8% a year. The deposits alone are 10,000 plus 120,000, which is 130,000. Compounding is everything above that. The compound interest calculator separates the money you put in from the growth, so you can see whether the extra came from the rate or from the monthly addition. Run the same 10 years with no monthly addition and the gap between the two charts is the value of the contributions.

What to change first

If two plans are close, change one input at a time. A higher rate and a higher monthly amount both lift the ending balance, but they are not interchangeable: the rate applies to money already there, and the contribution adds new money that then earns the rate. Time does both jobs. An age calculation is the other time question on the site — years lived, rather than years invested.

The curve is the point

Simple interest pays the rate on the original amount and stops there, so the growth is a straight line. Compound interest pays the rate on the original amount and on interest already added, so each period starts from a larger base. Early on the two lines are close. Over many years the compound line pulls away, because the base it is multiplying has itself been growing.

A monthly contribution is new money, and then it compounds for however many years are left. A contribution in year one has almost the full period. A contribution in the last year has almost none. That is why raising the monthly amount early changes the ending balance more than raising it near the end, and why a higher rate on a small balance can still lose to a lower rate on a balance you keep feeding.

What the projection leaves out

The chart uses the rate you type, every year, with no fees, no taxes, and no years when the market does something else. It is a comparison tool. Run two plans that differ by one input and the gap is the effect of that input. It is not a promise from a bank or a fund. Inflation is a separate question: a larger nominal balance can still buy less if prices rose. The calculator will not subtract that unless you model it yourself by using a lower rate.