By David Perry, a contributor.These days, more and more of us are self-employed. If you've recently joined the growing ranks of entrepreneurship, don't forget to plan for the self-employment tax. It's a substitute for the Social Security and Medicare taxes that an employer would ordinarily pay for you or withhold from your paycheck. If you don't ante up, you'll do more than just break the law. You'll also put your access to the Social Security system, and its medical benefits, at risk. The tax applies to anyone who earns more than $400 from self-employment during the year. You are definitely self-employed if you operate your own business, like a Chipotle ( CMG) franchise or a lawn-mowing service. But did you know that you might be self-employed if you have a regular job and do a little bit of work on the side? For example, if you're the IT manager of a Target ( TGT) store, but you fix people's computers on the weekend to earn some extra dough, the Internal Revenue Service wants to hear from you. How much is the tax? If you are lucky enough to be your own boss, you should expect to pay 15.3% of your first $102,000 of income. That figure includes 12.4% for Social Security and 2.9% for Medicare. You should use Form 1040-SE to calculate the magic number. It allows you to shave 7.65% off the amount subject to tax, which can result in considerable savings. Of course, it's not all bad news. In addition to paying for Social Security and Medicare in your golden years, half of your self-employment tax is also deductible on your Form 1040 whether or not you itemize. If you earn $50,000 from self-employment this year, the deduction for self-employment tax could lower your taxable income by more than $3,000, and that could buy a lot of burritos.