Why is there an estimated tax/interest penalty even if I file my tax return by the due date (i.e., 4/15/2019)?

[Opus 17]

There can be, if you are under-withheld or under-paid during the year.  To avoid a penalty, you need to either,

1. owe less than $1000

2. have paid into the system at least 90% of what you owe for this year,

3. have paid into the system at least 100% of your tax liability from last year (110% in some cases).

Wait for the IRS to bill you, the penalty that Turbotax will calculate for you is almost always off be a few dollars anyway.

[sonnet]

Thank you for answering the question, would you please give me the pointer of "110% of your tax liability from last year"? I only find 100% from https://www.irs.gov/taxtopics/tc306. Thank you!

[Opus 17]

Read the tax topic.  

There are special rules for farmers and fishermen, certain household employers and certain higher income taxpayers. For more information, refer to Publication 505, Tax Withholding and Estimated Tax.