A client who owes tax every year almost always has a withholding election that stopped matching their income, not a run of bad luck. The fix starts with pulling the actual numbers: total tax liability from the prior return against total withholding from all W-2s, then finding the gap before touching the W-4 itself.

Why does a client owe tax every year?

A recurring balance due means withholding is consistently running below total tax liability, usually because the W-4 on file no longer reflects the household's real income or filing situation. The redesigned Form W-4, in place since 2020, has no allowances to miscount, so the more common cause now is a stale entry: a second job that started after the form was filed, a spouse who also started working, freelance income with no withholding at all, or a marriage or divorce that changed the filing status assumption baked into the payroll system.

Multiple jobs are the single most common driver, because each employer's payroll system withholds as if that job is the household's only income. Two W-2 jobs paying $60,000 each will each withhold at the rate appropriate for a $60,000 earner, when the household is actually taxed as a $120,000 earner. Without Step 2 of the W-4 completed (either checkbox 2c or the multiple jobs worksheet), the shortfall compounds all year and shows up as a balance due the following spring.

How should a CPA diagnose a withholding shortfall?

Start by comparing the prior year's total tax (Form 1040, line 22 or 24 depending on year format) against total withholding reported on all W-2s and 1099s, then repeat that comparison using current-year pay stubs annualized to a full-year estimate. That two-point comparison tells you whether the gap is growing, shrinking, or holding steady, which determines whether the client needs a one-time W-4 update or an ongoing pattern of mid-year checks.

Run the numbers through the IRS Tax Withholding Estimator using current pay stub data rather than guessing at a fix, since the tool accounts for multiple jobs, self-employment income, and the standard deduction changes that manual W-4 math tends to miss. The estimator output tells the client exactly what to enter on a new W-4: an additional per-paycheck withholding amount under Step 4(c), a dependent credit adjustment under Step 3, or a change to filing status assumptions.

Can underwithholding cause a penalty on top of the balance due?

Yes. A taxpayer who owes $1,000 or more after subtracting withholding and refundable credits from total tax can trigger the estimated tax underpayment penalty under IRC Section 6654, separate from the balance due itself. The safe harbor that avoids this penalty is paying in, through withholding and estimated payments combined, the smaller of 90% of the current year's tax or 100% of the prior year's tax (110% if the prior year's adjusted gross income exceeded $150,000, or $75,000 if married filing separately). A client who owes $3,000 might be fine on the balance due but still owe a penalty if withholding fell short of that safe harbor threshold.

When should a client update their W-4?

A client should submit a new Form W-4 to their employer whenever a major life event changes the household's income or filing picture: a new job, a spouse starting or leaving work, a marriage or divorce, or the birth or adoption of a dependent. Waiting until the following tax season to fix a known gap guarantees at least one more year of the same shortfall, since W-4 changes only affect withholding going forward from the pay period the employer processes the update.

The year-end true-up is where this shows up on the return: any shortfall not caught mid-year becomes the balance due at filing, plus a possible penalty if the safe harbor wasn't met. For clients who don't want to touch their W-4 mid-year (some employers make this a hassle, or the client just won't get around to it), estimated quarterly payments can close the same gap without a payroll change, though that shifts the fix from the employer's system to the client's own calendar.

What is the CPA's role versus the client's decision?

The CPA's job is to run the numbers, show the client exactly where the shortfall is coming from, and hand over a specific corrective number, not just a general warning to "check your withholding." Once the client sees the dollar gap tied to a cause (the second job, the freelance income, the life event), the fix usually stops feeling arbitrary and the client actually follows through. A vague instruction to "adjust your W-4" gets ignored; a specific instruction to add $85 per biweekly paycheck under Step 4(c) gets acted on.

Document the diagnosis in the client file each year this comes up, because a client who owes for three consecutive years with no documented conversation about it is a liability for the firm as much as it is a headache for the client. Showing the client the numbers, not just telling them there's a problem, is what actually changes behavior year over year.