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Compound interest calculator

Monthly deposits supported

Enter a starting amount, an annual rate and the number of years to see the future value with compound interest, how much of it is interest, and the effective annual rate. Add a monthly deposit, or switch to simple interest.

Try:

Fill in the fields to see the result.

How to use it

  1. Enter the principal, annual rate and years.
  2. Pick how often interest compounds, and add a monthly contribution if you save regularly.
  3. Read the future value and interest earned; switch to Simple to compare.

Details

Compound
A = P(1 + r/n)^(n·t)
Simple
I = P × R × T ÷ 100
Effective rate
(1 + r/n)^n − 1
Not included
Tax on interest, fees and inflation

Questions

How often should interest compound?

Use what the bank states. Most savings accounts compound daily or monthly; Indian bank fixed deposits usually compound quarterly; many bonds pay half-yearly.

What is the effective annual rate?

The yearly growth once compounding is counted. 7% compounded monthly is about 7.23% a year, which is why APY is higher than APR.

How are monthly contributions counted?

Each deposit is added at the end of its month and compounds from then, so the last deposits earn little. Real accounts may credit at different times, so treat it as an estimate.

What's the rule of 72?

Divide 72 by the annual rate to estimate the years to double: at 8%, about 9 years. The calculator gives the exact figure.

Is what I type sent anywhere?

No. The calculation runs in this browser tab with the same code our MCP server uses. Nothing you type is sent to a server or saved, apart from settings like filing status, which stay in this browser.

Can an AI agent use this?

Yes. The same function is a tool on our free MCP server at tools.cuelark.com/mcp, so ChatGPT, Claude or Cursor get exactly the answer this page shows. 'Copy MCP call' gives you the arguments.