The latest analysis was conducted by McKinsey's Center for U.S. Health System Reform, whose independent research has been cited by the federal government and others.
"The whole point of the law was not only to cover the uninsured, but so people didn't have to make choices between food or drugs, or going to the doctor or dentist," said Karen Davis, a health policy expert at the Johns Hopkins Bloomberg School of Public Health. "It's what it is designed to do."
Many insurers tried to price their least expensive plans so they would become free or nearly free with the addition of subsidies that are set based on a person's income and the cost of a midlevel, or silver, plan.
Independence Blue Cross in Philadelphia has four plans that are free to some customers. But the company, along with other insurers, has been careful not to publicize its free coverage for fear of alienating customers who will need to pay more.
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"We're not advertising zero dollar," said Brian Lobley, a senior vice president at Independence Blue Cross. But the company is promoting monthly premiums in the $20 to $30 range, he said.
The Obama administration has also stressed affordability over coverage with no monthly charge, frequently saying that the cost of coverage will be less than a monthly cellphone bill for many consumers. Officials at the Department of Health and Human Services would not comment on the McKinsey analysis, saying in a statement that the goal of the health law was to provide a range of options for people with differing needs and budgets.
The analysis found that five million to six million people who are uninsured will qualify for subsidies that will be greater than the cost of the cheapest bronze or silver plan. A million more people with individual insurance could also be eligible, according to McKinsey, although estimates of the size of the market for private individual insurance vary widely. None of the people in the analysis qualify for Medicaid.
The availability of zero-premium plans may make the deal especially enticing to the healthy young people the marketplace needs to succeed, said Mark V. Pauly, a professor of health care management at the University of Pennsylvania's Wharton School. "This is such a good deal that you'd have to believe you were immortal not to really pick it up," he said.
Although they vary in their design, bronze plans generally cover about 60 percent of a person's medical costs. All plans, including bronze, must cover standard benefits like prescription drugs, maternity care and mental health treatment.
The availability of the zero-premium plans varies across the country. McKinsey found that about 40 percent of the uninsured in Missouri will be able to select a no-cost bronze plan, for example, compared with 2 percent of the uninsured in New Jersey.
(Read more: No easy answers as Obamacare drops health plans)
Its estimate, based on an analysis of premiums for plans offered in the marketplaces in all 50 states and the District of Columbia, is in line with two other estimates, by Credit Suisse and Morgan Stanley.
The McKinsey researchers also found that about half of the people eligible for zero-premium plans were under 39 and uninsured. The Obama administration has been emphasizing the affordability of its plans for young people, a critical group because their participation in the marketplaces will help keep overall premiums low.
It is impossible to know who will actually sign up, and whether they will choose a zero-premium plan.