Well, the rules for when you owe an underpayment penalty are a bit complex, start here. <a rel="nofollow" target="_blank" href="
https://www.irs.gov/taxtopics/tc306.html">https://www.irs.gov/taxtopics/tc306.html</a>
Most of the time, someone who switches from a W-2 withholding to owing their own payments, will fit one of the penalty exceptions in the first year. (That's not guaranteed though.) The bigger problem is potentially owing several thousand dollars in April not realizing you should have been paying all along.
Depending on your overall income level, other jobs, kids, etc., that could affect the amount you owe considerably. Only someone who knows your income numbers (you or your accountant) can know for sure.
