If I have under withheld Federal taxes in 2018, how long do I have to pay additional without penalty?

[VolvoGirl]

When the tax return is due on April 15, 2019.  You will either have a tax due or a refund.  Even if you get an extension to file or file after April 15 the tax due is still due by April 15.  

If you are worried you will owe too much on your tax return you can pay an estimated payment now at <a rel="nofollow" target="_blank" href="https://www.irs.gov/payments">https://www.irs.gov/payments</a>.

[ldjermyn]

The question remains, how long do I have to pay estimated taxes to avoid a penalty?

[DoninGA]

Fourth quarter 2018 estimated taxes are due on or before January 15, 2019.

[VolvoGirl]

Here’s an IRS article.  <a rel="nofollow" target="_blank" href="https://www.irs.gov/taxtopics/tc306">https://www.irs.gov/taxtopics/tc306</a>

Even if you don’t have enough withholding you might avoid the penalty.  Generally, most taxpayers will avoid this penalty if they owe less than $1,000 in tax after subtracting their withholdings and credits, or if they paid at least 90% of the tax for the current year, or 100% of the tax shown on the return for the prior year, whichever is smaller.

[VolvoGirl]

Did you have a penalty in a prior year?  Why will you be under withheld for this year?  Do you have other income besides W2?  

[Anonymous]

depending on your tax liability vs withholding, an estimated tax payment now would not eliminate the penalty for the 1st three quarters ,  the penalty for the 4th quarter would be less the sooner you make an estimated tax payment.    however, if possible, have more tax withheld for the rest of the year.   withholding is assume to occur evenly throughout the year, even if t isn't