The world is watching and waiting to see whether the U.S. Federal Reserve will raise interest rates this month, or at least by the end of 2015.

A rise in the “federal funds rate” will translate pretty quickly into higher mortgage interest rates, affecting any prospective homebuyer who can’t simply pony up a few hundred thousand dollars in cash. (The fed isn’t the only driver of mortgage interest rates, but it’s a factor in the mix.)

Given that nearly a decade has passed since the fed raised this rate, most experts don’t believe it will do anything more than inch it up. This isn’t a time for shock strategies.

The prospect of a rise in the rate nevertheless raises a question for prospective homebuyers. How much would a small rise in mortgage interest rates affect the amount you ultimately pay for your mortgage (assuming you choose a fixed-rate loan)?

The answer can be found by having some fun with Nolo’s “How much will my fixed rate mortgage payment be?” calculator.

Let’s assume you’re buying a $500,000 home, putting 20% down ($100,000, ouch) and will therefore be taking out a $400,000 mortgage. The reported 30-year rate on Bankrate.com today is 3.96%.

According to Nolo’s calculator, your monthly payment on this loan would be $1,900, and the total interest you’d pay over the life of the loan would be $284,161.

Now let’s assume interest rates go up a notch, to 4%. That would take your monthly payments to $1,910 and your total interest paid by the end of the loan term to $287,478 — $3,317 more than you would have owed in our first example. Not awful, over 30 years.

Let’s keep going, and take those interest rates up to 4.5%. (Could happen!) That would boost your monthly payments to $2,027 and your total interest paid by the end of the loan term to $329,627. Things are starting to look more serious.

What about 5% mortgage rates? They’re still within the realm of where experts predict rates might go in the coming months or year. A mortgage at 5% on your $400,000 loan would come with monthly payments of $2,147 and bring your total interest paid to $373,021.

Them’s real dollars. With a seemingly small rise interest rate on the day you close on your home, from just below 4% to 5%, you’d be looking at owing $88,860 more in total interest by the time you’d paid that loan off.

Of course, if housing prices were to go down as a result of the rate hike, that would offset part of the problem – but no one seems to think they will.

Now might be a good time to get serious about buying a home. A trusted mortgage broker can also help you think strategically about how to finance your purchase.