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Compound Interest Calculator.

The whole argument for starting now instead of later, in one number.

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Compound Interest

The whole argument for starting now instead of later, in one number. Watch what happens when you change the years and nothing else.

7% is a common long-run assumption for a broad stock index after inflation. It is an assumption, not a promise.

$0Ending Balance
$0You Contributed
$0Growth
–Multiple of Input

How the maths works

Monthly rate r = annual return ÷ 12 ÷ 100 Months n = years × 12 Future value = P(1 + r)ⁿ + C × [((1 + r)ⁿ − 1) ÷ r] └ lump sum ┘ └ monthly contributions ┘ Growth = Future value − total contributed

How to use it

  1. Enter what you already have invested. Zero is fine.
  2. Enter what you can add each month. $25 is a real starting number and does real work.
  3. Set the number of years, then change only that field and watch what happens.
  4. Leave the return at 7% unless you have a reason — it is a common long-run assumption for a broad stock index after inflation, and it is an assumption, not a promise.

A worked example

$500 to start, $100 a month, 7%, over 30 years becomes about $125,000 — of which only $36,500 came out of your pocket. Cut it to 20 years and it is roughly $54,000. Ten fewer years does not cost you a third of the outcome; it costs you well over half. That gap is the one advantage that cannot be bought back later.

Where this sits in the Financial Literacy resource

A calculator tells you where you are. It does not tell you what to do next, and a number without a plan behind it tends to produce anxiety rather than progress. These stages are free, need no account, and cover the decision this calculator is measuring.

Learn it Stage 3: Rebuild Stage 4: Invest

Common questions

What return rate should I use?

7% is a widely used long-run assumption for a diversified stock index after inflation; around 10% is the common figure before inflation. Both are averages over very long periods, and no individual year delivers them. Use 7% for planning and treat any single-figure projection as a model rather than a forecast.

How does compound interest actually work?

Your returns start generating returns of their own. In year one you earn on your contributions; in year twenty you are largely earning on previous years’ earnings. That is why the curve is nearly flat early and steep late, and why time in the market matters more than the amount you start with.

Is $25 a month worth investing?

Yes — and the habit is worth more than the amount. $25 a month at 7% over 30 years is roughly $30,000, of which about $21,000 is growth. More importantly, someone who starts at $25 raises it later; someone waiting until they can afford $250 usually never starts.

What this calculator is not

It is an educational model, not a projection and certainly not advice. It knows nothing about your income, your state, your debts or your benefits status, and it ignores taxes and fees unless the page says otherwise. If you receive SSI or SSDI some of this maths works differently and getting it wrong can cost you eligibility — start with the Disability Wealth Guide instead. Our sourcing and correction policy is on the editorial standards page.

Nothing here is stored. Every calculation runs in your browser. No numbers are transmitted, logged or saved to any server, and no account is required. Close the tab and it is gone.