15-Year Refinance Calculator: Estimate Your Payment

A 15-year refinance calculator estimates the monthly principal-and-interest payment and total interest on a replacement loan when you enter the new loan amount and APR. Enter the balance you plan to refinance, the rate quoted by a lender, and a 15-year term; the results are estimates, not a rate offer or a full closing-cost comparison.

Formula

The calculator uses the standard fixed-rate, level-payment loan formula. First it finds the monthly payment, then multiplies that rounded payment by the number of monthly payments and subtracts the amount borrowed to estimate total interest.

• p = new loan principal in dollars. This is the amount being financed, not necessarily the home’s current value.
• apr = annual percentage rate entered as a percentage, such as 6.5 for 6.5% per year.
• r = monthly interest rate, calculated as apr ÷ 1,200.
• n = total payments, calculated as years × 12.
• pmt = monthly principal-and-interest payment in dollars, rounded to cents.
• int = estimated total interest over the full term in dollars.

In symbols, pmt = round[p × r ÷ (1 − (1 + r)⁻ⁿ), 2], and total interest = pmt × n − p. The input is an APR expressed as a yearly percentage, so the formula converts it to a monthly rate by dividing by 1,200. The first result is the estimated payment; the second is interest over all scheduled payments.

Worked Example

Using the calculator’s default inputs—$200,000 borrowed, 6.5% APR, and 15 years—the monthly rate is 6.5 ÷ 1,200 = 0.0054167, and the payment count is 15 × 12 = 180.

1. Insert the values into the payment formula: $200,000 × 0.0054167 ÷ [1 − (1 + 0.0054167)⁻¹⁸⁰].
2. The monthly principal-and-interest payment, rounded to cents, is $1,742.21.
3. Multiply the rounded payment by the number of payments: $1,742.21 × 180 = $313,597.80.
4. Subtract the principal: $313,597.80 − $200,000 = $113,597.80 estimated total interest.

Assumptions

• The interest rate is fixed for the entire loan term.
• Payments are monthly and made at the end of each monthly period.
• Interest is modelled monthly at APR divided by 12.
• The estimate excludes lender fees, appraisal or other transaction costs, taxes, homeowners insurance, mortgage insurance, escrow, and any existing-loan payoff charges. It assumes no extra payments or missed payments.

What a 15-Year Refinance Means

Refinancing replaces an existing mortgage with a new loan. A 15-year term schedules repayment over 180 monthly payments. Compared with a longer term for the same principal and APR, the shorter term generally produces a larger required principal-and-interest payment because the balance is repaid more quickly. It also has fewer months in which interest can accrue. The actual comparison depends on the new principal, rate, fees, and the terms being replaced; this calculator does not decide whether refinancing is appropriate.

The amount refinanced may differ from the outstanding principal on a mortgage statement. A payoff quote can include interest accrued since the last payment and other amounts due. If eligible costs are added to the new loan, the principal used in the estimate may be higher. Get the proposed loan amount and APR from the lender’s written quote rather than assuming either figure.

How to Use and Interpret the Estimate

Enter the amount that would be financed by the new loan, its offered APR, and the term in years. For a 15-year estimate, use 15. The calculator’s default values are only an example, not a statement of available rates or costs. Replace them with the figures supplied for the loan you are evaluating.

The payment shown covers principal and interest only. A real monthly housing payment may also include taxes, insurance, mortgage insurance, or escrow collections, which can change over time. The total-interest result assumes every scheduled payment is made and does not add upfront costs. It is not the same as the total cost of refinancing, nor does it show the remaining balance at an earlier date.

Because the payment is rounded to cents before the total is calculated, multiplying the displayed payment by 180 gives the displayed scheduled-payment total. Actual loan servicing can make small adjustments, including a final payment that differs slightly, depending on lender rounding and the loan documents.

Costs and Comparisons to Check

A monthly-payment comparison alone can hide important differences. A refinance may involve charges paid upfront or financed into the new balance, and the existing loan may have a different rate, remaining term, or payment structure. For a useful side-by-side estimate, gather the current payoff amount, remaining payments, new loan amount, quoted APR, term, and an itemized estimate of transaction costs. Keep financed costs separate from cash-paid costs so they are not counted twice.

• Compare principal-and-interest payments on the same basis; do not treat escrow or insurance as loan principal.
• Account for how many months remain on the current loan, rather than comparing only its original term.
• Include any costs that would be paid or added to the balance when assessing overall cash outlay.
• Consider how long the replacement loan is expected to remain outstanding; a full-term interest estimate assumes all 180 payments occur.

A common misconception is that a lower monthly payment automatically means lower borrowing cost. A payment can fall because the term is extended, even if interest is paid over more months. Conversely, a 15-year schedule can raise the monthly payment while reducing the number of payments. Compare the full set of cash flows and loan disclosures, not a payment figure in isolation.

Limits and Common Input Mistakes

This is fixed-rate arithmetic, not an underwriting tool or a personalized quote. It cannot account for adjustable rates, lender-specific payment conventions, closing-cost details, escrow changes, or the exact payoff calculation on an existing mortgage. The calculator also cannot establish whether a borrower qualifies. Obtain actual loan terms and costs directly from the lender and check the final disclosure for the figures that apply.

• Enter 6.5 for 6.5%, not 0.065; the calculator expects a percentage.
• Use the new loan principal, not the home’s market value or the original mortgage amount.
• Enter years for the term. Entering 180 would mean 180 years, not 180 monthly payments.
• Do not assume the calculator’s APR includes every cost relevant to a particular quote; check how the lender presents its figures.

Frequently Asked Questions

What is the monthly payment on a 15-year refinance?
The payment depends on the new principal and APR. For the example here—$200,000 at 6.5% APR for 15 years—the estimated principal-and-interest payment is $1,742.21 per month. Taxes, insurance, and other housing costs are not included.

Does a 15-year refinance always have less total interest?
No single comparison is guaranteed by the term alone. A shorter schedule has fewer payments, but the result also depends on the new principal, APR, fees, and the terms of the existing loan. This calculator models only interest on the new loan under its assumptions.

Does this payment include closing costs or property taxes?
No. The estimate includes principal and interest only. Add costs to the loan amount only if they are actually financed, and review a lender’s itemized quote for transaction costs and any taxes, insurance, or escrow amounts.

Can I use the calculator for a 30-year refinance?
Yes. Change the term input to 30 years. The formula works for a fixed-rate term from approximately one month up to 50 years, but the result remains an estimate and excludes the items listed above.

Is APR the same as the interest rate?
Not necessarily. APR is a yearly measure presented by a lender and may reflect certain loan costs differently from the stated interest rate. Use the figure and definition specified in the lender’s quote, and do not assume all fees are captured by this simple payment formula.

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

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in p | New loan amount | $ | 200000 | 1 | 100000000
in apr | Interest rate (APR) | % | 6.5 | 0.01 | 100
in yrs | Loan term | years | 15 | 0.0833 | 50
out pmt | Monthly principal-and-interest payment | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2