A 15-year loan payoff calculator estimates the fixed monthly payment and total interest for a loan repaid over 15 years. Enter the amount borrowed and the annual APR to see the payment and the interest cost across 180 monthly payments; this estimate does not include fees or other charges.
Formula
The calculator uses the standard fixed-payment, fully amortizing loan formula. It assumes the same payment is made once a month and that interest accrues at a monthly rate derived from the entered annual percentage rate.
- Monthly payment: M = round[P × r ÷ (1 − (1 + r)−n), 2]
- Total interest: I = (M × n) − P
- Monthly rate: r = APR ÷ 1,200
- Number of payments: n = term in years × 12
P is the starting loan principal in dollars, APR is the annual percentage rate entered as a percent, r is the monthly rate as a decimal, n is the number of monthly payments, M is the monthly payment in dollars rounded to cents, and I is the calculated total interest in dollars. The total-interest output uses the rounded monthly payment shown by the calculator.
Worked Example
Using the defaults—$20,000 borrowed, an 8% APR, and a 15-year term:
- Convert the APR to a monthly decimal rate: 8 ÷ 1,200 = 0.0066667.
- Find the number of payments: 15 × 12 = 180.
- Substitute into the payment formula: $20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)−180] = $191.13 per month, rounded to cents.
- Calculate interest using the displayed payment: ($191.13 × 180) − $20,000 = $14,403.40.
The calculator therefore shows a monthly payment of $191.13 and estimated total interest of $14,403.40.
Assumptions
- The APR and payment amount stay fixed throughout the term.
- Payments are made monthly at the end of each monthly period.
- Interest is modelled monthly at APR divided by 12, with the APR entered as a percentage.
- The loan is paid in full through the scheduled payments; no extra payments or missed payments are modelled.
- Fees, taxes, insurance, PMI, escrow, and other charges are not included unless reflected in the entered loan amount or rate.
How to Use a 15-Year Payoff Calculator
Enter the loan principal—not the amount of interest you expect to pay—and the APR supplied for the loan. Keep the term at 15 years to model 180 monthly payments. If you are checking a loan already in progress, use its remaining principal and the applicable rate, then enter the remaining repayment term in years; a term other than 15 years will change the result.
Use the loan’s actual rate and any lender-provided payment details when available. An advertised or estimated rate may not match the rate in a specific loan offer. If fees are financed, include them in the principal only when they are actually added to the balance. Otherwise, this calculator will not include them in its payment or interest estimate.
How to Read the Results
The monthly payment is the modelled principal-and-interest amount for each of 180 payments. Early in repayment, a larger share of each payment generally goes to interest; as the balance falls, more goes to principal. Total interest is the sum implied by the rounded monthly payment over the full term, minus the original principal. It is not the total amount repaid: add the principal to the interest to get the estimated total of scheduled payments.
For the example, scheduled payments total $191.13 × 180 = $34,403.40. Of that amount, $20,000 is the original principal and $14,403.40 is estimated interest. Actual payment schedules can differ because of payment dates, rounding practices, fees, rate changes, or other terms in the loan documents.
What Changes a 15-Year Loan Payoff?
The principal, APR, and time to repay all affect the result. A larger principal increases the payment when the rate and term are unchanged. A higher APR increases the payment and generally raises total interest. A shorter term requires larger monthly payments but leaves less time for interest to accrue; a longer term spreads repayment over more months and changes both the payment and total interest.
A 15-year term means a scheduled payoff over 180 payments under the fixed-rate assumptions—not necessarily that every real loan is contractually fixed for that entire period. Variable rates, payment pauses, extra principal payments, and late or skipped payments are outside this calculation. For those cases, use the current balance, applicable rate, and the relevant payment schedule, or request an updated payoff figure from the servicer.
Common Mistakes and Limits
- Entering a percentage incorrectly: Enter 8 for 8% APR, not 0.08. The formula performs the conversion to a monthly decimal rate.
- Using the wrong balance: A payoff estimate for an existing loan should start with the current principal balance, not the original amount borrowed.
- Confusing payment with total cost: The monthly result does not include every possible charge. The interest result is also not a payoff quote for a particular date.
- Ignoring payment rounding: The monthly payment is rounded to cents, and the interest estimate multiplies that displayed figure by 180. A lender’s final payment can differ slightly due to its rounding and schedule.
- Assuming extra payments are included: Extra principal payments can change the payoff date and interest. This calculator assumes only the scheduled payment.
The formula is for a fixed-rate loan with level monthly payments and a fully amortizing balance. It does not calculate changing-rate loans, daily interest, a precise payoff amount on a chosen date, or a lender’s fees and contractual adjustments.
Frequently Asked Questions
How many payments are in a 15-year loan?
A loan paid monthly over 15 years has 180 scheduled payments, assuming one payment each month and no changes to the schedule.
Does a 15-year payoff calculator include taxes and insurance?
No. This calculation estimates principal and interest only. Taxes, insurance, PMI, escrow, and fees are not included unless they are represented in the loan amount or rate you enter.
Can I use this calculator for a loan I already have?
Yes, as an estimate: enter the current principal balance, the applicable APR, and the remaining term. A lender or servicer can provide the exact balance and payoff amount for a specific date.
Why might my lender’s payment differ?
The loan may have fees, a different rate or term, a non-monthly interest convention, or payment dates and rounding that this simplified formula does not model. Compare the calculator inputs with the lender’s disclosure.
Does paying extra shorten the payoff time?
Extra payments applied to principal can reduce the balance faster, which may change the payoff time and interest. The calculator does not model extra payments, so its results remain based on the scheduled payment only.
This result is an estimate; the lender’s disclosure gives the exact figures.
Related
Formula used
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 | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2 out int | Total interest | $ | pmt * yrs * 12 - p | 2
Each line is one input (in, sel, list) or one result (out: label, unit, formula, decimals). How the calculators work
Method: the standard loan formula for this calculation, fixed rate and monthly compounding unless stated, rounded as each result states. Not financial advice; no lender, rate or product is recommended. See formulas and assumptions, the disclaimer or report an error.
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