Just a short reminder that the final 2018 Estimated Tax Payment is due June 15, 2018. To avoid (further) penalties for underpayment of estimated tax, in general, by January 15, 2019 you should have paid (in combination with 2018 US taxes withheld and all other 2018 Estimated Tax Payments) the lower of:
1) 90% of your (eventual) 2018 tax liability, or
2) 100% of your 2017 tax liability (110% if your 2017 Adjusted Gross Income exceeded $150,000 or $75,000 if Married Filing Separately)
Note: You do not need to make estimated tax payments if your final 2018 tax liability does not exceed $1,000.
You can find the form and instructions at: http://www.irs.gov/pub/irs-pdf/f1040es.pdf