Compound interest

Compute future value of a principal under compound interest. Returns future value, total interest earned, and the effective annual rate (APY) given the compounding frequency. Matches Excel's FV(rate, nper, 0, -principal) and the classic (1+r/n)^(nt) textbook formula.

Input

Field	Type	Description
principal *	number	Starting amount (positive)
annualRate *	number	Annual interest rate as decimal (0.05 = 5%)
years *	number	Time horizon in years
compoundingPerYear	number	Compounding periods per year (1 = annual, 12 = monthly, 365 = daily). Default 12.

Example output

{
  "futureValue": 1647.01,
  "totalInterest": 647.01,
  "effectiveAnnualRate": 0.051162,
  "periods": 120
}

Try it — see the 402 challenge (free)

curl -i -X POST https://agent402.tools/api/compound-interest \
  -H "Content-Type: application/json" \
  -d '{"principal":1000,"annualRate":0.05,"years":10,"compoundingPerYear":12}'

The response is HTTP 402 Payment Required with exact payment requirements. Any x402 v2 client pays automatically and retries:

Paid call (JavaScript agent)

import { wrapFetchWithPayment } from "@x402/fetch";
import { x402Client } from "@x402/core/client";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";

const client = new x402Client();
registerExactEvmScheme(client, { signer: privateKeyToAccount(KEY) });
const payFetch = wrapFetchWithPayment(fetch, client);

const res = await payFetch("https://agent402.tools/api/compound-interest", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "principal": 1000,
    "annualRate": 0.05,
    "years": 10,
    "compoundingPerYear": 12
  }),
});

No wallet? Pay with compute

This is a pure-CPU tool, so an agent without a wallet can pay with proof-of-work instead of USDC: fetch a challenge, solve the sha256 puzzle (16 leading zero bits — a fraction of a second of CPU, no money, no AI tokens), and resend with the X-Pow-Solution header.

import { createHash } from "node:crypto";
const lz = (b) => { let t = 0; for (const x of b) { if (!x) { t += 8; continue; } t += Math.clz32(x) - 24; break; } return t; };
const c = await (await fetch("https://agent402.tools/api/pow/challenge?slug=compound-interest")).json();
let n = 0;
while (lz(createHash("sha256").update(c.challenge + ":" + n).digest()) < c.difficulty) n++;
await fetch("https://agent402.tools/api/compound-interest", { method: "POST", headers: { "X-Pow-Solution": c.token + ":" + n, "Content-Type": "application/json" }, body: JSON.stringify({"principal":1000,"annualRate":0.05,"years":10,"compoundingPerYear":12}) });

Part of these workflows

This tool is one step in 3 curated multi-tool workflows — agents can fetch the whole sequence as an MCP prompt or call https://agent402.tools/api/skill-packs/{slug}/prompt.

• Loan comparison — Compare two or more loan offers — different rates, terms, fees, prepayment structures — on the metrics that actually matter (monthly payment, total interest, year-1 equity build, NPV at your discount rate, effective rate). Apples-to-apples math without opening a spreadsheet.
• Investment decision — Should we do this project? Run a capital allocation decision (equipment, expansion, acquisition, build-vs-buy) through the textbook CFO workflow: NPV at your hurdle rate, IRR vs. cost of capital, opportunity cost against a passive benchmark, and levered cashflow analysis if the project is debt-financed. Deterministic answers, not a gut call.
• Retirement planning — Will my retirement plan actually work? Project the accumulation phase forward with compound interest, compute the target nest egg from your expected spending, then model the drawdown phase using the same PMT formula a mortgage uses — your retirement is mathematically a loan you're paying yourself. Deterministic numbers, no glossy advisor PowerPoint.

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