How much am I required to pre-pay in order to avoid a penalty?
You are generally required to pre-pay at least 90% of your tax either by having the tax withheld from your income, or by making timely estimated tax payments. A penalty will also not be charged if you pre-pay a “safe harbor” amount equal to 100% of the previous year’s tax. However, special rules exist for certain taxpayers:

• Qualifying farmers and fishermen are required to pre-pay only 2/3 (66.67%) of their tax, or the safe harbor amount, whichever is less. You are a qualifying farmer/fisherman if at least 2/3 of your gross income in either the current or previous year is or was derived from farming or fishing. See Farmers and Fishermen in Publication 505 for more information.
• The safe harbor amount for high income taxpayers is 110% of the previous year’s tax. You are a high income taxpayer if your previous year’s adjusted gross income was $150,000 or more ($75,000 or more if you were married, filing a separate return).

You also will not be charged a penalty if your total tax due, after withholding, is less than $1,000, or if you had no tax liability for the previous year.