If you have income that isn't subject to withholding taxes, then you probably should be paying estimated taxes.

It doesn't matter whether the untaxed money comes from a job, investments, alimony or prizes you've won. If Uncle Sam doesn't get his share close to the time you received the money, you could end up owing not only taxes but also penalties and interest.

Most people satisfy their tax obligations through payroll withholding. But when that doesn't happen, you have to get the money to the government yourself by filing Form 1040-ES vouchers.

The U.S. tax system is on a pay-taxes-as-you-earn goal, so the Treasury's goal is to get any estimated taxes regularly, too. The Internal Revenue Service has set up a timetable, calling for estimated tax payments four times a year. Although the payments are commonly called quarterly, they don't coincide with calendar quarters.

Estimated filing schedule
Estimated tax due For income received
April 15 Jan. 1 through March 31
June 15 April 1 through May 31
Sept. 15 June 1 through Aug. 31
Jan. 15 Sept. 1 through Dec. 31

The four estimated tax payments are generally due each year on the 15th of April, June, September and January. But if that date falls on a weekend or federal holiday, the 1040-ES filing deadline is pushed to the following business day.

There are several such deadline changes in 2012.

The fourth quarter estimated tax payment for the 2011 tax year was Tuesday, Jan. 17, because Jan. 15 was on Sunday and the following Monday was the Martin Luther King, Jr. federal holiday.

Similarly, the first estimated tax payment for 2012 was due Tuesday, April 17, because April 15, again, fell on Sunday and Monday was Emancipation Day, a holiday in Washington, D.C. When a holiday in the nation's capital coincides with a tax deadline, the tax day is moved to the next business day.

The June 15 estimated tax due date is fine, but in September, the 15th is a Saturday, meaning that 1040-ES will be due on Monday, Sept. 17.

The IRS prefers you figure the total estimated tax for the entire year, divide it by four and send in equal payments according to the schedule. There's a work sheet with the Form 1040-ES package or as part of your tax software to do it.

You can send a paper check along with the Form 1040-ES voucher. Alternatively, you can file electronically with a credit card or by enrolling in the tax agency's Electronic Federal Tax Payment System, or EFTPS.

However, many times, folks who receive a financial windfall immediately spend the proceeds without any thought to the tax implications. Even people who earn a steady stream of money that isn't taxed upfront tend to put off filing estimated taxes because they need the cash and figure they'll settle things with the IRS at the annual April filing deadline.

But ignoring your estimated tax duties is not wise. If you end up owing $1,000 or more in April, you might have underpaid your tax bill. And that could result in you owing added penalties and interest, says Linda Durand, a certified public accountant with Drolet & Associates PLLC in Washington, D.C. "The IRS wants people to be paying their taxes during the year," she says.

Alternate Payment Options

Eva Rosenberg, who is authorized to represent taxpayers before the IRS and offers tax advice on the TaxMama website, offers an alternative to continual calculations, as long as you expect your taxable income to be the same or higher than it was last year.

All you need is last year's tax return and statements showing current tax withholding.

Figuring estimated payments
Look at Page 2 of your last 1040, specifically the "total tax" entry. Let's say it was: $10,000
From that, deduct any withholding you expect to have from any sources (wages, unemployment). For this example, let's use: $3,000
That gives you the total amount to be made up by estimated tax payments: $7,000
Divide the result by 4, and that's what you'd pay each IRS quarter in this scenario: $1,750

Rosenberg's method works even if you expect to owe substantially more in taxes this year than you did the previous one. This is because the IRS considers estimated taxpayers compliant as long as they pay either 90% of their eventual tax bill or a "safe harbor" payment based on a percentage of the tax owed the previous year.

Safe harbor refers to a regulation that eliminates a person's liability as long as the party acted in good faith. In the case of income taxes, it's an amount that protects you from IRS penalties for income tax underpayment.

Many taxpayers opt to pay 100% of their prior year's tax bill because it gives them a specific number to work with. Even better, it protects them from penalties and interest, regardless of how high their upcoming final tax bill goes.

Navigating Safe Harbors

Still, the safe harbor payment is a little more costly if you make a lot of money.

If your previous year's adjusted gross income was more than $150,000 for married couples filing jointly and single taxpayers; $75,000 for married taxpayers filing separately, and you want to base your estimated tax payment on the prior year's amount, you'll have a higher safe harbor percentage to meet.

In these cases, the IRS expects the high-earning taxpayer to pay at least 110% of his or her previous year's tax bill. That means if your adjusted gross income on that previous return was $150,000 and you ended up with a $30,000 tax bill, the IRS expects you to pay $33,000 -- $30,000 plus 10% -- in estimated and withholding taxes to guarantee you don't encounter additional tax penalties.

"With both (partners) in a couple earning or people holding multiple jobs, the salary cap is not as out of reach as it may seem, especially if they had a good investment or sold a piece of investment property during the year," says Durand.

Paying Only When You Earn

Although the IRS prefers four equal payments of estimated taxes throughout the year, you don't have to pay estimated taxes until you receive untaxed income.

If most of your untaxed income comes in one quarter, such as stock dividends paid at year's end, or if you operate a business in which income fluctuates throughout the year, you might want to consider paying your estimated taxes under the annualized income system.

"The annualized method allows you to take a look at each quarter independently and pay the tax in the quarter that you earned it," says Durand. "Say your job is one where most income is in the summer, such as landscaping, rather than the winter. You want to pay the taxes when you have the money."

With this approach, your required estimated tax payment for one or more periods might be less than the amount figured using the four-equal-payments method. To find that out, you'll have to complete a work sheet found in IRS Publication 505, Tax Withholding and Estimated Tax. Sole proprietors need another work sheet found in IRS Publication 505 to determine annualized self-employment taxes that are included with the estimated payments.

And you'll need to file Form 2210 with your annual return to explain why you didn't send in the expected equal payments, Durand says. This will keep the IRS, which assumes you earned the money equally during the year, from charging you an underpayment penalty and interest for not paying enough in a particular filing quarter.

"It is a little more complicated," Durand says. "But for cash flow it's better, and it puts the tax in the quarter when it is earned."

Avoiding Estimated Taxes Altogether

Are you already panicking at the prospect of struggling through work sheets and filing even more tax returns? You might have yet another option.

If you have wage income in addition to untaxed earnings, file a new W-4 at work and ask your boss to start taking out more payroll taxes to cover any shortfall. This strategy also works for couples who file jointly, but only one spouse has wage income subject to withholding.

Your (or your spouse's) take-home pay will be a bit lighter, but you'll be off the hook for estimated tax payments.