How to Calculate Car Loan Interest (With Worked Examples)
The formula, a real month-by-month breakdown, and the shortcut that gets it badly wrong.
The short answer
Car loan interest is charged on your remaining balance each month, not on the original loan amount. Each month: interest = current balance × (annual rate ÷ 12). Whatever is left of your payment goes to principal. Because the balance shrinks every month, the interest portion shrinks with it — which is why total interest on a $30,000 loan at 7.14% over five years is about $5,761, not the $10,710 a flat-rate estimate would suggest.
- Monthly interest = balance × (APR ÷ 12). Recalculated every month on the new balance.
- Early payments are mostly interest; later payments are mostly principal. This is amortisation, not a trick.
- Do not use the flat simple-interest formula. It overstates the cost of a five-year loan by roughly 86%.
- APR includes lender fees; the interest rate does not. Compare offers on APR.
- Extra payments go straight to principal — and every dollar of principal removed stops accruing interest immediately.
The two formulas you need
There are two formulas worth knowing. One calculates your monthly payment; the other calculates how much of that payment is interest.
The monthly payment formula
The monthly interest formula
This is the one that actually explains where your money goes, and it is far simpler:
Everything left over reduces the balance. On that $596.02 payment, $178.50 is interest and the remaining $417.52 is principal.
A worked example, month by month
Here is the first two months of a $30,000 loan at 7.14% over 60 months, worked by hand so you can follow each step.
| Month 1 | Month 2 | |
|---|---|---|
| Starting balance | $30,000.00 | $29,582.48 |
| × monthly rate (0.00595) | $178.50 | $176.02 |
| Payment | $596.02 | $596.02 |
| Interest portion | $178.50 | $176.02 |
| Principal portion | $417.52 | $420.00 |
| Ending balance | $29,582.48 | $29,162.48 |
Notice what happened between month one and month two: the interest fell by $2.48 and the principal rose by exactly the same amount. That gap widens every single month. By the final payment almost the entire $596.02 is principal.
Over the full 60 months you pay $35,761.17 in total — $5,761.17 of it interest.
Get the complete month-by-month interest and principal split for your own loan, without doing 60 rows of arithmetic.
See the full scheduleThe mistake that overstates interest by 86%
This is the error that sends people into a dealership with the wrong number in their head.
The intuitive way to estimate interest is to multiply: loan × rate × years. On our example that gives $30,000 × 7.14% × 5 = $10,710. The real figure is $5,761.17. The flat estimate is off by 86%.
| Method | Total interest | Accurate? |
|---|---|---|
| Flat simple interest (P × r × years) | $10,710.00 | No — off by $4,949 |
| Amortised (actual) | $5,761.17 | Yes |
The reason is straightforward: the flat formula assumes you owe the entire $30,000 for the whole five years. You do not — you owe less every month. Interest is only ever charged on what remains.
Standard U.S. auto loans from banks, credit unions and captive lenders are simple-interest amortising loans, so the calculation above applies. But precomputed interest loans do exist, mainly in subprime and buy-here-pay-here financing. With precomputed interest, the total is calculated up front and baked into the contract, so paying early saves you little or nothing.
Check your contract for the words "precomputed" or "Rule of 78s" before assuming extra payments will help. If those terms appear, the maths in this guide does not describe your loan.
APR vs. interest rate
Two numbers appear on every loan offer, and they are not the same thing.
| Interest rate | APR | |
|---|---|---|
| What it covers | Cost of borrowing only | Interest plus lender fees |
| Typically includes | The rate itself | Origination and documentation fees rolled into financing |
| Which is higher | Lower or equal | Equal or higher |
| Use it to | Calculate monthly interest | Compare competing offers |
Because APR folds fees into a single percentage, it is the honest basis for comparison. A 6.0% rate with $600 of financed fees can cost more than a 6.3% rate with none. The Truth in Lending Act requires lenders to disclose APR for exactly this reason (CFPB, Auto Loans).
Your rate is driven mostly by your credit. On our $30,000 example over five years, a 10% rate costs $8,244.68 in interest and a 7% rate costs $5,642.16 — a difference of $2,602.52 for the identical car. These are example rates, not market averages.
Enter your rate and term to see the payment and total interest, then change the rate to price the difference a better credit tier would make.
Compare ratesHow to pay less interest
Because interest is charged on the balance, anything that lowers the balance sooner lowers total interest. In order of effectiveness:
1. Put more money down
Every dollar of down payment is a dollar that never accrues interest. It is the only lever that works from day one.
2. Take a shorter term
A shorter term raises the monthly payment but cuts total interest sharply, because the balance falls faster and shorter terms often carry lower rates.
3. Pay extra toward principal
Extra payments reduce the balance immediately, so next month's interest is calculated on a smaller number. The effect compounds over the remaining term. Confirm with your lender that extra amounts are applied to principal rather than held as a prepaid future instalment.
4. Refinance to a lower rate
If your credit has improved since you bought, a lower rate speeds principal repayment — provided you do not extend the term at the same time.
Most auto lenders do not charge one, but some do — Experian notes the fee is typically around 2% of the loan amount. It is a single line in your contract and worth finding before you make a large extra payment.
Test different extra monthly amounts and see exactly how much interest each one saves and how many months it removes.
Test extra paymentsOne caution worth carrying over from the wider picture: paying more interest than expected is a budgeting problem, but owing more than the car is worth is a bigger one. For how the interest schedule interacts with depreciation, see rolling negative equity into a car loan.
Methodology and sources
Every figure in this guide was computed from this site's auto loan amortization engine — the same code behind the linked calculators — and independently verified in Python before publication. The example loan uses 7.14%, the Federal Reserve's average 60-month new-car loan rate at commercial banks for the second quarter of 2026 (G.19).
Market averages are not quotes. Your actual rate depends on your credit profile, the lender, the term, your down payment and the vehicle itself.
- 1Consumer Credit — G.19 (commercial bank 60-month new-car loan rate, Q2 2026) — Federal Reserve, September 8, 2026 release
- 2Average Car Loan Interest Rates by Credit Score (prepayment penalties) — Experian, September 2026
- 3Auto Loans — consumer tools and guidance — Consumer Financial Protection Bureau
- 4What is a prepayment penalty? — Consumer Financial Protection Bureau
Frequently asked questions
Written by
Founder and developer of We Are Calculator. No lead selling, no lender rankings, no affiliate-pulled recommendations. Every guide pairs primary sources (IRS, CFPB, Federal Reserve, CRA) with the free calculators you can run yourself.
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