Auto Loan Comparison Calculator: Compare Payments

An auto loan comparison calculator estimates the monthly payment and total interest for a fixed-rate car loan from its amount, APR, and term. Enter the figures for a loan offer to see the estimated cost; change the inputs to compare a different offer on the same basis.

Formula

The calculator uses the standard fixed-payment loan formula. Let P be the amount financed in dollars, a the annual percentage rate (APR) entered as a percentage, r = a / 1200 the monthly rate as a decimal, and n = 12 × years the number of monthly payments. The unrounded payment is P × r / [1 − (1 + r)⁻ⁿ]. The calculator displays the payment to the nearest cent. Estimated total interest is the displayed monthly payment multiplied by the number of payments, minus the loan amount.

For the calculator’s default inputs, the payment formula uses a $20,000 principal, an 8% APR, and 60 monthly payments. Although the input is labelled APR, the calculation treats the monthly rate as APR divided by 12; it does not separately model lender fees or other charges that may affect a disclosed APR.

Worked Example

1. Loan amount: P = $20,000; APR: a = 8%; term: 5 years.
2. Convert APR to a monthly decimal rate: r = 8 / 1200 = 0.0066667. Convert the term to payments: n = 5 × 12 = 60.
3. Substitute into the payment formula: $20,000 × 0.0066667 / [1 − (1.0066667)⁻⁶⁰].
4. The result rounded to cents is a monthly payment of $405.53.
5. Total interest uses that displayed payment: $405.53 × 60 − $20,000 = $4,331.80.

Actual payment schedules can differ if the lender calculates interest differently, rounds each period, or includes amounts beyond principal and interest. Use the worked example as an illustration of this calculator’s stated assumptions, not as a loan quote.

Assumptions

• The interest rate stays fixed for the full term.
• Payments are equal monthly payments, made at the end of each monthly period.
• Interest is compounded monthly using APR divided by 12.
• The calculation excludes origination or other fees, taxes, insurance, optional products, and any amount not included in the loan amount. It does not model a down payment, trade-in, balloon payment, or changing balance from extra payments.

How to Compare Auto Loan Offers

To compare offers, use the amount actually financed, the APR provided for that offer, and its repayment term. Keep the loan amount and term fixed when isolating the effect of different APRs; then compare terms separately if they differ. The lender’s written figures are the source for the actual rate, amount financed, payment schedule, and charges. Rates, fees, and eligibility vary, so enter the specific figures you have rather than relying on a general estimate.

• Amount financed: This is the principal used in the calculation, not necessarily the vehicle’s advertised price. A down payment, trade-in credit, taxes, fees, or optional products can change the amount financed.
• APR: Enter the annual percentage rate as a percent, such as 8 for 8%, not 0.08. This calculator does not add fees separately.
• Term: A longer term means more monthly payments. Even if the payment is lower, total interest can be higher because the balance is repaid over more months.
• Payment and total interest: These are estimates for principal and interest under the formula above. They are not a complete measure of all vehicle ownership costs or the total amount due at signing.

For example, if one offer has a lower monthly payment because it runs for more years, that payment alone does not show which offer has lower total interest. Compare the same loan amount and note both the number of payments and the total interest. A separate offer with different fees or a different amount financed also cannot be compared fairly by looking only at its payment.

Common Comparison Mistakes and Limits

A common misconception is that a lower payment automatically means a lower-cost loan. The payment is spread across the term, so a longer term can reduce each monthly amount while increasing the cumulative interest. Conversely, the calculator’s total interest is not the same as the total cost of buying and operating a vehicle: it leaves out charges and costs that are not entered.

• Do not enter the vehicle price in place of the amount financed if the final loan principal differs.
• Do not enter APR as a decimal or enter a monthly rate in the APR field.
• Check that the term is in years, not months. The calculator converts years to months by multiplying by 12.
• Expect small differences from a lender’s payment quote when the lender uses different rounding, dates, fees, or payment timing.

The calculation assumes a fixed rate and equal periodic payments. It cannot represent a variable rate, a skipped or late payment, a balloon payment, negative equity rolled into a later transaction, or extra payments that alter the balance. To evaluate a real offer, use the lender’s disclosed amount, APR, payment amount, number of payments, and itemized charges as applicable. Do not treat this estimate as a promise of approval or as a statement of the amount a lender will collect.

Frequently Asked Questions

How do I compare two auto loans?
Enter each offer’s amount financed, APR, and term. Compare monthly payment and total interest, and separately check lender disclosures for charges and terms this calculator does not include.

Does a longer auto loan term lower the monthly payment?
For the same principal and rate, spreading repayment across more months generally lowers each calculated payment, but it can increase total interest. The result depends on the inputs and the fixed-payment assumptions.

Is total interest the same as the total amount I pay?
No. The calculator reports estimated interest separately. The sum of its principal and interest payments is the amount financed plus the estimated interest; other fees, taxes, insurance, and vehicle costs are excluded.

Should I enter the car price or the amount financed?
Enter the loan principal—the amount being financed—not automatically the sticker price. Check the lender’s documents because a down payment, trade-in, or amounts added to the loan can make these figures different.

Why does the calculator differ from my lender’s quote?
Possible reasons include a different amount financed or APR, fees, payment dates, rounding conventions, or a payment structure other than equal monthly principal-and-interest payments. Compare the inputs and ask the lender how its disclosed payment is calculated.

This result is an estimate, and the lender’s disclosure gives the exact figures.

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in p | Loan amount | $ | 20000 | 1 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in yrs | Loan term | years | 5 | 0.0833 | 50
out pmt | Monthly payment | $ | p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2