Q. If I make a 30k payment for the first quarter of 2021 estimated, then I won't get an underpayment penalty for 2021? Even if at the end of the year I end up owing 500k more in taxes?
A. No. But, if you pay $33,000, yes. You can pay $33K in the first quarter or spread it over 4 equal quarterly payments.
The safest option to avoid an underpayment penalty is to aim for "100 percent of your previous year's taxes." If your previous year's adjusted gross income was more than $150,000 (or $75,000 for those who are married and filing separate returns last year), you will have to pay in 110 percent of your previous year's taxes to satisfy the "safe-harbor" requirement. If you satisfy either test, you won't have to pay an estimated tax penalty, no matter how much tax you owe with your tax return.
IRS Interest rates: https://www.irs.gov/newsroom/interest-rates-remain-the-same-for-the-second-quarter-of-2020
