Why do I have an underpayment penalty all of the sudden this year? I pay in full every year. Besides W2, there is only savings interest.

[jaime-vignali]

It is not clear to me what is triggering an underpayment penalty this year. This is very unfair. I have not received any IRS notices. I pay in full every year.

[AmyC]

If you owe more than $1,000, you owe a penalty. The IRS expects you to pay them when you make the money, not once a year. If you owe more than $5,000, you have substantially underpaid and have penalties and interest. You need to withhold more at work or make an estimated payment any time you come into additional money.

Payments can be made throughout the year to the IRS.