Two kinds of penalties
Late penalties can be a bit tougher to calculate and depend on whether you've filed your return. If you owe the IRS a balance, the penalty is calculated as 0.5% of the amount you owe for each month (or partial month) you're late, up to a maximum of 25%. This late penalty increases to 1% per month if your taxes remain unpaid 10 days after the IRS issues a notice to levy property.
On the other hand, if you don't file your tax return on time, the penalty is much more severe. The penalty for failing to file is 5% of the amount you owe per month (or partial month), which combines a 4.5% late filing fee with the 0.5% late payment fee. Now, the late filing fee also maxes out at 25% of the unpaid balance, but the late payment fee can keep running up to a combined total of 47.5% of the unpaid tax.