According to research by Zillow Mortgage Marketplace, any potential borrower with a credit score less than 620 is unlikely to receive a 30-year fixed-mortgage, even if they offer a relatively high down payment. Yet, according to, 29.3% of Americans have a credit score below that number. This means that nearly one-third of Americans would likely be turned down for the nation's most popular mortgage product. Zillow tracked over 25,000 loan quotes and purchase requests in the first half of September. Potential borrowers credit scores 720 or higher received the lowest interest rates on, an average annual percentage rate of 4.3% for a 30-year FRM. Midrange credit scores, between 620 and 719, received APRs from 4.44% to 4.73%. Those with credit scores below 620 received too few loan quotes to calculate the average APR, Zillow said. Zillow's chief economist Stan Humphries attributes the trend to a tightening of credit standards, which he sees as a good thing. "Four years ago, in the era of easy-to-get subprime loans, many borrowers with low scores did buy homes, which in turn helped contribute to a housing bubble," said Humphries. "Today's tighter credit is a predictable response by banks after the foreclosure crisis, but also keeps a cap on housing demand, which is important for the greater housing market recovery." Zillow entered into a partnership with today, an online personal finance service from Intuit Inc. (INTU) Now registered Mint users will receive a valuation quote, also known as a Zestimate, for their house as part of their online portfolio. Write to Christine Ricciardi.