It’s that time of the year again. The time when we all have to hope we withheld enough money, and don’t have to pay the piper. It’s tax time!

If you’ve spent your whole life as an employee, this may be an easy — dare I say, happy — moment. If you know what you’re going to earn in advance for the year, you can set things up so that you have more taxes taken out than you need to pay. This usually means that you file your 1040EZ and wait for a fat refund check to appear in your bank account.

Let’s start at the very beginning. As someone else’s employee, when you get a job, you are given a W4 form to fill out as part of the onboarding process. This federal form allows you to tell your employer how much (or how little) you want them to take out of your check each pay period and set aside to pay your income taxes. This, along with FICA (Medicare and Social Security), and any other pretax items such as retirement savings and health insurance, are deducted straight out of your paycheck. You never even see them in your take-home pay. At the end of the year you receive a W2 form, which shows how much you earned, and how much was held back. Pretty easy math!

For freelancers, it’s different.

Depending on your business model, you might have one client, or 20, or more. You don’t fill out a W4, although you might provide a W9 that simply tells your client what your tax ID number is (if you are set up with an LLC) or your Social Security number.

Each of your clients should send you a 1099 form at the end of the year showing how much they paid you over the course of the year. Keep in mind that when they did pay you, nothing was taken out for taxes, at all.

Each quarter you have to do the math (usually using an online calculator) to figure out how much you should send off to the government — both federal and state. This payment covers your income taxes, as well as both halves of FICA (as an employee, you pay half and your employer pays half), which is called self-employment tax, and totals about 15 percent of your total income.

These payments are due at intervals throughout the year, and you pay based on a combination of what you actually earned, and your best guess of what you will earn over the course of the year, as well as what you think you’ll be deducting for business expenses. If your income varies, you might pay very little one quarter, and thousands of dollars the next.

On the plus side, as a freelancer you get to deduct expenses that employees can’t claim. Every mile that I drive to see a client counts as a deduction. If I’m an employee, and I commute to work, those miles are not deductible. If I work out of my house, I can deduct a percentage of my home maintenance costs as a home office expense. Even if I work from home, as an employee I don’t get to deduct those expenses.

It can be a challenge to keep track of your income and expenses as a free agent, but it’s an important element of working for yourself. So, get yourself some software, keep all of those receipts, and use a dedicated credit card to help keep all of your business expenses in one place. If this is your first time doing your taxes as a freelancer, it’s well worth a trip to your friendly local CPA to make sure you’re doing things right. Better a few hundred dollars spent on some professional advice than a call from the IRS.