As April 15 approaches, most people who don’t already have an accountant on speed dial begin to feel a little bit of dread. It marks the last day you can file your income taxes on time. But while death and taxes may be the only true certainties in life, somehow, Tax Day always seems to sneak up on us. So what happens if tax season slips your mind, and you just don’t file anything? 

It depends. If you already know you’re not going to get your taxes done by this year’s Tax Day, go ahead and file for an extension now. You can get an extra six months to file federal taxes by filling out a form and estimating (and paying) how much you’ll owe for that year.

While you may be granted a filing extension, you are still required to pay your taxes by the regular due date: If you don’t hand the IRS at least an estimated amount by April 18, 2017, you’ll be charged a fee equal to .5 percent of the tax you owed in the first place per month it was left unpaid (up to 25 percent). If you ignore repeated notices from the IRS, that .5 percent increases to 1 percent per month. And you’ll have to pay interest on the money you haven’t given over (3 percent plus the federal short-term rate, which changes every three months, compounded each day).

If you don’t file any federal income tax return at all by mid-April, you’ll be slapped with a fine—5 percent of the amount you already owe for each month you're overdue, up to 25 percent. If you file your return more than two months late without a good excuse, you’ll pay a minimum of $135 in penalty fees, or the balance of the tax you owe, if that total is less than $135. (According to the IRS's website, "The total penalty for failure to file and pay can be 47.5% [22.5% late filing and 25% late payment] of the tax owed.")

If you do a little math, you’ll see that it usually pays to go ahead and file your return or get an extension, even if you can’t pay your taxes immediately. Here’s how Turbotax explains it:

Example: Let's say you didn't file your return or an extension by April 18, 2016, and you still owe the IRS an additional $1,000.

Best-case scenario: You file your return on April 29 (2 weeks late) and submit your payment for $1,000. You would likely owe an additional $50 for the late-filing penalty ($1,000 x .05 = $50).

(Had you filed an extension by April 18, 2016, your late-payment penalty would be only $5 instead of $50. It definitely pays to file an extension!)

Worst-case scenario: You finally file your 2015 return in April of 2021, 5 years late, and submit your payment for $1,000. You would likely owe an additional $250 for filing late ($1,000 x the maximum .25), plus possible interest.

If you don’t owe any taxes because your employer withheld more than necessary and you are due to get a tax refund, you have three years to file your taxes before the IRS will keep that money. So as long as you get around to it by April 18, 2019, you’ll still get that money back. After those three years, the IRS will keep your whole refund, and it won’t count toward next year’s tax bill, either.

Say you just don’t want to pay your taxes (a crime, just to be clear). How long before the IRS will come after you?

If your penalties and back-taxes add up to more than $25,000, someone from the IRS is going to come knocking at your door. In 2015, the IRS investigated 223 people for regularly failing to file their taxes, and put 167 people in jail for an average of three years. (Remember, it was the IRS that took down Al Capone.) 

If you are a chronic non-filer, and don’t file your taxes even after warnings from the IRS, the government will go ahead and estimate what you owe, calculating what’s called a “substitute for return.” This total doesn’t include deductions that you might have been eligible for, meaning that if you let the government do your taxes for you, you’ll probably end up with a heftier bill. 

And that’s just at the federal level. While states vary on how they treat people who don’t file their taxes, they slap penalties and interest on late returns and payments, too. Some states will even take your federal tax refund to pay your state back taxes. However, in many states, being approved for a federal tax extension also gets you an automatic extra six months on your state income taxes. 

The lesson: If there’s any chance you’ll be late filing your return this year, ask for an extension ASAP. 

A version of this story was first published in 2016.