Payments will be sent in numerical order, based on the last two digits of U.S. taxpayer's social security numbers, the agency said.

USA Today reported Tuesday that the IRS will also put a calculator on their Web site, so those receiving checks can calculate how large they will be.

Most parents will receive a check of $300 every child under 17 years of age. But, millions of people exempt from filing taxes will need to remember to file this year to receive the rebate.

"People just need to file their tax returns," IRS Acting Commissioner Linda Stiff told the newspaper.

The agency expects to send more than 34 million rebate checks in May and have most checks mailed by July 11, the report said.

Those filing late will also receive checks. The agency said Oct. 15 would be the deadline for filing for anyone to receive a check this year.

The online service to determine eligibility and calculate the size of a check is at

www.irs.gov/app/espc.