If you haven’t filed your tax returns yet, you’re not alone. The IRS is expecting to receive 20 million returns this week, which means about 16% of Americans put it off until the last minute. It’s not too late to file on time, but the midnight deadline is definitely approaching. If you owe the government any money, even if you can’t afford to pay right now, you should definitely get your return filed today.

So what happens if you don’t file before the deadline? The AP has a handy overview of the fees and penalties facing tardy taxpayers:

Taxpayers who don’t file returns or extensions by midnight on April 15 face penalties of 5 percent a month on any unpaid taxes, up to a maximum of 25 percent.

For taxpayers with small outstanding tax bills, there is a minimum penalty of $135, or 100 percent of the unpaid taxes, whichever is smaller. For example, if you owe $100 and don’t file a tax return, the penalty is $100. Plus, you still have to pay the taxes.

And that’s not all: You’ll also owe interest on the unpaid amount. The government is currently charging a 3 percent annual interest rate, compounded daily.

If you file a return but cannot pay all you owe, the penalty is just 0.5 percent of the unpaid taxes a month. That’s a tenth of the penalty for not filing. Though you’ll still have to pay that interest, of course.

Of course, not everyone owes taxes. If you’re owed a refund, the late-filing penalties don’t apply. However, you should still try to file as soon as possible, as you’re effectively giving the government an interest-free loan by letting them hold on to your refund money.

If your refund is pretty big, it may also be a good idea to talk to your HR department about adjusting your withholding. You could end up with fatter paycheck all year, instead of waiting until next April to get your hard-earned money. This article from USA Today lays out the advantages and disadvantages of changing your withholdings, and is worth a read.