To calculate mortgage refinance savings, compare your current monthly payment with the payment on the new rate over the same remaining term. For example, refinancing a $300,000 balance with 25 years remaining from 7% to 5.5% cuts the monthly payment from $2,120.34 to $1,842.26 — a saving of $278.08 a month and $83,422.53 of interest over the full remaining term, before closing costs. The free Refinance Calculator runs the same comparison with your own balance, rates and term. General information only — not financial advice, and not tax advice.
What Refinancing Actually Changes
Refinancing replaces your existing mortgage with a new one, usually at a different rate, term or both. The saving comes from paying less interest on the same balance — but only if the new deal is genuinely cheaper once costs are counted. Closing costs, fees and any prepayment charges on the old loan all eat into the saving, and extending the term can lower the monthly payment while increasing total interest, even at a lower rate. That is why a refinance decision needs three numbers, not one: the monthly saving, the total interest saving, and the time needed for the monthly saving to repay the costs of refinancing.
The cleanest comparison holds the term constant: same balance, same remaining years, only the rate changes. That isolates the rate effect, which is the effect most borrowers are actually shopping for. Once you understand the rate effect, you can separately judge whether changing the term helps or hurts — a shorter term usually raises the payment but cuts total interest, while a longer term does the reverse.
Using the Refinance Calculator, Step by Step
The Refinance Calculator compares two loans side by side:
- Enter your current balance and rate. Use the remaining balance, not the original loan amount, and the rate you actually pay now.
- Enter the remaining term in years. Comparing over the same remaining term keeps the comparison honest. A new 30-year term on a loan with 25 years left is a different comparison — test it separately.
- Enter the new rate. Use the rate you have actually been offered, not an advertised headline rate you may not qualify for.
- Read the monthly saving and the total interest saving. The monthly figure tells you the cash-flow effect; the total figure tells you the lifetime effect.
- Set the saving against closing costs. Divide the total closing costs by the monthly saving to get the break-even time in months. If you may move or refinance again before break-even, the deal may not pay.
The Formula Behind the Saving
Each monthly payment comes from the amortisation formula: PMT = P × r ÷ (1 − (1 + r)−n), where P is the balance, r is the monthly rate and n is the number of months. For $300,000 at 7% over 300 months, r is 0.07 ÷ 12 = 0.005833 and the payment is $2,120.34. At 5.5% over the same 300 months, r is 0.055 ÷ 12 = 0.004583 and the payment is $1,842.26. The monthly saving is $2,120.34 − $1,842.26 = $278.08.
Total interest over the remaining term is total payments minus the balance. At 7%, total payments are $2,120.34 × 300 = $636,101.28, so interest is $336,101.28. At 5.5%, total payments are $1,842.26 × 300 = $552,678.74, so interest is $252,678.74. The total interest saving is $336,101.28 − $252,678.74 = $83,422.53. That figure assumes you stay in the loan for the full 25 years and excludes closing costs — both assumptions matter, and the calculator's break-even step addresses the second one.
Work out break-even before you sign: if closing costs are $5,000 and the monthly saving is $278.08, break-even is $5,000 ÷ $278.08 = about 18 months. A refinance that pays for itself in under two years is a very different proposition from one that takes eight years — especially if a move is possible.
Worked Example: $300,000 With 25 Years Remaining
The table below holds the balance and term fixed and varies only the new rate. All figures assume the same 25-year remaining term, monthly compounding, no fees and no prepayment charges — an illustration of the rate effect, not a lending offer.
| New rate | New monthly payment | Monthly saving vs 7% | Total interest saving over 25 years |
|---|---|---|---|
| Current rate: 7% | $2,120.34 | — | — |
| New rate: 6% | $1,932.90 | $187.43 | $56,230.02 |
| New rate: 5.5% | $1,842.26 | $278.08 | $83,422.53 |
Two patterns stand out. First, even a single percentage point — from 7% to 6% — saves $187.43 a month and $56,230.02 over the term, which is why small rate differences matter enormously on large balances. Second, the saving is not proportional to the rate cut: the second half-point, from 6% to 5.5%, adds $90.65 a month, less than the first point's $187.43. Diminishing returns apply here too — but on a $300,000 balance, even diminished returns are large in absolute terms.
Monthly payment on a $300,000 loan over 25 years, by interest rate — hover or tap each bar for details.
The chart shows the monthly effect at a glance: the payment falls with the rate, but not by the full rate difference, because the payment also repays principal. That is why the calculator matters — guessing the saving from the rate gap alone usually overstates it. The exact payment formula, applied to your own balance and term, is the only reliable guide.
Common Mistakes With Refinancing
1. Ignoring closing costs. A $278.08 monthly saving looks different if closing costs are $9,000 and you may move in two years. Always compute break-even in months before deciding.
2. Extending the term without noticing. Restarting a 30-year term on a loan with 25 years left lowers the payment but adds five years of interest. Compare on the same term first, then judge any term change separately.
3. Comparing an advertised rate you cannot get. Headline rates often assume top credit and a low loan-to-value ratio. Compare using the rate actually offered to you, with fees included where possible.
4. Forgetting prepayment charges. Some existing loans charge for early repayment. A prepayment charge is part of the cost of refinancing and belongs in the break-even calculation.
5. Refinancing repeatedly for small gains. Each refinance carries costs. Chasing every small rate dip can leave you paying fees faster than you save interest. A clear break-even rule — for example, only refinancing when break-even is under two years — prevents fee churn.
Key takeaway: refinancing $300,000 with 25 years left from 7% to 5.5% saves $278.08 a month and $83,422.53 of interest over the term, before closing costs. Compare on the same term, then check break-even before you sign.
Frequently Asked Questions
How do I calculate mortgage refinance savings?
Compute the monthly payment at your current rate and at the new rate over the same remaining term, using PMT = P × r ÷ (1 − (1 + r)−n). For $300,000 over 25 years, 7% gives $2,120.34 and 5.5% gives $1,842.26 — a saving of $278.08 a month. The Refinance Calculator does the same comparison with your figures.
How much can I save by refinancing from 7% to 5.5%?
On a $300,000 balance with 25 years remaining, the monthly payment falls from $2,120.34 to $1,842.26, saving $278.08 a month and $83,422.53 of interest over the full term, before closing costs. Your saving depends on your balance, rates and remaining term.
What is the break-even point for refinancing?
Divide total closing costs by the monthly saving. With $5,000 of costs and a $278.08 monthly saving, break-even is about 18 months. If you sell or refinance again before break-even, the refinance may cost more than it saves.
Does extending the loan term when refinancing save money?
It lowers the monthly payment but usually increases total interest, because you pay interest for more years. Compare first on the same remaining term, then judge any term extension as a separate decision with its own total-interest cost.
Should I refinance for a 1% rate reduction?
Often yes on a large balance, but check break-even. On $300,000 over 25 years, a cut from 7% to 6% saves $187.43 a month and $56,230.02 of interest before costs — a strong saving if closing costs are recovered well within your expected time in the loan.
Do closing costs change the saving?
Yes. The headline interest saving excludes closing costs, fees and any prepayment charges on the old loan. Subtract those costs, or use the break-even calculation, to get the net benefit of refinancing.
Does the calculator guarantee a new rate?
No. The calculator compares rates you enter; it does not offer a rate or approve a loan. The rate you actually qualify for depends on credit, loan-to-value, property and lender criteria. This article is general information, not financial advice.
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