Reminder: Estimated taxes are still due tomorrow. The May 17 deadline only applies to 2020 returns.
While the IRS and many states recently postponed the tax filing deadline to May 17, that only applies to 2020 tax returns.
Your 2021 first-quarter tax payment is still due tomorrow, April 15.
There's a lot more detailed info on the IRS website, but you can avoid estimated tax penalties in one of two ways:
1. Figure out your total tax for 2021, divide it by four, and give the IRS at least 90% of that amount. The caveat is that if you only pay 90% of your estimated taxes, you'll probably still owe something when you file next year.
2. Figure out your total tax for 2020 (line 24 of your 1040) and give the IRS 1/4 of that amount. The caveat here is that your 2021 and 2020 taxes might be quite a bit different; if this year's tax is higher, then you could still be in for a big bill when you file your 2021 return next spring (on top of your estimated payment for Q1 2022).
You might find form 1040-ES helpful, but it's not required.
If your employer, pension administrator, broker, etc. withheld federal income taxes between January 1 and March 31, then that counts toward your Q1 payment (only your federal withholding - not your Social Security or Medicare withholdings).
State estimated taxes are another story altogether.
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