What a mortgage rate actually does to a monthly payment

By Austin Zehnder, San Francisco finance and real estate professional, founder of Zehndra.

Most buyers I talk to shop the price of a house really hard, and then treat the rate like a footnote. I get it. Price is the number on the listing. But the rate, quietly, shops you right back.

Here is the math, in plain English. On a 30 year fixed loan, your principal and interest payment comes from the loan amount, the monthly rate, and 360 payments. You do not need to memorize the formula. You just need to feel what it does.

Take an $800,000 loan, principal and interest only. At 3 percent, that is about $3,373 a month. At 6 percent, about $4,797. At 7 percent, about $5,322. Same house. Same loan amount. A four point rate move added roughly $1,950 a month, which is about $23,400 a year. That is not a rounding error. That is a different life.

Flip it around, because this is the hopeful part. On that same $800,000 loan, dropping from 7 percent to 6 percent saves about $525 a month, or $6,300 a year, without renegotiating the price at all. A small rate win is a big money win.

Three rules I actually use

Compare principal and interest first. Taxes, insurance, and HOA dues matter, but they do not move with the rate. Isolate the rate effect before you add the rest.
Turn the rate into dollars per month. "Rates are up 1 percent" sounds abstract. "$525 more a month" is a decision you can feel.
Do not confuse a lower rate with a good deal. Price, loan amount, how long you will hold, and whether you will refinance all matter too. The rate is one input, just a very large one.

Want a real world version? On my @housingcharted page, we ran a payment duel on a $1 million home with 20 percent down. Principal and interest was about $3,224 a month at 2.65 percent, versus about $5,539 a month at 7.40 percent. That is a $2,315 monthly gap, on the same house.

If you want to run your own numbers, Zehndra by Austin Zehnder has the Rental Deal Analyzer, built for exactly this kind of payment math. Also read: How to underwrite a rental deal in 15 minutes and Rent vs. buy: the numbers that matter.