Loan comparison calculator
Borrowing 25,000 at 5.5% over 36 months costs 2,176.4 in interest; at 7.9% over 48 months it costs 4,239.2, even though the payment is 145.75 lower. Compare two offers on total interest, not on the headline rate. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Principal a
- 25,000
- Annual rate a
- 5.5
- Term months a
- 36
- Principal b
- 25,000
- Annual rate b
- 7.9
- Term months b
- 48
- Monthly payment a
- 754.9
- Total repayment a
- 27,176.4
- Total interest a
- 2,176.4
- Monthly payment b
- 609.15
Two loan offers cannot be ranked by looking at a single advertised annual rate. A cheaper percent on a longer term can cost more interest than a slightly higher percent paid off sooner, and a lower monthly payment can hide a larger total cost of credit. This loan comparison calculator accepts a principal, an annual nominal interest rate as a percent, and a term as a whole number of months for each of two loans, applies the standard reducing-balance installment formula to both, and returns the monthly payment and total interest of each together with which loan is cheaper on total interest.
What a two-loan comparison means and how monthly payment and total interest are computed
A loan comparison starts from two independent amortizing notes rather than from a headline percent. The number of payments n is the term in whole months. Money fields round to two decimal places so they match currency.
The cheaper loan is the one with the lower total interest, which is the true extra cost of credit; if the two rounded totals match, the result is a tie. A lower monthly payment does not automatically win: stretching a note can cut the installment while raising the interest paid over the life of the loan. Origination fees, discount points, and insurance are not invented: add financed fees to principal if they go into the note, otherwise leave them out of the comparison.
Where a loan comparison calculator earns its keep
Affordability worksheets already know two financed amounts; posting both principals, rates, and terms fills a card with monthly payment, total interest, and which loan is cheaper so a counselor can see whether a longer cheaper-rate note actually costs more interest than a shorter one. Educators teaching amortizing versus interest-only credit can seed classroom examples that fail loudly when either term is not a positive integer, instead of silently treating a fractional month as a valid loan.
Failed validation, including a term that is not a positive integer, does not consume balance. Keep both principals strictly positive and both terms whole numbers of months, pin the rounded money fields in tests, and you get reproducible loan comparisons for widgets, LMS modules, and edge workers without a loan-origination stack of your own.
What you can do with it
Compare two personal-loan offers
Turn two principals, annual rates, and terms in months into monthly payments, total interest, and which loan is cheaper before signing.
Lender and marketplace landing pages
Let visitors enter two advertised products and see monthly payment, total interest, and the cheaper loan next to each card.
FAQ
How does the loan comparison calculator decide which loan is cheaper?
A lower monthly payment does not by itself win.
What happens if a term is not a positive integer?
Fractions, zero, booleans, and non-numeric values are rejected and not billed.
Can the two loans have different principals or different terms?
Yes. Each loan is amortized independently. Different amounts and tenures are valid; cheaper still follows total interest, so a smaller principal or a shorter term can win even at a higher rate.
Does a 0% rate on one or both loans work?
Total interest on that side is then zero aside from rounding.
Why can the loan with the lower monthly payment cost more?
Because a longer term spreads the balance over more payments, so each one is smaller while interest keeps building. In the example, the 48-month loan saves 145.75 a month but costs 2,062.8 more in interest.
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/loan-comparison \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"principal_a":25000,"annual_rate_a":5.5,"term_months_a":36,"principal_b":25000,"annual_rate_b":7.9,"term_months_b":48}'const res = await fetch("https://api.kit.forhosting.com/fin/loan-comparison", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"principal_a": 25000,
"annual_rate_a": 5.5,
"term_months_a": 36,
"principal_b": 25000,
"annual_rate_b": 7.9,
"term_months_b": 48
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/fin/loan-comparison",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"principal_a": 25000,
"annual_rate_a": 5.5,
"term_months_a": 36,
"principal_b": 25000,
"annual_rate_b": 7.9,
"term_months_b": 48
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/fin/loan-comparison", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"principal_a":25000,"annual_rate_a":5.5,"term_months_a":36,"principal_b":25000,"annual_rate_b":7.9,"term_months_b":48}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"principal_a":25000,"annual_rate_a":5.5,"term_months_a":36,"principal_b":25000,"annual_rate_b":7.9,"term_months_b":48}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/fin/loan-comparison", 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_a": 25000,
"annual_rate_a": 5.5,
"term_months_a": 36,
"principal_b": 25000,
"annual_rate_b": 7.9,
"term_months_b": 48
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "fin.loan_comparison",
"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.
Limits
max_term_months | 600 |
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
A term that is not a positive integer fails with invalid_input instead of inventing a schedule.
What a two-loan comparison means and how monthly payment and total interest are computed
For each loan the monthly rate r is the annual nominal percent divided by twelve hundred: r equals annual_rate divided by twelve divided by one hundred. When r is greater than zero the closed form is monthly_payment equals P times r times (1 plus r) to the n, divided by (1 plus r) to the n minus one. That identity is the standard reducing-balance installment lenders publish: each installment covers interest on the outstanding principal first, then reduces the balance, so the interest share falls even though the cash payment never changes. When the annual rate is zero the denominator would be zero, so the calculator switches to the honest special case monthly_payment equals P divided by n. Total repayment is monthly_payment times n, and total interest is total repayment minus principal.
Input contract, the positive-integer term rule, and the response shape
POST a JSON body with six required fields. For loan A: principal_a, the financed amount as a finite number strictly greater than zero (aliases amount_a and loan_amount_a); annual_rate_a, the annual nominal interest as a percent such as 5.5 for five and a half percent per year, finite and at least zero (aliases rate_a and interest_rate_a); and term_months_a, the term as a positive integer number of months from one through six hundred (aliases term_a and months_a). For loan B the same three fields are principal_b, annual_rate_b, and term_months_b, with matching aliases. Each numeric field may arrive as a JSON number or as a numeric string. Missing fields, empty strings, non-finite values, a non-positive principal, and a negative annual rate fail with invalid_input naming the field and are not billed. The distinctive product rule is the term: term_months_a and term_months_b must each be a positive integer. Zero, a fraction such as thirty-six point five, a boolean, or a non-numeric string never produce a fake schedule, because a loan term that is not a whole number of months is not a valid comparison input. On success the body echoes both principals, rates, and terms, and adds monthly_payment, total_repayment, and total_interest for each side, plus cheaper (loan_a, loan_b, or tie), interest_difference, monthly_difference, and a formula string restating the amortizing identity. Example: principal twenty-five thousand on both sides, annual_rate_a 5.5 with term_months_a 36 versus annual_rate_b 7.9 with term_months_b 48 yields two rounded monthly payments and a cheaper flag on the side with lower total interest.
Where a loan comparison calculator earns its keep
Quote tools and origination CRMs can call the same pure module from the browser for free previews and from the API when storing a scenario, guaranteeing that the cheaper flag a visitor saw matches the cheaper flag stored after they save a pair of offers. Content teams can illustrate zero-percent seller financing on one side with annual_rate zero, where monthly_payment collapses to principal over term_months without a divide-by-zero.
Integration use cases
Quote and CRM pipelines
Store two notes in a quote tool whose API cheaper flag matches the free browser preview byte for byte.
Technical questions
What does the loan comparison calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input, including a term that is not a positive integer, is not billed.