With the tax filing deadline just a couple of short weeks away, now's the time to get moving on your return if you haven't yet started. For most folks, this means combing through paperwork, gathering documents, and -- in some cases -- frantically calling tax preparers in the hope that they have some last-minute availability.

But what if, despite your best efforts, you're unable to complete your return by April 17, which is when your 2017 taxes are due? Thankfully, there's a solution that can help you avoid the penalties associated with not filing on time: the tax extension.

Tax forms with pen and calculator

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How tax extensions work

The beauty of tax extensions is that you don't need a specific reason to request one. Simply submit IRS Form 4868, and you'll be granted an automatic six-month extension on your federal return, pushing its due date well into October.

Sounds simple enough, right? But don't get too excited just yet. While a tax extension will give you more time to submit your return, it won't extend your deadline for paying your taxes. This means that if you underpaid your taxes in 2017 and therefore owe money, you're required to send that cash to the IRS by April 17 or otherwise face a late payment penalty.

Now, keep in mind that the penalty for paying taxes late isn't nearly as bad as the penalty for filing a return late. If you don't submit your return by the April 17 deadline or get an extension, and you owe money on your taxes, you'll be hit with a penalty equal to 5% of your unpaid IRS bill per month, up to a total of 25% of that outstanding amount. If you request an extension but don't pay your tax bill by April 17, the penalty will be much lower -- just 0.5% of your outstanding tax debt per month, up to a total of 25%. Still, you're better off avoiding the late payment penalty if you can.

Paying your tax bill without filing a return

At this point, you're probably thinking: "How on earth am I supposed to pay my tax bill when I haven't completed my return?" It's a valid question, and the answer is: Do your best. Start by looking at last year's tax return and seeing how much you owed then. If your income hasn't changed substantially, that'll give you a ballpark estimate as to what you're on the hook for.

If your earnings went up significantly in 2017, estimating your taxes will be a bit trickier, but you can try calculating 2016's effective tax rate, boosting it a bit, and paying the IRS the difference. For example, if your effective tax rate in 2016 was 25% but your earnings went up quite a lot in 2017, assume that your new effective tax rate will be 30%. Then, add up your income -- including your wages from your primary job, bank account interest, capital gains, and any other money you took in -- and compare that total to the amount of tax you already paid for the year, which you'll find on your W-2. If the total is less than 30%, pay the IRS the difference by April 17 to lower your chances of racking up large late-payment penalties.

Is this formula perfect? Absolutely not. But it's a simplified means of avoiding hefty late penalties in the event you need a tax extension and are pretty certain you owe the IRS money.

While a tax extension will buy you extra time to submit your return, it won't get you out of paying your tax bill. If your primary motivation behind an extension is to avoid having to come up with that money sooner, then there's no sense in going through the process of requesting an extension. You're better off getting your taxes done with, and signing up for an IRS installment plan to repay your tax debt as painlessly as possible.