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 Tax Day, you can file for an extension. 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 15, 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 percent [22.5 percent late filing and 25 percent 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 15, and you still owe the IRS an additional $1000.

Scenario 1: You file an extension on or before April 15 and pay your $1000 bill on April 25 (10 days late). Your penalty would be $5 (the 0.5 percent late-payment penalty applied to $1000), plus another dollar or so for the interest.

Scenario 2: You didn't file an extension, and you file your return on April 25 (10 days late) along with your $1000 payment. Your penalty would be $50 (the 5 percent late-filing penalty applied to $1000), plus another dollar or so for the interest.

Scenario 3: You file your return five years late, along with your $1000 payment. Your penalty would be around $534 (the maximum late-filing penalty of 25 percent applied to $1000, plus 5 percent interest compounded daily assuming the interest rate doesn't change).

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 2022, 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 2016, the IRS investigated 206 people for regularly failing to file their taxes, and put 159 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.