How Much Does Your Interest Rate Change Your Mortgage Payment?

On a $400,000 30-year mortgage, moving from 6% to 7% adds $263.01 to your monthly payment and $94,682.84 to the total interest over the life of the loan. One percentage point. That is why locking a rate even a quarter-point lower, or buying it down with points, is rarely a rounding error — it is a car, or a year of college.

This article shows the exact sensitivity so you can judge any rate quote, lock decision, or points offer against real dollars. All figures come from the calculator.

Payment and interest at every rate

A $400,000 loan over 30 years, principal and interest only:

Rate    Monthly P&I   Total interest
5.00%   $2,147.29     $373,023.14
5.50%   $2,271.16     $417,616.16
6.00%   $2,398.20     $463,352.76
6.50%   $2,528.27     $510,177.95
7.00%   $2,661.21     $558,035.59
7.50%   $2,796.86     $606,868.89
8.00%   $2,935.06     $656,620.99

Run a $400k loan at 6.5% and try other rates.

The rule of thumb: ~$25/month per 0.1%

The jump per full percentage point grows slightly as rates rise, because interest compounds on a bigger number:

Rate move        Added monthly payment   Added lifetime interest
5% → 6%          $250.92                 $90,329.62
6% → 7%          $263.01                 $94,682.84
7% → 8%          $273.85                 $98,585.39

So on a $400,000 loan, a useful shortcut is about $25–27 a month for every 0.1% of rate, and roughly $90,000–$99,000 of lifetime interest per full point. Scale it to your loan: a $200k loan moves about half as much, a $600k loan about 1.5x as much, because payment and interest both scale linearly with the amount borrowed.

Why this makes rate shopping worth real effort

A single extra lender quote that comes in 0.25% lower is worth about $66/month and roughly $23,000 over the life of a $400k loan. Most people spend more time choosing a TV. Three concrete moves:

• Get at least three quotes on the same day. Rates move daily, so quotes from different days are not comparable. A cluster of same-day quotes shows you the real market.
• Compare APR, not just the rate. APR folds in lender fees and points, so it is the better apples-to-apples number.
• Quote both with and without points. Buying points is prepaying interest for a lower rate; whether it pays off depends on how long you keep the loan.

Should you buy points?

A "point" costs 1% of the loan and typically buys roughly 0.25% off your rate. On the $400k loan, one point costs $4,000 and might drop your payment by about $66/month (the 0.25% sensitivity from above). That is a break-even of about 60 months — five years. If you will keep the loan and the rate longer than the break-even, points win; if you might move or refinance sooner, they do not. Test any specific points offer by entering both rates in the calculator and comparing the monthly payment and total interest directly.

Rate vs. term vs. extra payments

Rate is one of three levers on the total cost of a mortgage, and they interact:

• Rate sets the baseline — see the table above.
• Term compresses time: a 15-year loan more than halves lifetime interest. See 15- vs 30-Year Mortgage: The Real Math.
• Extra payments let you beat your own rate after closing, no refinance required. See How Much Do You Save by Paying Extra on Your Mortgage?.

If you already have a loan and rates have dropped, the question becomes whether refinancing is worth the closing costs — the break-even math is in Refinance Break-Even: When Refinancing Actually Makes Sense.

The bottom line

On a $400,000 30-year loan, every percentage point of rate is about $250–$275 a month and $90k–$99k in lifetime interest — roughly $25/month per tenth of a point. That sensitivity is why a few hours of rate shopping, or a well-timed lock, returns more per hour than almost anything else in the homebuying process.

Plug your loan amount and a few rate quotes into the calculator to see your own sensitivity, and read How Much Interest Will You Pay on a Mortgage? for the full lifetime-cost picture.