15 Year Loan Payment Calculator: Monthly Cost and Interest

This 15-year loan payment calculator estimates a fixed monthly payment and the total interest for a loan repaid over 15 years. Enter the loan principal and annual APR to see the payment and interest estimate; compare the figures with the lender’s quoted terms.

Formula

The calculator uses the standard fixed-payment amortization formula. It converts the annual APR to a monthly rate, then calculates the level payment needed to repay principal and interest over the selected number of monthly payments.

Monthly payment: M = P × r / [1 − (1 + r)⁻ⁿ]. Here, P is the loan amount in dollars, r is the monthly rate as a decimal (APR ÷ 1200), and n is the number of monthly payments (years × 12). The APR is entered as a percentage per year. The payment is displayed to the nearest cent.

Total interest: I = M × n − P. I is the estimated interest paid in dollars across all scheduled payments, using the displayed monthly payment M. The calculator does not add fees or other charges to principal or interest.

Worked Example

Using the defaults, the principal is $20,000, APR is 8%, and term is 15 years. First convert the APR to a monthly decimal rate: r = 8 ÷ 1200 = 0.0066666667. The number of payments is n = 15 × 12 = 180.

Substitute those values into the payment formula: M = 20,000 × 0.0066666667 ÷ [1 − (1.0066666667)⁻¹⁸⁰]. This gives an unrounded payment of about $191.13, or $191.13 per month when rounded to cents.

Then calculate total interest from the rounded payment: I = $191.13 × 180 − $20,000 = $34,403.40 − $20,000 = $14,403.40. These are estimates under the assumptions below; the payment can differ if a lender uses different terms or includes additional amounts.

Assumptions

• The interest rate stays fixed for the full term.
• There are 12 equal monthly payments per year, with payments made at the end of each month.
• Interest accrues monthly using APR divided by 12; this is the calculator’s standard monthly-rate convention.
• Fees, taxes, insurance, private mortgage insurance, escrow, and any additional payments are not included.

How to Use the Calculator

Enter the amount borrowed in dollars, not the home or vehicle’s purchase price unless that entire price is being financed. Enter the APR as a percentage: for example, enter 8 for 8%, not 0.08. The term field is in years, so 15 represents 180 monthly payments.

The first result is the estimated scheduled monthly principal-and-interest payment. The second is total interest over the full term, calculated by multiplying the cent-rounded payment by the number of months and subtracting the original principal. It does not mean that each month’s payment is divided evenly between principal and interest: in an amortizing loan, interest is generally a larger share earlier in the schedule, while the principal share grows over time.

Use the APR and amount that apply to the specific loan offer when available. The example values are only inputs to demonstrate the arithmetic, not a statement about rates available from lenders. If a quote includes an upfront fee, financed costs, a variable rate, a final balloon payment, or a different payment schedule, this simplified calculation will not reproduce its complete payment terms.

When a 15-Year Payment Estimate Helps

A fixed 15-year term provides a consistent timeline for comparing payment arithmetic across different principal amounts or APR assumptions. Changing the amount changes the balance being repaid; changing the APR changes the monthly interest rate; changing the term changes the number of payments. To compare scenarios clearly, adjust one input at a time and keep the other two unchanged.

The total-interest figure makes the term’s full-schedule arithmetic visible, but it should be read alongside the monthly payment and any costs excluded from the model. The calculator is not a household budget assessment and cannot determine whether a payment is manageable or whether a lender will approve an application. For an actual offer, use the lender’s itemized payment and cost disclosures as the source for its terms.

Common Input and Reading Mistakes

A frequent error is entering APR as a decimal rather than a percentage. With this calculator, an APR of 8% is entered as 8; entering 0.08 means 0.08% and produces a very different estimate. Another mistake is entering 15 in the term field while assuming it means months: the field is years, so 15 means 180 months.

Rounding also matters. The calculator rounds the monthly payment to cents and uses that displayed payment to calculate total interest, as required by the specified method. A lender’s actual final payment can be adjusted to clear a small remaining balance caused by payment rounding, and its schedule may use exact dates or a different accrual convention.

Limits of the Calculation

This formula assumes one fixed APR, equal payments, and a fully amortizing balance. It does not model variable-rate changes, interest-only periods, missed or late payments, extra principal payments, fees, or a lender’s day-count rules. For a loan with any of those features, request a payment schedule based on the contract’s actual terms rather than treating this estimate as an exact quote.

Frequently Asked Questions

What does a 15-year loan payment include?
The result includes estimated principal and interest only. It excludes costs such as fees, taxes, insurance, and escrow unless those costs are separately reflected in the amount borrowed.

How many payments are in a 15-year term?
With one monthly payment, there are 15 × 12 = 180 scheduled payments.

Why can my lender’s payment differ?
The lender may use a different principal, APR, payment date convention, financed fees, or loan structure. Compare the calculator inputs with the lender’s written payment details.

Does the calculator show the payoff balance after a particular month?
No. It reports the scheduled monthly payment and total interest for the full term, not a balance at an interim date.

Does the APR include every borrowing cost?
This calculator uses APR only to estimate monthly interest. It does not separately model fees or other costs, so check the lender’s disclosure for the figures and definitions that apply to the offer.

Final Thoughts

Use the estimate to understand the payment and interest arithmetic for a fixed 15-year schedule, then verify the inputs and excluded costs against the actual loan documents. 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 | 15 | 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