How to Read a W-2, Box by Box
Why box 1 and box 3 disagree, what box 12 codes are, and how twenty-four pay stubs become one form. Worked on one practice year.
A W-2 is not written in January. It is the year-to-date column of your last pay stub, sorted into boxes that deliberately count different things. Read it in this order: box 1 is wages after everything that dodged income tax. Boxes 3 and 5 are wages after only the things that dodged Social Security and Medicare, which is why they usually disagree with box 1. Boxes 2, 4 and 6 are what was actually sent in against each of those three taxes. Box 12 is a set of coded footnotes. On the practice year below, box 1 reads $54,000.00, boxes 3 and 5 read $56,400.00, and the $2,400.00 difference has exactly one cause.
The practice year
One salaried worker, paid twice a month, $2,500.00 of gross pay per check. A pre-tax health premium of $150.00 a check leaves before any tax is figured. Retirement contributions across the year total $2,400.00. That is the entire setup, and every box below falls out of it.
The worked example: twenty-four stubs into one form
| Step | Working | Amount |
|---|---|---|
| 1. Gross for the year | $2,500.00 times 24 checks | $60,000.00 |
| 2. Health premium leaves | $150.00 times 24 | −$3,600.00 |
| 3. Boxes 3 and 5 are set | the wages those two taxes may count | $56,400.00 |
| 4. Retirement contributions leave | dodge income tax, never dodge the payroll tax | −$2,400.00 |
| 5. Box 1 prints | federal taxable wages for the year | $54,000.00 |
One salary, two honest answers. The health premium came off both bases. The retirement money came off one. That is the whole reason the boxes disagree, and it is printed on the form as a fact rather than explained anywhere on it. That mechanism is the single most useful thing to know about a paycheck, and it is why the two wage figures on this form disagree for an honest reason.
The boxes that matter, in plain English
| Box | What it holds | This practice year |
|---|---|---|
| 1 | wages subject to federal income tax | $54,000.00 |
| 2 | federal income tax already sent in | $4,300.08 |
| 3 | wages Social Security may count | $56,400.00 |
| 4 | Social Security tax withheld | $3,496.80 |
| 5 | wages Medicare may count | $56,400.00 |
| 6 | Medicare tax withheld | $817.92 |
| 12 | coded footnotes, including retirement contributions | $2,400.00 |
Boxes 3 and 5 hold the same figure here because this practice household sits well below the wage ceiling that applies to the Social Security portion. Above that published ceiling they part company, because one of the two taxes stops for the year and the other does not. The ceiling for the current year is published at ssa.gov and irs.gov and is re-set annually.
The tax boxes are sums, not percentages
This is the check that confuses people most, so it is worth doing slowly.
- Box 4 is twenty-four rounded Social Security lines added together: 24 times $145.70 is $3,496.80.
- Box 6 is twenty-four rounded Medicare lines: $817.92.
- Together that is $4,314.72.
- A flat 7.65% of $56,400.00 would be $4,314.60.
- The difference is $0.12.
Payroll rounds every check to the cent and never rounds the year, so twenty-four small roundings add up to a few cents of difference. That is not an error and it does not need fixing. The twenty-four stubs are what add. The FICA line has its own page if you want the rest of that machinery.
Box 12 is a footnote, not a wage
Box 12 names money that has to be reported but does not belong inside a wage box. Each entry is a letter code plus an amount. On this practice year, $2,400.00 of retirement contributions sits there under one code, and that same $2,400.00 is exactly the gap between box 1 and box 3.
What each letter means is published in the W-2 instructions at irs.gov, and the list changes. Do not guess a code from memory or from a forum, because a guessed code is a wrong code, and a wrong code in the wrong place on a return produces a letter a year later.
The five-minute check worth doing
- Find the last pay stub of the year. Its year-to-date column should reproduce most of the form.
- Compare gross year-to-date against box 1. The difference should equal your pre-tax deductions for the year. If it does not, one of the two is describing something you have not accounted for.
- Compare box 3 against box 1. If they differ, the difference should be the deductions that dodge income tax only. On this file that is $2,400.00.
- Check your name, your address and the last digits of your identifying number. Boring, and the single most common cause of a genuinely painful spring.
- Check box 2 against what you expected. That is what you sent in, not what you owe, and the difference between those two is the whole subject of a refund.
If any of the five does not reconcile, the correct next step is your payroll office, not a tax forum. Employers issue corrected forms; that is a normal process rather than a crisis.
Where the form goes next
Box 1 from every W-2 in the household adds up to the top of a return. Box 2 from every W-2 adds up to what was already paid, which is compared against the finished tax right at the bottom. Everything between those two points is the return itself, and one complete practice return, worked line by line follows the same invented household all the way through.
The other document that feeds the same picture is the stub the form was built from, and reading one properly is the fastest way to stop January being mysterious. Reading a pay stub line by line is where this whole chain starts.
Gross year-to-date, minus your pre-tax lines, should land on box 1. Five minutes, and it catches almost everything that goes wrong with this form.
What to do if a form is late, wrong or missing
Three situations, three different answers, and none of them is panic.
- Late. Employers have a published deadline for getting these out, and it is at irs.gov along with what to do if the date passes. Start with the employer.
- Wrong. Ask the employer for a corrected form. This is a routine process with its own form number, not an accusation, and payroll offices handle it every year.
- Missing because the employer has gone. That is the one case where the published route at irs.gov genuinely matters, and it is worth reading rather than improvising.
In all three cases the year-to-date column on your last stub is the evidence you already hold, which is the practical reason to keep the last stub of every year somewhere you can find it.
Why the form has more than one wage box at all
It is easy to read the disagreement between box 1 and box 3 as a design flaw. It is the opposite. Three different taxes with three different rules need three different bases, and a form that printed one wage figure would be hiding two of them.
The same logic runs through the rest of the system. Every tax on a household has its own definition of income, its own set of things that come off first, and its own published rules. A form that shows the bases separately is a form that lets you check the arithmetic. One that flattened them would look simpler and would be unusable.
Going deeper
How Taxes Work spends a full chapter on this one document and then follows the same practice household through an entire return, line by line, with every figure computed in code. It names no software, no preparer and no product, and it says on every page that it is education rather than tax advice.
For anything about your own return, irs.gov holds the current rules and a tax professional who can see your paperwork is the right person to read it.
Questions people actually ask
Why is box 1 different from box 3 on my W-2?
Because they count different things on purpose. Box 1 is wages after every deduction that dodges income tax. Boxes 3 and 5 are wages after only the deductions that dodge Social Security and Medicare. On the practice year here, box 1 reads $54,000.00 and boxes 3 and 5 read $56,400.00. The $2,400.00 gap is exactly the year's retirement contributions.
Where does the W-2 get its numbers from?
From the year-to-date column on your own pay stubs. The form computes nothing new. It sorts what payroll added up all year into boxes with published definitions, which is why the last stub of the year is the best cross-check you have.
What is box 12 on a W-2?
A set of coded footnotes naming money that has to be reported but does not belong in a wage box. On the practice year, $2,400.00 of retirement contributions sits there under a letter code. What each letter means is published in the W-2 instructions at irs.gov and it changes, so a guessed code is a wrong code.
Why does box 4 not equal exactly 6.2% of box 3?
Rounding, and it is normal. Payroll rounds every single check and never the year. On the practice year, twenty-four rounded lines add to $4,314.72 while a flat rate on the yearly wage would give $4,314.60. That is $0.12 of difference and it is arithmetic, not an error.
Is this tax advice?
No. This is general financial education about how a document is built. It is not financial advice, not tax advice or investment advice, and not a recommendation about your return. Current rules, box definitions and limits are published at irs.gov and change annually, and a tax professional is the right person for your own return.