IRS Link to Notice — Found Here

Notice 433 describes penalties and the applicable interest rates for various years of non-payment when corporate taxes are not paid in a timely manner.

This does not apply to individuals unless they are incorporated, and is not to be confused with Forms 433-A, -B, -D, or -F which are for individual purposes and concern applications for a Compromise Notice 746 updates the interest rates for more recent years.

When corporations do not pay their taxes on time, the IRS will charge penalties and interest as described in Notice 433 and Notice 746. 433 is the older version, for years prior to 2002. The information which follows is from 433. The penalty for late filing is 5% a month, the penalty for late payment is .5% a month, but combined they are limited to 5% of the unpaid tax for the month.

It also describes what interest is due to corporations which overpaid in the past and have filed a carry-back deduction for a refund. C Corporations who have a tax bill larger than $100,000 will be charged an additional 2% penalty for nonpayment.

There is also an additional penalty tax for failing to contribute to railroad retirement and pension funds that may range from 2% to 15%, depending on how late the contribution is. This is not to be Confused with Forms series 433, which has to do with Offers in Compromise, which is a petition to repay a lesser amount of taxes that what is due.