One of the most common questions tax professionals hear each filing season is:
"How can I owe taxes? They're already taking taxes out of every paycheck."
It's a fair question, and one that's becoming even more common.
Many taxpayers assume that federal income tax withholding automatically matches what they'll owe when they file their return. Unfortunately, that's not always the case. In fact, it's one of the biggest reasons taxpayers are surprised by an unexpected balance due in April.
Understanding why withholding falls short is an important part of helping clients avoid tax surprises and improve their year-round tax planning.
Why Withholding Isn't Always Enough
A paycheck doesn't know your entire tax picture.
Your employer calculates withholding based primarily on the information provided on Form W-4 and the IRS withholding tables. While those calculations work well for many employees, they don't always account for everything happening in a taxpayer's financial life. Employers use IRS withholding tables in Publication 15-T to calculate federal income tax withholding based on the employee's Form W-4.
As life changes, withholding may no longer reflect the taxpayer's actual tax liability.
Five Common Reasons Clients Still Owe Taxes
1. They Have More Than One Job
Multiple jobs remain one of the most common causes of under-withholding.
Each employer calculates withholding independently. Neither employer knows what the other is paying, so each may withhold as though that job is the employee's only source of income.
When the income is combined on the tax return, the taxpayer may move into a higher tax bracket than either employer anticipated.
2. Both Spouses Work
Dual-income households frequently experience the same issue.
Each employer withholds based only on that employee's wages. Without proper adjustments on Form W-4, the combined household income may result in too little federal income tax being withheld throughout the year.
3. Other Income Isn't Subject to Withholding
Many taxpayers earn income that doesn't automatically have federal income tax withheld.
Examples include:
- Rental income
- Self-employment income
- Interest and dividends
- Side businesses
- Gig work
- Investment income
Unless additional withholding or estimated tax payments are made, these income sources often create an unexpected balance due at tax time. The IRS explains that taxpayers generally satisfy their pay-as-you-go obligations through withholding, estimated tax payments, or a combination of both.
4. Life Changes Affect Tax Liability
Major life events can change a taxpayer's withholding needs.
Examples include:
- Marriage
- Divorce
- A new child
- Children no longer qualifying for tax benefits
- Retirement
- A new job
- Significant changes in income
The IRS recommends reviewing Form W-4 each year and whenever a taxpayer's personal or financial situation changes.
5. Tax Law Changes
The 2026 Form W-4 includes updates to reflect provisions from the One Big Beautiful Bill Act (OBBBA).
These changes include new withholding considerations related to:
- Qualifying tips
- Qualified overtime compensation
- Vehicle loan interest
- The senior deduction
- Charitable contributions for non-itemizers
While these provisions may create planning opportunities, they don't automatically change withholding. Taxpayers who want their withholding to reflect these changes generally need to submit an updated Form W-4. The IRS has also updated its Tax Withholding Estimator to account for several OBBBA-related provisions.
Why Form W-4 Reviews Create Valuable Planning Opportunities
Many tax professionals think about Form W-4 only when onboarding a new employee.
In reality, it can be one of the most valuable planning tools available.
A mid-year withholding review provides an opportunity to:
- Identify under-withholding before year-end.
- Reduce the likelihood of underpayment penalties.
- Improve cash flow by avoiding excessive withholding.
- Discuss changing income, deductions, and credits.
- Help clients make informed decisions before filing season.
Rather than reacting after a return has been filed, tax professionals can proactively help clients improve their tax position throughout the year.
Don't Forget the IRS Tax Withholding Estimator
When reviewing withholding, the IRS encourages taxpayers to use the IRS Tax Withholding Estimator to determine whether changes to Form W-4 may be appropriate. The estimator helps taxpayers compare their projected tax liability with current withholding and determine whether adjustments should be made.
For many clients, a quick withholding review today can prevent an unpleasant surprise next filing season.
Is There A Way To Learn More?
Of course! Withholding isn't a "set it and forget it" decision.
Changes in income, family circumstances, multiple jobs, tax law, or other sources of income can all affect whether enough federal income tax is being withheld throughout the year.
Helping clients review Form W-4 before year-end not only reduces surprises at filing time but also creates valuable opportunities for proactive tax planning and stronger client relationships.
If you want to better understand the 2026 Form W-4, withholding regulations, and practical planning strategies, take a look at our course:
2026 Withholding Checkup: W-4 Regulations, Planning, and OBBBA Updates
You'll learn how to identify common withholding issues, understand the latest Form W-4 changes, and apply practical strategies to help clients make more informed withholding decisions throughout the year. Click here to learn more.
