The IRS will deliver tax-free stimulus checks to millions of Americans over the next several weeks.

To qualify for a direct payment, you need to have a Social Security number, meet certain adjusted-gross-income thresholds, and not be claimed as a dependent on someone else's tax return.

Americans who haven't had to file a tax return in 2018 or 2019 but get Social Security payments for retirement will also be paid $1,200 each.

Dependents older than 16, people without a Social Security number, and those with incomes above $99,000 (or $136,500 if you file as a head of household) won't get a stimulus check.

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Americans started receiving their stimulus checks from the government this week as part of a massive coronavirus relief package.

These "recovery rebates" will pay up to $1,200 per person, plus an extra $500 per child to parents — and they're completely tax-free. But not everyone is entitled to a stimulus check.

The Urban Institute and Brookings Institution's Tax Policy Center estimated that nine in 10 households would get some payment.

Here's who doesn't qualify:

1. People with incomes above $99,000, or single parents with incomes above $136,500

The size of each stimulus payment is based on the adjusted gross income (AGI) listed on your 2019 tax return. If you haven't filed yet, it will be based on your 2018 return.

The maximum payment is $1,200 for single filers with an AGI below $75,000 or single parents (head-of-household filers) with an AGI below $112,500. Married couples with no children who file jointly and have an AGI below $150,000 will get a total of $2,400 (double the maximum individual payment).

The $1,200 payment will begin to phase out at a rate of $5 for each $100 over the AGI threshold before ceasing at an AGI of $99,000 for single filers, $136,500 for heads of household (single parents), and $198,000 for married filers with no children.

Bear in mind that the stimulus payment is technically a tax credit to apply to your 2020 tax bill that the government is providing ahead of time. Since the IRS has only past tax returns to go off of, there are bound to be clerical errors.

For instance, if your past income disqualifies you for a stimulus payment, but your earnings are below the qualifying threshold in 2020 — which is likely the case for people who suddenly lost their jobs or had their hours reduced because of the coronavirus — you won't get a payment right now, but you should be able to claim the refundable credit on next year's tax return.

Note that Americans who get Social Security payments for retirement or railroad retirement benefits will also be paid $1,200 each via the direct deposit or home address provided on their statements, as long as they are not claimed as a dependent on someone else's tax return (more on that below).

People who aren't required to file a tax return because they don't have enough income will also get payments. The IRS is urging non-filers to submit their payment information via a new tool on its website so they don't have to wait for a paper check. TurboTax also has its own online portal where low-income Americans who don't file a tax return can send their information to the IRS.

2. Dependent children over 16

Parents who have a Social Security number will receive $500 for each child who is age 16 or younger. There is no limit on the number of children that qualify.

If you have a dependent child who is 17 or 18 years old, they don't qualify for the $500 payment.

3. Dependent adults

Any person over the age of 18 who is claimed as a dependent on another person's tax return is not eligible for an individual or child stimulus payment.

That means college students ages 19 to 23 whose parents pay for more than half of their expenses and claim them on their tax returns won't get payments. Likewise, adults with disabilities and elderly people who are claimed as a dependent on a family member's tax return are not eligible.

4. Anyone without a Social Security number

To get a stimulus payment, you need to have Social Security number. The Social Security Administration issues Social Security numbers to all citizens, permanent residents, and some noncitizens who are authorized to work in the US.

If a couple files jointly, but only one person has a Social Security number, neither will be eligible for a stimulus payment, except in the case of military families, according to the Tax Policy Center.

This post was updated on April 15, 2020 to include new information from the IRS.