Maybe one of the best things about the federal tax rebates headed for millions of households starting this spring is that most people don't have to do anything special to get one.

Simply file your tax return as usual and sit back and wait. There are no extra forms to fill out or questions to answer. Uncle Sam will do the calculations for you and automatically send you the money.

While the majority of households will receive the maximum $600 for singles, $1,200 for couples and an extra $300 per child, high and low wage earners will receive less. Some will get nothing.

If you're not sure how much you will be getting and don't want to wait to find out, Kiplinger.com has posted a handy online calculator that figures a household's rebate based on income, filing status, number of children and tax liability. It's available at www.kiplinger.com/tools/rebate/.

The Internal Revenue Service is working on its own online calculator, but isn't sure when it will be ready, spokesman David Stewart said.

What is for certain is that no one gets a rebate unless they file a federal tax return. So people who normally aren't required to file a return will have to do so this year if they want to get their money.

In general, to qualify for the maximum rebates of $600 for singles and $1,200 for couples, you must have paid at least that much in federal income taxes during the year. The majority of Americans pay that much or more. (Married couples filing jointly can qualify for the maximum $1,200 even if one spouse does not work.)

Still, even people who don't earn enough to pay any federal income taxes, but make at least $3,000 -- including people whose only income is from Social Security or veterans' disability payments -- will get a minimum of $300.

The top rebate begins to phase out for singles with adjusted gross income of more than $75,000 and couples with adjusted incomes over $150,000. The rebates end for singles making more than $87,000 and couples making over $174,000.

The ceilings rise for households with children so that even if a couple makes too much money to get a rebate for themselves, they can receive part or all of the rebates for their children. Qualifying children must be under the age of 17 as of Dec. 31, 2007.

The checks -- which President Bush authorized Wednesday hoping to boost the sagging economy -- will start going out in May and probably continue through the summer. Exactly how long it will take before everyone receives their money depends in part on when people file their returns, Mr. Stewart said.

The IRS will be updating information about the rebates on a special Web page, available by clicking the "Rebate questions?" link on the agency's home page at www.irs.gov.

The agency hasn't yet decided in what order the checks will be processed, Mr. Stewart said, so there is no way to tell right now who will be among the first to get their money. The rebates are in addition to any regular tax refund.

If you're wondering if you must declare the rebates as income on next year's federal tax returns, the answer is no. The money is classified as a tax credit and not considered income.