Compound Interest Calculator

Interest on interest is the whole point

A 7% year on $10,000 is $700 if nothing else happens. Leave it invested and next year’s 7% also hits the $700. That is compounding. Add $200 a month and the snowball has two engines: deposits and interest on a rising balance. This compound interest calculator shows the ending value, how much you put in, and how much is interest.

It is classroom math with a constant rate. A real brokerage account will not print a straight 7% line. Use it to compare “what if I start now versus in five years,” not to pick a fund.

What the calculator does

The starting amount grows at the annual rate divided by the compounding frequency. Monthly contributions are treated as end-of-month deposits and grown at the equivalent monthly rate implied by that same annual rate.

Total contributions are the start plus monthly amount times months. Interest is future value minus that total. If the rate is 0%, you simply get principal plus deposits back.

How to use it

Put today’s balance in starting amount. Put the amount you can actually auto-transfer in monthly contribution—$0 is fine. Pick a rate you can defend: a high-yield savings APY for cash, a conservative illustration for long-run stocks. Ten years is a useful default; stretch it if the goal is retirement.

Monthly compounding matches most U.S. savings and contribution habits. Switch to annual if a CD credits once a year.

Worked examples

$10,000 at 7% for 10 years with $200 a month and monthly compounding lands near $54,700 on this model. $5,000 at 5% for 20 years with no extra deposits is about $13,560. Starting from $0 and putting $300 a month at 6% for 15 years is about $87,200.

If the $200/month plan is delayed five years, you lose both the deposits and the interest those deposits would have earned. That gap is usually larger than monthly versus daily compounding.

What this will not do

It will not model sequence-of-returns risk, inflation, capital-gains tax, or an employer 401(k) match. A match is free principal—add it to the monthly field if you want a rough picture. For historical purchasing power, use the inflation calculator.

For a house payment or a car note, use the mortgage calculator or the car payment calculator. Those pages discount a debt. This page grows an asset.

Typical examples

Frequently asked questions

What is the compound interest formula?

For a lump sum, A = P(1 + r/n)^(n t), where P is principal, r is the annual rate, n is compounds per year, and t is years. Monthly deposits use the future value of an annuity on top of that.

How much is $10,000 at 7% for 10 years with $200 a month?

On this page’s default monthly compounding, about $54,700. Roughly $34,000 of that is money you deposited; the rest is interest. Change the rate and you will see how sensitive the ending number is.

Is monthly or daily compounding a big gap?

At typical savings rates the gap is small. At 7% over 10 years on $10,000 with no contributions, monthly versus daily is a few dozen dollars. Contributions matter more than daily versus monthly.

Does this include taxes or fees?

No. A brokerage expense ratio, a 401(k) wrap fee, or tax on interest all reduce the real ending value. Treat the result as a pre-fee, pre-tax sketch.

Is 7% a promise?

No. 7% is a round long-run stock-market illustration people use in planning, not a savings-account APY and not a forecast. Use the rate your account actually pays if you are modeling cash.