The numbers help determine where to send HHS Secretary Kathleen Sebelius to pitch the law. They're guiding the placement of television ads aimed at younger people, scheduled to start airing as the Winter Olympics open this week.
Washington is largely bypassing states that are leading their own sign-up efforts, including California, New York and Illinois.
The research for AP found that just 13 counties account for 20 percent of the uninsured. The top county, Los Angeles, has more than 2 million people without health coverage, or about 5 percent of the national total.
(Read more: Medicare cost-cutting program reaps $380 million)
"The administration is well-aware of where the uninsured population lives," said Lynn Blewett, the director of the Minnesota health data center. "It's to their benefit to get out to the states where they are going to have the biggest bang for their buck."
Uninsured Americans generally live in metropolitan areas, but data-driven research can also help in rural states with seemingly low numbers of uninsured people, said Brett Fried, a senior researcher at the center. Census files that provide coverage information according to ZIP codes can be used to tease out concentrations of uninsured.
Bataille said the government also has an outreach effort tailored to rural areas.
The Minnesota researchers used Census data from the 2011 Small Area Health Insurance Estimates, the only source of annual estimates of uninsured people for all counties. They were not able to filter out people who entered the country illegally and thus are not eligible for coverage under the law. Blewett said that group, although numerous, is not large enough to skew the overall geographic pattern.
(Read more: Counting the costs of a global epidemic)
Cook County, Ill., which includes Chicago, ranks third nationally in the total number of uninsured, and third in uninsured young adults, with more than 460,000, the study found.
Among them is Katina Rapier, who recently filled out her paperwork during an Enroll America event at her community college. Enroll America is a nonprofit that works closely with the administration.
Rapier, 25, aspires to own a chain of women's clothing stores, but she has been uninsured since she turned 18 and says it's a struggle to afford her crucial asthma medication. She thought she had missed the deadline to apply for coverage, not realizing that open enrollment runs through March.
"If it can help me get safe medication, I'm all for it," Rapier said of the health care law.
No matter what the numbers say, she doesn't think the administration will have an easy time signing up young adults.
"They think health is something that you worry about when you get older," Rapier said.
The White House originally set a goal of 7 million enrollees in the new insurance markets, and the administration says it has reached the 3 million mark.
—By The Associated Press