From your post, it appears that even with your estimated tax payment the total tax withheld and estimates paid were insufficient to prevent an underpayment penalty due to your large capital gain in December, If your estimated payment was larger then you would not have incurred an underpayment penalty.
The U.S. tax system operates as a pay-as-you-go system. What this means is that you must pay your taxes as they occur. For example, If your estimated tax payment for the first quarter is less than the amount required for the income earned you will incur an underpayment penalty. This penality is not deleted by any future estimated tax payments even when the estimates exceed the minimum required to avoid a penalty for the quarter.
The U.S. tax system operates as a pay-as-you-go system. What this means is that you must pay your taxes as they occur. For example, If your estimated tax payment for any quarter is less than the amount required for the income earned you will incur an underpayment penalty. This penality is not deleted by any future estimated tax payments even when the estimates exceed the minimum required to avoid a penalty for the quarter.
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