(NEXSTAR) – May 17 was the last day to request an extension to file your tax returns. So if you missed that deadline — and still haven’t made any payments — you can expect to owe a little extra to the IRS.

The Internal Revenue Service expects taxpayers to file and pay their taxes by the annual deadline unless the taxpayer can provide valid “reasonable cause” for not being able to do so. Otherwise, interest begins to accrue on any unpaid balance, and penalties can be imposed for failing to pay and file your taxes.

Interest accrues daily until the entire unpaid balance is paid in full. The interest rate (for non-corporate entities) is determined by the federal short-term rate, plus 3 percentage points. The federal short-term rate, which is calculated every three months, has remained the same (3%) for the quarter beginning on April 1, 2021, the IRS confirmed in March.

Failing to pay, in and of itself, can also be penalized with a charge of 0.5% of the taxes owed after the due date. This penalty is charged “for each month or part of a month” that taxes aren’t paid, according to the IRS. This rate increases to 1% if the IRS has informed a taxpayer of a final notice to levy or seize property (beginning 10 days after the notice is issued). However, taxpayers with IRS installment agreements will be charged interest at only half the standard rate (0.25%).

Similarly, failure to file taxes (barring any valid reasonable cause, such as incapacitation or a natural disaster) is penalized with a charge of 5% of the unpaid tax amount for each month, or part of a month, that the return is late. The charge will not exceed 25% of the unpaid tax amount, which is calculated based on the amount owed less any withholding, any estimated tax payments or any refundable credits. However, there is a “minimum failure-to-file penalty” for taxpayers whose filings are not received after 60 days of the due date: $435, or 100% of the required taxes shown on the return, whichever amount is less.

If a taxpayer fails to pay and file taxes in the same month, the total penalty still will not exceed 5%, up to a total of 25%. After five months, when the 25% threshold is reached, the penalty “will max out” for failure to file, but penalties for failing to pay will continue. The maximum penalty, for both failing to pay and file, can reach 47.5% of the tax, in total.

In other words, the sooner you file your late tax returns, the better.

On the other hand, if you owe no taxes or you’re owed a refund, the IRS won’t penalize you for failure to file.

“If, however, you wait too long to file your return and claim a refund, you risk losing it altogether,” David Alito, the IRS Wage and Investment Division Deputy Commissioner, explained in a May 14 post to the agency’s website. As Alito noted, returns generally need to be filed within three years of the due date for any refunds to be issued.

“If you don’t, the money is forfeited, by law, and becomes property of the U.S. Treasury,” he wrote.