How much would the underpayment penalty be on $100,000.00 if gotten during the last half of the year.

Current rate is 4% per quarter so $4K to $8K ... 

Failure to pay proper estimated tax: Internal Revenue Code §6654

• Estimated tax payments are generally required, if you expect to owe at least $1,000 in tax after subtracting withholding and refundable credits.
• Generally calculated on Form 2210
• We calculate the penalty separately for each required installment. The number of days late is first determined and then multiplied by the effective interest rate for the installment period.
• See Publication 505 for more information.

Actually, it is 4% per year.

It also depends on what your prior year tax was, and if you mean $100,000 of income or $100,000 of tax.

The sooner you make an Estimated Tax payment, the smaller your penalty will be.

Go to this IRS website for your payment options - <a rel="nofollow" target="_blank" href="https://www.irs.gov/payments">https://www.irs.gov/payments</a>

Deleted. See new comment below

OP just asked about Capital Gains:
<a rel="nofollow" target="_blank" href="https://ttlc.intuit.com/questions/4138109-i-have-some-large-capital-gains-for-2017-do-i-need-to-pay-estimated-taxes-on-these-amounts-before-year-end">https://ttlc.intuit.com/questions/4138109-i-have-some-large-capital-gains-for-2017-do-i-need-to-pay-estimated-taxes-on-these-amounts-before-year-end</a>

Log term capital gains receive favorable tax treatment, and are usually taxed at 15%.  Some may be taxed at 0% and some at 20% or 23.8%, depending on the tax payer's tax bracket.

So 4% for half a year on  $15,000 tax on $100,000  of income is about $300. But, if you wait til April to pay that's 3+ more months at 4%.

For taxes, the 4th Quarter starts September 1st . So the 4th quarter payment is not due until January 15, 2018, if the gain was realized in the 4th quarter. The under payment penalty will be measured from Jan. 15.

Remember state taxes as well.

For federal (and most state) taxes, the 4th Quarter starts September 1st (yes the "quarter" is 4 months long, not 3). So the 4th quarter payment is not due until January 15, 2018. Th under payment penalty will be measured from Jan. 15.
Furthermore, if you file, and pay, before Jan 31, no penalty will be due (if the gain was realized in the 4th quarter)