Congress' official scorekeeper said Wednesday that roughly two million more Americans will pay penalties under President Obama's health care law for lacking insurance than had previously been estimated.

Under the law, Americans must be insured starting in 2014 or pay a penalty assessed on their tax returns.

Shortly after the legislation passed in 2010, the Congressional Budget Office, working alongside the Joint Committee on Taxation, estimated that in 2016 roughly four million people a year would opt to pay the penalty instead of getting coverage. On Wednesday, the CBO and JCT revised that figure up to six million, citing legislation passed since 2010 as well as the weaker economic outlook.

The groups also pointed to the Supreme Court's decision earlier this year to make the health care law's expansion of Medicaid optional for states.

Of those people who opt for the penalty, 10% are projected to be below the federal poverty level for 2016, which the CBO and JCT estimate will stand at about $12,000 for an individual or $24,600 for a family of four.

Related: How health insurance mandate will work

In 2014, the penalty will be no more than $285 per family, or 1% of income, whichever is greater. In 2015, the cap rises to $975, or 2% of income. And by 2016, it reaches $2,085 per family, or 2.5% of income, whichever is greater.

The dollar amounts for a single adult would be $95, $325 and $695 during that same time period.

Roughly 30 million non-elderly Americans are projected to remain uninsured in 2016, though most will not be subject to the penalty tax. For instance, the penalty will be waived for people with very low incomes who don't have to file tax returns, those who are members of certain religious groups, or people who face insurance premiums that would exceed 8% of family income even after including employer contributions and federal subsidies.

Penalty payments collected in 2016 are expected to total $7 billion, about $3 billion more than previously estimated.