estimated tax

Hi:

Last year, I got refund from Fed and State tax. That was a normal year.

This year is different. My company was sold in Sept and it is now a private company. All my previous company stocks are sold. My gain is huge. But I do not know  how much the gain is since the calculation is complicated.

Do I need to pay additional tax by the end of the year so that there is no underpayment penalty?

I was told there are two rules:

• 90% of the amount of tax you will pay in total for the year; or
• 100% of the amount of tax you paid for the previous year.

 

So far, the Fed withhold is already more than what I paid for last year. 

But my estimation for this year's tax will be at least 3-4 times of last years'. 

Will I get underpayment penalty if I do not pay extra by the end of year? 

 

Thanks