The underpayment penalty is about 2% of what you owe, but it is calculated quarterly. (The penalty you owe for tax you should have paid in April will be more than the penalty you owe for tax you should have paid in September.). Then, you owe interest on all monies owed to the IRS for any reason (including underpaid taxes and penalties) at a variable rate which is currently about 5% APR.
To avoid a penalty, you must owe less than $1000 when you file your return, or the amount you paid in via withholding and estimated payments, must be at least 90% of your tax liability or 100% of your prior year tax liability.
