En español | When times are tough, everybody's looking for a little extra cash. And that's also true of Uncle Sam.

See also: 10 ways to give yourself a tax cut

So if you've found a way to earn substantially more income, and federal taxes are not being withheld from it, beware! You may be required to estimate your tax bill and send a quarterly check to the IRS even before your federal return is due in April. If you don't send money that's required, you could get socked with interest and a penalty.

You probably owe estimated tax if your withholdings add up to less than 90 percent of what you expect your total tax bill will be. — Photo by Moxie Productions/Blend Images/Corbis

Why do some people have to make estimated tax payments? "The federal income tax is a pay-as-you-go tax," notes the IRS on its website. Anyone with a regular job is aware of this, because employers withhold part of every paycheck for income taxes. The potential problem arises when you have income that's not subject to withholding.

Maybe you're an empty-nester who's renting out a spare bedroom or baking cookies to sell at a local farmers' market. Or your writing skills are bringing in extra cash from freelance assignments.

Triggers for estimated tax

Other sources of income from which taxes are often not withheld include interest, dividends, capital gains, alimony, prizes, awards and gambling winnings.

But even a regular salary, a pension, an IRA distribution or a Social Security benefit can trigger the requirement for estimated payments if the tax you've chosen to have withheld from that check is too low.

If the IRS determines that your extra income is from self-employment, you may owe even more tax. Generally, if you're in business for yourself and earn more than $400 a year, you must pay not only income taxes, but what's known as self-employment tax. That's a substitute for Medicare and Medicaid payroll taxes that you and an employer would jointly pay were the income from a regular job. It adds up to a 15.3 percent bite of self-employment earnings.

How can you tell whether your extra income is enough to trigger payment of estimated taxes? The rules are complex, and full details can be found in IRS Publication 505, Tax Withholding and Estimated Tax.

The 90 percent rule

But for most people, quarterly payments are required if all the taxes provided to the federal government in advance — most often from withholding in a regular job — add up to less than 90 percent of what they expect their tax bill will be.

If you qualify for any refundable tax credits, you're allowed to add the amount of the credits to your withheld tax. Some of the most common ones are the earned-income credit, the American Opportunity credit for undergraduate college expenses, and the adoption credit.

For example, let's say that last year you earned $40,000 at your job and your federal income tax bill was $3,779. You had $3,800 withheld from your pay during the year, so you were square at tax filing time in April.

Next: Software to assist you with estimated tax payments. >>