Interest Rate Calculator

Find the interest rate needed to grow your money to a target amount.

Calculate Interest Rate

Compound Interest Simple Interest

Principal (starting amount)

Target Amount (final amount)

Time Period (years)

Compounding Frequency

Use this tool from AI agents

This tool's engine is available to AI agents over MCP as interest_rate. Add the Clean.tools server, then your agent can call it directly.

Claude Code

claude mcp add --transport http clean-tools https://mcp.clean.tools/mcp

Claude Desktop (mcpServers)

{"mcpServers":{"clean-tools":{"command":"npx","args":["-y","@clean-tools/mcp"]}}}

Cursor

https://mcp.clean.tools/mcp

Or call it over plain HTTP from any script. Same engine, JSON in and JSON out, no key:

curl

curl -s https://api.clean.tools/v1/interest_rate \
  -H 'content-type: application/json' \
  -d '{"type":"compound","principal":10000,"target":15000,"years":5,"compoundingPerYear":12}'

JavaScript

const res = await fetch('https://api.clean.tools/v1/interest_rate', {
  method: 'POST',
  headers: { 'content-type': 'application/json' },
  body: JSON.stringify({ type: 'compound', principal: 10000, target: 15000, years: 5, compoundingPerYear: 12 })
});
const data = await res.json();

OpenAPI spec npm: @clean-tools/mcp Full developer guide

How it works

This interest rate calculator determines the annual interest rate needed to grow a principal amount to a target amount over a given time period. It supports both simple interest (rate = (target/principal - 1) / years) and compound interest (rate = n × ((target/principal)^(1/(n×t)) - 1)) calculations.

All calculations run entirely in your browser. No data is sent to a server. No cost, no signup, no tracking.

Need the same math in a script or an AI agent? The interest_rate engine is also an interest-rate API at https://api.clean.tools/v1/interest_rate and an MCP server you can add with the @clean-tools/mcp package from npm.

Frequently asked questions

Why must the target amount be greater than the principal?

Both formulas solve for the positive annual growth rate that turns your starting amount into a larger one, so a target equal to or below the principal has no rate to report and the tool asks you to raise it. To model a decline you would need a negative rate, which this calculator does not output.

Why is the compound rate lower than the simple rate for the same numbers?

Compounding reinvests earned interest, so it reaches the same target at a lower nominal rate than simple interest, which pays only on the original principal. Increasing the compounding frequency (from annually toward daily) lowers the reported rate a little more for identical inputs.

Is the compound result an APY or a nominal rate?

It is a nominal annual rate compounded at the frequency you choose, not the effective annual yield (APY). If you set compounding to annually the two are the same; at any higher frequency the nominal rate here will read slightly below the equivalent APY.

Which compounding frequency should I pick?

Match it to how the account or investment actually credits interest, since the same principal, target, and time produce a different nominal rate at each frequency. If you are unsure, monthly is the most common convention for consumer accounts.

Can I use this from the command line or an AI agent?

Yes. The same solver is exposed as a REST endpoint at https://api.clean.tools/v1/interest_rate, so you can POST your principal, target, and years as JSON from curl or any script and get the rate back as JSON. It is also an MCP server you can add to Claude, Claude Code, or Cursor, which lets an AI agent run the calculation directly. To run it locally as a stdio server, install the @clean-tools/mcp package from npm.

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Use Clean.tools from your AI agent

This tool is also interest_rate on the Clean.tools MCP server — the same engine, callable by your AI agent directly. No key, no signup: requests are processed in memory and request contents are never stored.