Form 2210 won't let me enter amounts manually. Therefore, the sum of the four columns in Line D Withholding is not equal to my total withholding for the year?

If TurboTax is showing an underpayment penalty on your return, visit or revisit the Underpayment Penalty section under Other Tax Situations and answer the interview questions. The penalty calculation may change based on your answers, including information about when your withholding was applied. In addition, if you believe you qualify for a waiver of the penalty, you can request a waiver in that section.

 

The IRS may reduce an underpayment penalty if any of the following apply:

• You or your spouse (if you file a joint return) retired in the past 2 years after reaching age 62 or became disabled and you had reasonable cause to underpay or pay your estimated tax late. See Waiver of Penalty in Instructions for Form 2210PDF.
• You had most of your income tax withheld early in the year instead of spreading it equally through the year. 
• Your income varies during the year. Complete Form 2210, Schedule AI, Annualized Income Installment Method PDF(found within the form).

The penalty for underpayment of estimated tax can also be removed to the extent that the underpayment is the result of a casualty, local disaster, or other unusual circumstance such that it would not be fair to impose the penalty.

 

See here for information about different TurboTax tools you can use to calculate withholding and estimated taxes