When you start a new job in the United States, the main tax form you fill out is Form W-4, the Employee’s Withholding Certificate, which tells your employer how much federal income tax to take out of each paycheck. You’ll also complete Form I-9 to prove you’re authorized to work, and if your state has an income tax, a state withholding certificate as well. Everything else in a typical onboarding packet is either optional or handled by your employer.
What the W-4 Does
Federal law requires your employer to withhold income tax from your wages, and the W-4 is how they figure out the right amount.1Office of the Law Revision Counsel. 26 USC 3402 – Income Tax Collected at Source It doesn’t calculate your actual tax bill. It gives payroll enough information to estimate what should come out of each check so that when you file your return, you’re close to even. Withhold too little and you owe a lump sum in April. Withhold too much and you’ve been lending the government money all year at zero interest.
Your employer will hand you the form during orientation or send you to it through an onboarding portal. You can also download it directly from the IRS.2Internal Revenue Service. About Form W-4, Employee’s Withholding Certificate The old “allowances” system is gone. Since 2020, the W-4 uses a simpler step-based layout that tracks how your return is actually calculated.
How to Fill Out the W-4
The 2026 W-4 has five steps. Most people only need to complete two of them.3Internal Revenue Service. Form W-4, Employee’s Withholding Certificate (2026)
Step 1 is your personal information. Name, address, Social Security number, and filing status. Filing status (single, married filing jointly, or head of household) has the biggest single effect on how much is withheld, so pick the one that matches how you actually plan to file.
Step 2 covers multiple jobs or a working spouse. Complete it only if you hold more than one job at the same time, or you’re married filing jointly and your spouse also works. You have three choices: use the IRS Tax Withholding Estimator at irs.gov, fill out the Multiple Jobs Worksheet on page 3, or check the box in Step 2(c) if there are exactly two jobs total with roughly similar pay. Skip the whole step if it doesn’t apply.
Step 3 handles dependents. If your total household income will be $200,000 or less ($400,000 or less filing jointly), multiply each qualifying child under 17 by $2,200 and each other dependent by $500. Enter the total. This lowers your withholding to reflect the credits you’ll claim on your return.
Step 4 is optional adjustments. Line 4a is for non-job income like interest or dividends, line 4b is for itemized deductions above the standard deduction, and line 4c is for extra withholding per pay period. New hires who take the standard deduction and have no side income usually leave this blank.
Step 5 is your signature. The form isn’t valid without it.
The most common mistake is overthinking Step 2. If you have a single job and file single, you complete Step 1, sign in Step 5, and you’re done. The defaults are built for straightforward situations.
What Happens If You Don’t Turn It In
If you never submit a W-4, your employer doesn’t guess. The IRS requires them to withhold as if you were single or married filing separately with nothing filled in on Steps 2 through 4.4Internal Revenue Service. Topic No. 753, Form W-4, Employee’s Withholding Certificate For many people that pulls more tax than necessary out of each paycheck. For others, especially those with a working spouse or a second job, it pulls too little, and you can end up with an underpayment penalty at tax time.
The IRS charges an underpayment penalty if you owe more than $1,000 when you file and haven’t paid at least 90% of your current-year tax or 100% of last year’s tax through withholding.5Internal Revenue Service. Underpayment of Estimated Tax by Individuals Penalty Filling out the W-4 correctly on day one is the easiest way to stay clear of that.
When to Update Your W-4
Your W-4 isn’t set in stone. When your life changes (marriage, divorce, a new child, a spouse starting or stopping work), your withholding should change with it. If a change means you’re entitled to less withholding than you’ve been claiming, the IRS requires a new W-4 within 10 days.6Internal Revenue Service. Publication 505, Tax Withholding and Estimated Tax Changes that increase your withholding, like adding a dependent, can be made whenever you want.
You can submit a revised W-4 to your employer any time, as many times as you need. If you claimed exempt from withholding and your situation shifts so that you’ll owe tax after all, you must give your employer a new W-4 within 10 days.6Internal Revenue Service. Publication 505, Tax Withholding and Estimated Tax Updates take effect by the start of the first payroll period ending 30 or more days after you submit the revised form.
Form I-9: The Other Form Every New Hire Fills Out
The I-9 isn’t a tax form, but it goes in the same onboarding pile and you can’t skip it. Every U.S. employer must verify that each new hire is authorized to work, regardless of citizenship.7U.S. Citizenship and Immigration Services. I-9, Employment Eligibility Verification Section 1 is yours; Section 2 is your employer’s.
You must complete and sign Section 1 no later than your first day of work, meaning the day you actually start performing work for pay.8U.S. Citizenship and Immigration Services. Instructions for Form I-9, Employment Eligibility Verification You can fill it out earlier, after accepting the offer, but not before. You’ll provide your name, address, date of birth, and Social Security number, then check a box showing whether you’re a U.S. citizen, a noncitizen national, a lawful permanent resident, or a noncitizen authorized to work.9Office of the Law Revision Counsel. 8 USC 1324a – Unlawful Employment of Aliens
Your employer then finishes Section 2 within three business days of your start date, examining your identification documents in person or, if they’re enrolled in E-Verify, through a live video interaction under a DHS-authorized remote examination procedure.10U.S. Citizenship and Immigration Services. Remote Examination of Documents
Which Documents Work
You have two paths. Present one document from List A (which covers both identity and work authorization), or one from List B (identity only) plus one from List C (work authorization only).11U.S. Citizenship and Immigration Services. Form I-9 Acceptable Documents Everything must be unexpired.
The common List A documents are a U.S. passport or passport card and a Permanent Resident Card. Any one of these finishes the requirement on its own, and your employer can’t ask for anything more.11U.S. Citizenship and Immigration Services. Form I-9 Acceptable Documents Without a List A document, the usual combination is a state-issued driver’s license or ID (List B) plus an unrestricted Social Security card (List C).12U.S. Citizenship and Immigration Services. 13.0 Acceptable Documents for Verifying Employment Authorization and Identity A Social Security card marked “NOT VALID FOR EMPLOYMENT” or “VALID FOR WORK ONLY WITH DHS AUTHORIZATION” doesn’t qualify.
If a required document has been lost, stolen, or damaged, you can present a receipt showing you’ve applied for a replacement. The receipt is good for 90 days, during which you must show the actual replacement.13U.S. Citizenship and Immigration Services. Receipts If you can’t complete the I-9 or produce valid documents within three business days, your employer can terminate you, so it pays to have everything ready before your start date.
State Withholding Forms
If your state collects income tax, expect a state withholding form alongside the federal W-4. Most states use their own certificate, tailored to their tax brackets and deductions. A handful accept the federal W-4 for state purposes, and nine states have no income tax, so workers there skip this step.
Your employer or their onboarding system provides the right form. If you work in one state but live in another, you may need forms for both, depending on reciprocity agreements. State forms generally ask about filing status, dependents, and extra withholding, but the details differ enough that you shouldn’t assume your federal choices carry over.
If You’re Handed a W-9 Instead
If a company gives you a Form W-9 rather than a W-4, that’s a signal they’re treating you as an independent contractor, not an employee.14Internal Revenue Service. About Form W-9, Request for Taxpayer Identification Number and Certification The difference is big. An employee has taxes withheld from every paycheck. A contractor gets paid in full and is responsible for setting aside money for taxes, paying quarterly estimates, and covering both the employer and employee shares of Social Security and Medicare.
The W-9 collects your taxpayer identification number so the company can report what they paid you on a 1099 at year-end. Nothing is withheld from your pay unless you fail to provide your taxpayer ID, in which case the company must withhold 24% of each payment as backup withholding.15Internal Revenue Service. Instructions for the Requester of Form W-9 If you thought you were being hired as an employee and the company hands you a W-9, ask about your classification before signing anything.
Other Forms in the Onboarding Packet
Beyond the W-4 and I-9, your packet may include a direct deposit authorization to route your pay to your bank, and benefits enrollment forms for health insurance and retirement plans. Some of those benefit choices, like traditional 401(k) contributions, reduce your taxable income.
Some employers also include Form 8850, a pre-screening questionnaire for the Work Opportunity Tax Credit. The credit belongs to the employer, not you, but it needs answers from you about veteran status or participation in certain assistance programs. The employer should give you this form on or before the day of the job offer.
Keep a personal copy of every form you sign. If a question ever comes up about your withholding, your classification, or your work authorization, your own records make it far easier to sort out.