How to Calculate Mortgage Refinance Savings: Six Steps

What refinance savings really means

Refinance savings is not one number. It is three: how much less you pay each month, how long that monthly saving takes to repay the closing costs, and how much less you pay in total over both loans. This guide works through all three on one example. The example is a $300,000 balance, refinanced from 7.5% with 25 years left to 6.0% over a new 30-year term, with $6,000 of closing costs. Every figure is checked against the mortgage refinance calculator.

Step 1: Work out your current monthly payment

Start with the payment you already have. The monthly payment on a fixed-rate loan comes from the payment formula: pmt = B·r/(1−(1+r)⁻ⁿ), where B is the balance, r is the monthly rate, and n is the number of monthly payments left. The monthly rate is the APR divided by 12, so at 7.5% it is 7.5% ÷ 12 = 0.625% a month, and with 25 years left n is 300 payments. Put the $300,000 balance, that monthly rate, and those 300 payments into the formula and the current payment is $2,216.97/mo. If you already know your payment from your statement, use that instead — but check it is principal and interest only, with taxes and insurance stripped out.

Step 2: Work out the new monthly payment

Now run the same formula on the new loan. The balance is the same $300,000, because a straight refinance does not change what you owe, but the rate and the term both change: the new APR is 6.0%, so the new monthly rate is 6.0% ÷ 12 = 0.5% a month, and the new term is 30 years, so n is 360 payments. Put those into pmt = B·r/(1−(1+r)⁻ⁿ) and the new payment is $1,798.65/mo. The lower rate pulls the payment down, and the longer term pulls it down further, because the same balance is spread over more payments — which is why Step 6 matters.

Step 3: Subtract to find the monthly saving

The monthly saving is the simplest step: current payment minus new payment. Here that is $2,216.97/mo minus $1,798.65/mo, which gives a monthly saving of $418.32/mo. That is the figure lenders quote first, and it is the figure most borrowers stop at, which is a mistake. It says nothing about the $6,000 of closing costs, the extra years you will pay, or the total interest. Treat $418.32/mo as the starting point, not as the answer.

Step 4: Divide the closing costs by the monthly saving — the break-even

Closing costs are the price of the refinance, and the break-even is how long the monthly saving takes to repay that price. Divide the closing costs by the monthly saving: $6,000 divided by $418.32/mo gives a break-even of 14.34 months, which lands in month 15. Until that month you are still behind. After it, every further monthly saving is real money kept. The practical test is strict: if you expect to move, sell, or refinance again before month 15, this refinance loses money for you, whatever the monthly saving says. If you expect to stay well past it, the saving has time to work.

Step 5: Compare the total cost over both loans

The total cost is where the refinance is finally judged, because it counts every payment on each loan, plus the closing costs on the new one. The current loan costs $665,092.06 in total, and the new loan costs $653,514.57 in total, including the $6,000 of closing costs. Subtract the new total from the current total and the total saving is $11,577.49. In interest alone, the current loan charges $365,092.06, while the new loan charges $347,514.57. Two thresholds sharpen the picture. If the closing costs rose to $17,577.49 (exact $17577.4929), the total saving would fall to zero. And the rate for a $500 monthly saving is 5.57% (exact 5.5722%); at 5.56% the saving would be $502.30/mo. Totals are computed from unrounded values, so rounded parts may differ by a cent from their rounded sum.

Step 6: Check the term trap

The term trap is the refinance mistake that hides inside a good-looking monthly saving: stretching the loan over more years can make the payment fall while the total cost rises, because you pay interest for longer. Our example stretches 25 years left into a fresh 30 years and still saves $11,577.49 in total, so the trap does not bite here — but change the terms and it does. On a 15-year new term instead, the new payment is $2,531.57/mo and the monthly saving is -$314.60/mo — the payment goes up by $314.60 a month — yet the total saving is $203,409.37, far larger, because the loan ends sooner. On the opposite edge, a borrower with only 20 years left who refinances into 30 years saves a tempting $618.13/mo a month, yet the total saving is -$73,487.47, because ten extra years of interest swamp the monthly gain. Judge every refinance on Step 5, the total.

What the saving leaves out

The saving in this guide is a principal-and-interest saving only, and several real costs sit outside it. Taxes and insurance are not in either payment, so they do not change the saving, but they do change what you actually pay each month. PMI is not counted: dropping it is an extra saving, adding it an extra cost. Fees beyond closing costs, such as any prepayment penalty, are not counted. And the comparison assumes you stay put for the full new term: sell early, and the total saving shrinks. For a closing-cost-only view, run the numbers through the refinance break-even calculator.

What to do next

Run your own balance, rates, remaining term, new term, and closing costs through the mortgage refinance calculator and read the three answers in order: monthly saving, break-even month, and total saving. Then apply the two tests: will you stay past the break-even month, and is the total saving positive after the closing costs? If either answer is no, change one input at a time — a lower rate, lower closing costs, or a shorter new term.