Even small differences in the interest rate will add up over time.

Here we show a 15-year, $100,000 mortgage at 6%, 6.5%, and 7%.

With each half-point change in the interest rate, the monthly payment rises by a few dollars.

The total change in the monthly payment is $55, from $844 to $899.

This may not seem like much, but note how the total interest rises by a few thousand dollars instead of a few dollars.

The increase in the total interest rises by $9,895 as the rate changes from 6% to 7%. That's almost $10,000 out of your pocket and into the bank's.