Auto loan calculator
Finance 25,000 at 6.5% for 60 months and your payment is 489.15 a month, yet the first one sends 135.42 to interest and only 353.73 to the loan. See the total interest too, before you sign. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Principal amount
- 25,000
- Annual rate
- 6.5
- Term months
- 60
- Monthly rate
- 0.0054166667
- Monthly payment
- 489.15
- Payment count
- 60
- Total paid
- 29,349.2
Shopping for a car is a monthly-payment decision long before it is a sticker-price decision. Lenders quote a principal (the amount actually financed after down payment, trade-in, and rebates), an annual percentage rate, and a term in months—thirty-six, forty-eight, sixty, seventy-two, or eighty-four are the notes most buyers see. This auto loan calculator turns those three numbers into the fixed monthly installment, the total interest paid over the life of the loan, and a compact amortization summary that shows how the first payment and the last payment split principal versus interest. Zero-percent promotional APR is supported.
How the monthly auto loan payment and interest are computed
An auto loan in this calculator is a fully amortizing installment contract with equal monthly payments. When r is positive the standard annuity formula applies: monthly payment equals principal times r times (1+r) to the n, divided by (1+r) to the n minus one. When the APR is exactly zero—a common dealer promotion—the payment collapses to principal divided by n, because there is no interest component to front-load.
After the closed-form payment is rounded to cents, the capability walks every remaining month: interest for the period is the opening balance times r, principal retired is payment minus that interest, and the new balance is what is left. The last installment is not blindly copied from the rounded payment; it pays leftover principal plus the last month of interest so the balance lands on zero instead of a few leftover pennies.
That walk is why totals can differ slightly from n times the advertised payment, which is the same behaviour a servicer uses on a real note. Typical consumer terms are thirty-six to eighty-four months; the contract accepts any positive integer term from one through three hundred sixty so short bridge notes and longer commercial lengths still compute without a second tool. Principal must be greater than zero and the annual rate must be finite and non-negative.
What you can do with it
Dealer desk and buyer worksheets
Turn amount financed, quoted APR, and a 36–84 month term into monthly payment, total interest, and first versus last amortization rows before signing.
FAQ
How is the monthly auto loan payment calculated?
For a 0% APR, payment is principal/n. The payment is rounded to cents, then the schedule is walked so the last installment clears the balance.
What happens if the term is not a positive integer?
Zero, negatives, and values such as 48.5 are rejected and are not billed.
What amount should I enter as the loan principal?
The amount financed, not the sticker price: subtract your cash down payment, positive trade-in equity and rebates, then add taxes, title fees or negative equity only if they are rolled into the loan.
Does a 0% dealer financing offer work?
Yes. With a zero rate there is no interest to front-load, so the monthly payment is simply the amount financed divided by the number of months, and the total interest is zero.
Why is the last car payment slightly different?
Because the payment is rounded to cents. The final installment pays the leftover principal plus one last month of interest so the balance lands exactly on zero; in the example it is 489.35 instead of 489.15.
Related tools
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
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/fin/auto-loan \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"principal":25000,"annual_rate":6.5,"term_months":60}'const res = await fetch("https://api.kit.forhosting.com/fin/auto-loan", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"principal": 25000,
"annual_rate": 6.5,
"term_months": 60
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/fin/auto-loan",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"principal": 25000,
"annual_rate": 6.5,
"term_months": 60
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/fin/auto-loan", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"principal":25000,"annual_rate":6.5,"term_months":60}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"principal":25000,"annual_rate":6.5,"term_months":60}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/fin/auto-loan", 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": 25000,
"annual_rate": 6.5,
"term_months": 60
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "fin.auto_loan",
"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
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. |
Technical notes
The term must be a positive integer; a fractional, missing, or zero term fails with invalid_input and is not billed.
How the monthly auto loan payment and interest are computed
The monthly periodic rate r is the quoted annual_rate divided by one hundred and then by twelve. The number of payments n is term_months, which must be a whole number of months because auto notes are written that way, not as a fraction of a year.
Input contract, schedule summary, and error behaviour
Each installment object has period, payment, principal, interest, and remaining balance so a UI can show how little of payment one is principal on a long, high-APR note and how the last row clears the book. Example: twenty-five thousand financed at 6.5 percent for sixty months yields a fixed monthly payment near four hundred eighty-nine dollars, with first-month interest taken from the opening balance and a last payment that may differ by cents. POST a JSON body with three required fields. principal is the amount financed, not the window sticker: subtract cash down, positive trade equity, and factory rebates that reduce the note, then add taxes, title, and negative equity only if they are rolled in. annual_rate is the nominal APR as a percent (six point five means 6.5 percent a year); aliases apr and rate are accepted so dealer forms can reuse their column names. term_months is the term in whole months; alias term maps to the same field. term_months must be a positive integer from 1 to 360—zero, negatives, and values such as 60.5 fail with invalid_input naming term_months, and those failures are not billed. Each numeric field may arrive as a JSON number or a numeric string; missing fields, empty strings, and non-finite values fail with a message that names the field. On success the body echoes principal, annual_rate, and term_months, reports monthly_rate and the rounded monthly_payment, then the schedule summary: payment_count, total_paid, total_interest, first_payment, and last_payment. Pair the JSON with your own year-by-year chart if you need every row; this endpoint returns the summary, not a sixty-line table, so responses stay small for widgets and APIs.
Where an auto loan calculator earns its keep
Buyers use an auto loan calculator at the desk to convert a quoted APR and a 60- or 72-month term into a payment they can actually budget, then to see total interest before saying yes to an extra year that only drops the installment a little. Credit unions and brokers drop the same three fields into origination worksheets so a 0 percent promo can be compared with a rebate that reduces principal instead. Failed validation does not consume balance. Refinance shops re-run principal as the current payoff, annual_rate as the new offer, and term_months as the remaining or reset term, then read total_interest against the old note. Keep term_months a positive integer, pin the rounded currency fields in tests, and you get reproducible auto-loan figures for deal jackets, LMS plugins, and edge workers without standing up a servicing engine of your own.
Integration use cases
Zero-percent promo versus rebate
Compare a 0% term against a cash rebate that lowers principal: same calculator, different annual_rate and principal, honest totals either way.
Refinance and payoff checks
Re-run the current payoff as principal with a new APR and term_months to see whether resetting the note actually saves interest.
Technical questions
Does a 0% promotional APR work?
Yes. Set annual_rate to 0. Monthly payment is principal divided by term_months; interest each period is zero; the last payment absorbs any leftover cents from rounding.
What does the amortization schedule summary include?
Besides monthly_payment, total_paid, and total_interest, the response includes first_payment and last_payment objects with period, payment, principal, interest, and remaining balance. It does not return every intermediate month.
What does the auto loan calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.