The penalty for the late filing of Form 940 (Employer's Annual Federal Unemployment Tax Return) is typically calculated based on the number of days the return is late. The penalty amount increases with the number of employees and the number of late filings within a calendar year. As of 2021, the penalty is generally 5% of the unpaid tax per month or part of a month the return is late, up to a maximum of 25% of the unpaid tax. Additionally, if the return is more than 60 days late, there is a minimum penalty of $435 or 100% of the unpaid tax, whichever is smaller. It is always advisable to consult the official IRS guidelines or a tax professional for the most accurate and up-to-date information.