Here is how the IRS computes it:

https://www.irs.gov/newsroom/calculating-the-economic-impact-payment

Example: "For eligible individuals with no qualifying children, the payment is reduced to $0 if their AGI is at least:

• $99,000 if filing as single or married filing separately
• $136,500 if filing as head of household and
• $198,000 if filing married filing jointly

For individuals with qualifying children, these total-phaseout amounts increase by $10,000 for each qualifying child. For example, for an eligible individual filing as married filing separately with 3 qualifying children, the individual’s payment is reduced to $0 if his or her AGI is $129,000 or more."