What do I enter for estimated tax liability if I don't know what I owe?

Your best estimate is fine — the IRS does not penalize you for an inaccurate estimate on your extension.

• If you expect a refund or to break even, enter $0.
• If you know you owe but aren't sure how much, use last year's tax return as a reference.
• If you are self-employed, estimate your net profit for the year and apply your typical tax rate.

The purpose of this field is to authorize any payment with your extension — an imperfect estimate is always better than not filing.