Mortgage & Loan Amortization Cal

Give this API a loan amount, an interest rate, and a repayment term, and it hands back the precise monthly payment plus a complete schedule showing — for every single month — how much goes to interest, how much reduces the balance, and what you still owe. You can also ask 'what if I pay an extra $300 a month?' or 'what if I make a $10,000 lump payment in month 24?' and it will tell you exactly how many months you save and how much interest you avoid. Every number is computed with banker's rounding and a pinned arithmetic rule so that two correct systems running the same inputs produce identical results, down to the cent. No rate lookups, no market data — you supply the rate, the API does the math.

Call it in 30 seconds

// npm i x402-fetch viem
import { wrapFetchWithPayment } from "x402-fetch";
import { createWalletClient, http } from "viem";
import { privateKeyToAccount } from "viem/accounts";
import { base } from "viem/chains";

const account = privateKeyToAccount(process.env.PRIVATE_KEY); // funds USDC on Base
const wallet = createWalletClient({ account, chain: base, transport: http() });
const fetchWithPay = wrapFetchWithPayment(fetch, wallet);

// Pays the $0.01 USDC automatically on the 402, then returns the real response:
const res = await fetchWithPay("https://mortgage-amortization.underscoredone.com/calculate", { method: "POST" });
console.log(await res.json());

# pip install x402 eth-account httpx
import os, asyncio
from eth_account import Account
from x402.clients.httpx import x402HttpxClient

account = Account.from_key(os.environ["PRIVATE_KEY"])  # funds USDC on Base

async def main():
    async with x402HttpxClient(account=account) as client:
        # x402 handles the 402 -> pay ($0.01 USDC) -> retry automatically
        r = await client.get("https://mortgage-amortization.underscoredone.com/calculate")
        print((await r.aread()).decode())

asyncio.run(main())

# 1) An unpaid request returns HTTP 402 with the exact payment terms:
curl -i -X POST "https://mortgage-amortization.underscoredone.com/calculate" \
  -H 'content-type: application/json' \
  -d '{"annual_rate_percent": 6.5, "extra_monthly_payment": 300, "one_time_payments": [{"amount": 10000, "month": 24}], "principal": 400000, "rounding": 2, "start_date": "2026-07-01", "summary_only": false, "term_months": 360}'

# 2) Paying + retrying requires an x402 client that can sign the USDC
#    authorization (curl alone cannot). See the JS / Python tabs below.

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