Car refinance calculator
Move an 18,000 balance from 8.5% to 5.5% with 36 months left and you save 24.69 a month, 888.84 in total. Enter the balance, both rates and the months remaining to see whether an offer is worth taking up. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Current balance
- 18,000
- Old rate
- 8.5
- New rate
- 5.5
- Remaining term
- 36
- Old monthly payment
- 568.22
- New monthly payment
- 543.53
- Monthly savings
- 24.69
- Old total payment
- 20,455.92
Refinancing a car loan is a rate-and-payment decision, not a new-vehicle shopping problem. Drivers who still owe a remaining principal on a used or new car want to know whether a lower annual percentage rate from a credit union, bank, or captive lender will cut the monthly installment and the total cash paid over the months still left on the note. Spreadsheets that only subtract two rates ignore amortization: the same remaining balance at a new rate still has to be paid down over the remaining term, so the savings live in the installment formula, not in a simple-interest guess. This car refinance calculator accepts the current remaining balance, the old annual rate, the new annual rate, and the remaining term as a whole number of months, computes both amortizing payments, and returns monthly savings together with total savings.
What car refinance savings are and how the two payments are compared
Car refinance savings are the cash you stop paying when you replace the current auto-loan rate with a lower one and keep paying the remaining principal over the same remaining term. Each payment uses the standard reducing-balance identity. Let r be the annual percent divided by twelve hundred, and n the remaining months.
When r is greater than zero the monthly payment is principal times r times one plus r to the n, divided by one plus r to the n minus one. When r is exactly zero the payment collapses to principal divided by n, which is how a genuine zero-percent refinance is billed. Monthly savings equal the old payment minus the new payment.
Total savings equal that monthly gap times n, which is also the difference between the two totals of payments. Money fields round to two decimal places so they match currency.
What you can do with it
Should I refinance my car loan?
Enter remaining principal, the rate on the current note, an offered refinance rate, and months left to see monthly savings and total savings before calling the lender.
Credit-union and bank landing pages
Let visitors compare a published auto-refinance APR against the rate they already pay, with the same remaining term, next to each product card.
FAQ
How is car refinance savings calculated?
PMT uses r = rate/12/100, or principal/n when r is zero.
What happens if the new rate is higher than the old rate?
Equal rates return zero savings and are billed as a successful comparison.
Are origination fees, taxes, and insurance included?
No. The identity is pretax rate-drop savings on the remaining principal. Gap insurance and sales tax are not applied.
Does the comparison stretch my car loan over a new term?
No. Both payments are worked out on the same remaining balance and the same months left, so the savings come only from the lower rate, not from spreading the debt over more time.
Which balance should I enter for a car refinance?
The principal you still owe on the current car loan today, not the amount you originally borrowed. Pair it with the number of months still left on that same loan.
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/car-refinance \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"current_balance":18000,"old_rate":8.5,"new_rate":5.5,"remaining_term":36}'const res = await fetch("https://api.kit.forhosting.com/fin/car-refinance", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"current_balance": 18000,
"old_rate": 8.5,
"new_rate": 5.5,
"remaining_term": 36
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/fin/car-refinance",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"current_balance": 18000,
"old_rate": 8.5,
"new_rate": 5.5,
"remaining_term": 36
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/fin/car-refinance", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"current_balance":18000,"old_rate":8.5,"new_rate":5.5,"remaining_term":36}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"current_balance":18000,"old_rate":8.5,"new_rate":5.5,"remaining_term":36}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/fin/car-refinance", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"current_balance": 18000,
"old_rate": 8.5,
"new_rate": 5.5,
"remaining_term": 36
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "fin.car_refinance",
"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
If the new rate exceeds the old rate the task fails with invalid_input instead of advertising a negative saving as a benefit.
What car refinance savings are and how the two payments are compared
The comparison is not a lease residual, not a new-car money factor, and not a mortgage recast: it is two amortizing installments of the same current_balance over the same remaining_term, one at old_rate and one at new_rate. The endpoint echoes current_balance, old_rate, new_rate, and remaining_term, and it reports old_monthly_payment, new_monthly_payment, monthly_savings, old_total_payment, new_total_payment, and total_savings so a shopper can see both the monthly relief and the lifetime cash kept in pocket. Origination fees, gap insurance, and sales tax are not invented: subtract a cash fee from total_savings yourself if the lender charges one, otherwise leave this pretax rate-drop identity as published. Equal rates produce zero savings rather than an error; only a new_rate that exceeds old_rate is rejected, because that offer is not a refinance that saves money.
Input contract, response shape, and error behaviour
POST a JSON body with four required fields: current_balance, the remaining principal as a finite number strictly greater than zero (aliases balance, remaining_balance, and principal); old_rate, the current annual nominal interest rate as a percent such as 8.5 for eight and a half percent a year, a finite number from zero through one hundred (aliases old_apr, current_rate, and current_apr); new_rate, the offered refinance annual percent on the same scale, also from zero through one hundred, and it must not exceed old_rate (aliases new_apr and refinance_rate); and remaining_term, the months left on the current note as a positive integer from one through one hundred twenty (aliases remaining_months, term, months, and term_months). Each numeric field may arrive as a JSON number or as a numeric string. Missing fields, empty strings, non-finite values, a zero or negative balance, a negative or above-one-hundred rate, a boolean, and any remaining_term that is not a positive integer—zero, a fraction such as thirty-six point five, or a non-numeric string—fail with invalid_input naming the field and are not billed. On success the body includes the echoed inputs, both monthly payments, both totals of payments, monthly_savings, total_savings, and a formula string restating the two-PMT identity. Example: current_balance eighteen thousand, old_rate eight point five, new_rate five point five, remaining_term thirty-six yields two thirty-six-month amortizations of the same principal and a positive monthly and total gap. Pair the figure with your own fee table if the refinance charges origination; this endpoint computes rate-drop savings only over the remaining term.
Where a car refinance calculator earns its keep
Auto-loan brokers and dealer F&I desks can seed side-by-side offers that fail loudly when the new rate exceeds the old rate, instead of silently printing a negative saving that a customer might misread as extra cash. Educators teaching installment versus simple interest can show why subtracting two APRs is not the same as subtracting two PMT results on the remaining principal. Failed validation does not consume balance. Quote tools and marketplace comparison pages can call the same pure module from the browser for free previews and from the API when storing a scenario, guaranteeing that the savings a visitor saw on a landing page match the savings stored after they save a quote. Household budget apps already know a payoff balance and a remaining term from a statement PDF; posting current_balance, old_rate, new_rate, and remaining_term fills a card with old payment, new payment, monthly savings, and total savings so a household can decide whether a three-point drop is worth the paperwork. Keep remaining_term a whole number of months, pin the rounded money fields in tests, and you get reproducible car refinance savings for widgets, LMS modules, and edge workers without a lending stack of your own.
Integration use cases
Quote and broker comparison tools
Store current_balance, old_rate, new_rate, and remaining_term in a quote pipeline whose API savings match the free browser preview byte for byte.
Technical questions
Does the calculator restart the loan term?
No. remaining_term is the months left on the current note, and the refinance is compared over that same horizon (a rate drop only). To model a different term, this endpoint is the wrong tool; it does not restart or extend the note.
What does the car refinance calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.