Background

The Employee’s Withholding Certificate (Form W-4) sets how much federal tax your employer withholds from paychecks. The IRS redesigned the W-4 in 2020 to reflect tax law changes; errors or life changes (marriage, a new job, or child care credits) can leave you underwithheld and exposed to penalties (see Form W-4 and instructions: https://www.irs.gov/forms-pubs/about-form-w-4).

Quick checklist to fix a W-4 mistake

  1. Confirm the shortfall. Compare year-to-date withholding on recent paystubs to projected tax liability using the IRS Tax Withholding Estimator (https://www.irs.gov/individuals/tax-withholding-estimator) or Publication 505 (Tax Withholding and Estimated Tax: https://www.irs.gov/publications/p505).
  2. Decide the remedy. Either increase withholding via a new Form W-4 (Step 4(c) lets you request extra dollar withholding per pay period) or make estimated quarterly tax payments (Form 1040-ES).
  3. Submit a revised W-4. Give the updated Form W-4 to your payroll or HR department — employers generally implement withholding changes on the next payroll after receiving a valid W-4 (employer guidance: https://www.irs.gov/forms-pubs/about-form-w-4).
  4. Recalculate and monitor. After changes take effect, re-run the IRS estimator or review paystubs to ensure projected underpayment disappears.

When to consider estimated tax payments

  • If you have income not subject to withholding (self-employment, investment, rental), estimated quarterly payments may be necessary to avoid penalties. Use Form 1040-ES worksheets or the IRS estimator.
  • If you have two jobs or your spouse works, adjusting withholding on the higher-paying job (or using the special multiple-jobs worksheet) can be simpler; see our guide on Federal Withholding for Multiple Jobs for more detail (https://finhelp.io/glossary/federal-withholding-for-multiple-jobs-how-to-avoid-underwithholding/).

Safe-harbor rules and penalties (brief)

To avoid an underpayment penalty, the IRS generally requires that you pay either 90% of the current year’s tax or 100% of the prior year’s tax (110% if your adjusted gross income was over $150,000) through withholding and estimated payments (IRS Pub. 505). If you expect to miss those thresholds, act now to increase withholding or make an estimated payment.

Practical example

A client (“Sarah”) discovered in March she had underwithheld because she left a second job off her W-4. We ran the IRS estimator, then filed a new W-4 at her current employer and added a specific extra withholding amount in Step 4(c) to spread the catch-up across remaining paychecks. She avoided a large April bill and kept her cashflow smoother.

Common mistakes to avoid

  • Waiting until tax time. Corrections are most effective when done as soon as you notice a change in income or family status.
  • Only relying on your last-year refund. A refund last year doesn’t guarantee correct withholding this year, especially after job changes or law updates.
  • Forgetting multiple income sources. If you and your spouse both work or you have side income, run the estimator for the full household.

Step-by-step sample W-4 actions

  • Single job, small shortfall: enter an extra dollar amount on Step 4(c) of Form W-4.
  • Multiple jobs or spouse works: use the estimator and follow the multiple-jobs guidance or check the checkbox in Step 2(c) if both jobs have similar pay.
  • Self-employment or large nonwage income: plan estimated quarterly payments with Form 1040-ES.

When to contact a pro

If your situation involves complex investments, self-employment, or large changes in income, consider a tax professional. In my practice, a short planning call to model withholding vs. estimated payments often prevents both surprise taxes and rushed year-end fixes.

Related reading

Authoritative sources

Disclaimer

This article is educational and does not replace personalized tax advice. For guidance tailored to your situation, consult a certified tax professional or CPA.