Zillow says nearly a third of mortgaged homes in the Baltimore region are worth less than the loan amount -- far more underwater homeowners than other estimates suggest.

The most frequently quoted figures, from real estate data firm CoreLogic, put the underwater crowd in the region at just under 20 percent.

The big spread between the two companies' estimates are national, not just local. Zillow, a real estate site, says it worked with credit bureau TransUnion to get the exact loan balance for mortgaged homes -- including home equity lines -- so it didn't have to start with the original loan balance and estimate the amount paid off.

Both companies estimate home values, Zillow with its Zestimate.

Nationally, Zillow says 31 percent of mortgaged homes were underwater during the first three months of the year, same as its estimate for the Baltimore metro area. Within the region, negative equity varied dramatically, from more than half in a handful of Baltimore and Baltimore County ZIP codes to less than 10 percent in some of the pricey suburbs.

Zillow has an interactive map here.

These are the extremes:

MOST

21205 (area around the Johns Hopkins Hospital in East Baltimore), 53 percent

21202 (from the Inner Harbor in Baltimore to North Avenue), 52 percent

21230 (from Locust Point on the west to the city line on the east in Baltimore), 52 percent

21213 (just north of 21205 in East Baltimore), 52 percent

21223 (the Union Square area of Baltimore), 51 percent

(The only other ZIP code over the halfway mark is 21244, Windsor Mill in Baltimore County, at nearly 51 percent.)

LEAST

21036 (Dayton in Howard County), 8.6 percent

21042 (Ellicott City in Howard County), 9.1 percent

21051 (Fork in Baltimore County), 9.6 percent

21405 (Annapolis in Anne Arundel County), 9.7 percent

21029 (Clarksville in Howard County), 9.8 percent