The government requires you to pay as you go. Employers do it for employees by withholding payroll taxes. The self-employed must withhold for themselves.

Quarterly estimated tax payments are the solo practitioner’s version of tax withholding—prepayments of what you will owe next April 15. It’s like doing tax returns four times a year. For the newly self-employed, estimating how much to pay and when can be befuddling.

Your current year’s estimated taxes are due are April 15, June 15, September 15 and January 15 of the following year. To calculate how much, figure your adjusted gross income, taxable income, taxes, deductions and credits for the year. There are two approaches.

1. Regular installments: As long as your four quarterly payments end up equal to 100 percent of your previous year’s tax liability, you avoid penalties, even if you make a million dollars more this year. (If you make a million dollars more, you’ll have to cough up the additional taxes when you file your 1040 form.)

If the previous year you had a $10,000 federal income tax liability (line 58 on your 1040), you can meet this year’s obligation by paying one fourth, or $2,500, on each of the four due dates. This method is simpler, and the IRS prefers it. For details, check IRS Publication 505, “Tax Withholding and Estimated Tax.”

Tip: One exception to relying on line 58 is if your adjusted gross income last year (line 33 of 1040) exceeded $150,000. Then your four quarterly payments must total 112 percent of that figure.

2. Annualized income installments: If your income is unevenly distributed during the year, you still need to make payments equal to 90 percent of expected tax. Complete the IRS 2002 Estimated Tax Worksheet to annualize your tax at the end of each period based on a reasonable estimate of your income, deductions and other items. Use the result to make your estimated tax payments and complete your payment-vouchers. If you use this method, you also must file Form 2210 with your 1040.