You can find that information here. 

https://www.irs.gov/taxtopics/tc653

The formulas for penalties and interest are all published on the IRS website IRS.gov

@Nero1 

if you have already paid in 100% of your 2021 tax liability, there are no penalties for late filing or late payment of estimated tax assuming the ones made were timely.  if not, then there will be penalties for late payment of estimated tax that stops on 4/18/2022, late filing penalties of 4.5% for up to 5 months after the 4/18 deadline, and a late payment penalty of .5% per month after 4/18/2022 until paid.

https://www.irs.gov/payments/penalties 

I haven't paid my 2021 taxes yet, but I don't know if I need to pay or not. I haven't completed the filing, so I don't know if I need to pay. 

I know that if I don't owe taxes, and I'm getting a refund, then I don't need to file an extension. But if I owe taxes, and I didn't file an extension, then will I get a penalty for that?

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if not, then there will be penalties for late payment of estimated tax that stops on 4/18/2022

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Are you saying that penalties for late payment of estimated taxes stop on 4/18/22, so I'm not penalized every month I don't file/pay after that?

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late filing penalties of 4.5% for up to 5 months after the 4/18 deadline

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So the late filing penalties end 9/18/22? But the extension ends in 6 months, in Oct '22. 

Can all these penalties be waived for any reasons? I had late estimated tax penalties waived before, a few years ago, and I don't know if they'll waive anything again.