There is an underpayment penalty that may be imposed if you owe more than $1,000 in tax for the current year.

Exceptions to the underpayment penalty are:

• You earn at least 2/3 of your gross income from farming or fishing activities and are filing your return and paying all taxes due on or before March 1
• You owe less than $1,000 in tax
• You have paid in at least 90% of the tax for the current year
• The tax paid in the current year is at least 100% of the tax shown on your prior year's return

The amount of the penalty will depend of how much the underpayment is.  It is calculated on Form 2210, and will appear on your 1040 on line 79 (in the Amount You Owe section at the bottom of the 2nd page).