Hong Life CalculatorsUnderstanding the car refinance calculator — concept and principle
This calculator compares your current car loan with a refinanced one. It works out the current and new monthly payments, the interest left on each, the net savings after fees and how many months it takes to recover the fees. Use it before applying to refinance an auto loan at a lower rate or a different term.
How it is calculated
- Monthly payment = B × i ÷ (1 − (1 + i)^−n), with i = APR ÷ 12 and n months.
- Current payment uses the payoff amount, current APR and months left (assuming the loan is on schedule).
- The new loan starts from the payoff amount, plus fees if you roll them in.
- Interest = payment × months − amount financed.
- Net savings = total still to pay on the current loan − (total on the new loan + fees paid up front).
- Months to recover fees = fees ÷ monthly payment drop.
The principle
Refinancing replaces the old loan with a new one. A lower rate reduces the interest charged on each month's balance, but the total interest also depends on how long you borrow: stretching the term lowers the monthly payment while adding months of interest. Comparing total cost, including fees, shows whether the deal really saves money. The break-even in months tells you how long the lower payment takes to repay the fees.
Worked example
Default inputs: payoff $18,000, current APR 8.9%, 42 months left; new APR 5.9% for 48 months; fees $300 paid up front.
- Current payment = 18,000 at 8.9% over 42 months ≈ $500.36; interest left ≈ $3,015.
- New payment = 18,000 at 5.9% over 48 months ≈ $421.91; interest ≈ $2,251.
- Monthly payment change ≈ −$78.45.
- Net savings = (500.36 × 42) − (421.91 × 48 + 300) ≈ $463.
- Months to recover the fees = 300 ÷ 78.45 ≈ 3.8.
| Current interest rate (APR %) | Net savings from refinancing |
|---|---|
| 2.23 | -$1,825 |
| 4.45 | -$1,080 |
| 6.67 | -$317 |
| 8.9 | $463 |
| 11.1 | $1,262 |
| 13.3 | $2,078 |
| 17.8 | $3,763 |
How to read the result
Positive net savings mean refinancing lowers your total cost even after fees. Here the saving is smaller than the interest difference because the new loan runs six months longer. Keeping the new term equal to or shorter than the months left usually saves the most. If savings are negative, the lower payment is coming from stretching the loan, not from saving money.
Common mistakes
- Looking only at the monthly payment.
- Forgetting fees or a prepayment penalty on the current loan.
- Extending the term well beyond the remaining months.
- Refinancing a car worth less than the loan, which lenders may refuse.
Frequently asked questions
When is refinancing a car loan worth it?
When the rate drop saves more than the fees over the remaining time, without stretching the term too much.
Does a longer term save money?
Usually not overall; it lowers the payment but adds interest.
What is the break-even on fees?
The number of months of lower payments needed to repay the refinancing fees.
Sources: Level-payment loan amortization (standard annuity formulas; OpenStax, Principles of Finance (2022), Ch. 8)