Actual CPA here, you’ll owe an underpayment of estimated tax penalty if you don’t pay in at least the lesser of 90% of current year/100% of prior year (110% if AGI >$150K) tax. The penalty is based on the Federal short term rate and prorated based on amount underpaid and time outstanding (i.e. Q1 is overdue for the whole year but only 1/4 of the underpayment, Q4 is the whole amount but only late by one quarter). When all is said and done, it usually works out to like 2% and we have plenty of clients that would rather hold the cash and pay the penalty because it’s generally cheaper than borrowing, or they figure they can get a greater rate of return by interesting it (depending on their situation/perspective).

If you miss April 15th it goes to credit card rates though (failure to pay penalty, which is very different from underpayment of estimated tax). Don’t do that. Remember: an extension is for time to file, not time to pay.