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Penalty for Underpayment of Estimated Tax

Definition of Penalty for Underpayment of Estimated Tax:

Has to do with the tax estimation component of filing an extension. If you owe taxes greater than $1,000 and did not pay at least 90% of the taxes you owe (or 100% of the tax amount you owed the year before, whichever is less) you will be fined 0.5% per month on the difference.