ForHosting KIT · Developer Utilities

Loan total interest calculator

Borrowers, advisors, and product teams often ask a single plain question before signing: how much interest will this fixed-rate amortizing loan cost over its full term?

● BetaFree · in your browser

Run — free How it works & API

Use it from WebAPIEmailTelegramApp soon

Headline rates and monthly payments hide the lifetime cost of capital, especially on long mortgages and multi-year installment plans where interest can approach or exceed the principal. This loan total interest calculator accepts principal, annual rate as a percent, and term in years, optionally adjusts payments per year, and returns the fixed periodic payment, total amount paid, total interest, and interest as a percent of principal. The math is the classic fully amortizing closed-form payment formula with a clean zero-rate branch, fully deterministic, and identical in the free browser widget and the prepaid API so quotes, education pages, and automation never disagree on the same inputs.

How to use it

Enter your values in the form above. The tool checks them before calculating and shows the result on the same page.

Check your inputs

Use the labels and units shown next to each field. If something is missing or outside the allowed range, the page points to the field to fix.

Use it again or automate it

Use the browser tool for individual checks and the API when you need the same capability in an automated workflow.

What you can do with it

Get an answer now

Enter one set of values and see the result without building a spreadsheet or script.

Compare scenarios

Change one value at a time and rerun the calculation to understand what affects the result.

Automate repeated work

Use the API when the same calculation needs to run inside your product or workflow.

FAQ

How do I use this capability?

Complete the fields above and run it on this page. The form highlights anything that needs attention.

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/finance/loan-total-interest

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/finance/loan-total-interest \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"principal":250000,"annual_rate":6.5,"term_years":30}'

const res = await fetch("https://api.kit.forhosting.com/finance/loan-total-interest", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "principal": 250000,
    "annual_rate": 6.5,
    "term_years": 30
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/finance/loan-total-interest",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "principal": 250000,
    "annual_rate": 6.5,
    "term_years": 30
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/finance/loan-total-interest", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"principal":250000,"annual_rate":6.5,"term_years":30}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"principal":250000,"annual_rate":6.5,"term_years":30}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/finance/loan-total-interest", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "principal": 250000,
  "annual_rate": 6.5,
  "term_years": 30
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "finance.loan_total_interest",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Errors

HTTP	Code	Meaning
`401`	`unauthorized`	Missing or invalid API key.
`402`	`insufficient_balance`	Your balance doesn't cover the task price.
`404`	`unknown_type`	That task type doesn't exist.
`429`	`rate_limited`	Too many requests. Use the webhook instead of polling.

Read the full KIT documentation →