The size of a homebuyer’s down payment can make the difference between an affordable monthly mortgage payment and one that squeezes a household’s budget, Zillow, a real estate and rental marketplace, reported Friday.

A 20% down payment on a home’s purchase price is a longstanding benchmark, Zillow said. This keeps monthly payments lower, saves on interest and negates the need for costly mortgage insurance.

Forty-three percent of buyers — of whom 42% are millennials, the largest contingent — put down 20% or more, according to Zillow’s analysis of homebuyers nationally and in five major metropolitan areas: Atlanta, Chicago, Washington, D.C., Phoenix and San Francisco.

The survey revealed that just as U.S. housing markets vary greatly by region, so do down payment trends.

In Atlanta and Phoenix, slightly less than a third of homebuyers put down 20%. Zillow found that buyers in Phoenix were just as likely to put down 5% or less as they were to put down 20% or more.

Even more concerning were buyers in Atlanta, who put down less than 5% more often than they put down at least 20%, opening low payers to greater risk of becoming underwater on their mortgages, Zillow said.

In contrast, buyers in Chicago, San Francisco and Washington, D.C., were at least as likely as the typical national buyer to put down at least 20%.

Seven-Year Itch

Saving for a down payment is the chief hurdle to homeownership, Zillow research showed.

It takes a typical American homebuyer more than seven years to save a 20% down payment on the typical-valued home. Zillow defined today’s “typical American homebuyer” as a 41-year-old, college-educated couple with an annual median household income of $72,500 and aspirations for single-family living.

In some markets, the time it takes is much longer. In San Francisco, that is a daunting 18 years or more to save the $193,440 needed. Still, 51% of buyers there put down 20% or more.

How do they manage to pull together that amount?

According to the report, 70% of buyers nationwide reported that savings had made up at least some portion of their down payment. Thirty-nine percent said it was proceeds from a previous home sale, which typically accounted for about a fifth of the total down payment.

Fifty-one percent of first-time buyers who lacked an equity nest egg reported that their down payment included a gift and/or loan from family or friends.

For about half of millennial buyers, a gift or loan from family or friends accounted for 20% of the down payment on average. Other millennials reported that they had cashed out investments and or used retirement funds for their down payment.

“Saving up for a down payment can be tough and requires good budgeting and long-term planning, especially when for many of us the cost of rent and everyday life outpaces what we’re able to put in the bank,” Zillow senior economist Aaron Terrazas said in a statement. “Even if you don’t have plans to buy a home in the next year or two, it never hurts to start setting aside savings for a future home purchase.”

Terrazas noted that many mortgage options exist that require less than 20% down, but cautioned that buyers should be careful not to set themselves up to be underwater.

“Interest rates are rising, of course, but for many, waiting a bit longer and saving for a larger down payment might still be the way to go as they weigh their current stability and housing needs against their long-term futures,” he said.

— Check out Top 10 Cities for Millennial Homebuyers on ThinkAdvisor.