As per Bsch477 the main reason for having an underpayment of taxes is not having enough withheld from your pay or not paying enough in your first three quarters of estimated taxes.
You can adjust this by filing a new W-4 with your employer, assuming you are a W-2 employee. This is especially important if you work multiple jobs or had fluctuating income because each job withholds your pay like it is your only job, often resulting in an underpayment of tax.
Click here for "Why Am I Getting an Underpayment of Tax Penalty?"
Click here for "Guide to IRS Tax Penalties."
Click here for "Underpayment of Estimated Taxes"
Click here for How Does TurboTax Calculate my Estimated Tax Penalty?"
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