What Does Box 12 Code M Mean on a W-2?
Uncollected Social Security tax on group-term life insurance. What it means, whether it is taxable, and what to do with it.
By the Easy Guides Editorial Team · Updated for 2026
Box 12 Code M is uncollected social security tax on group-term life insurance.
What Code M means
Social Security or RRTA tax your former employer could not collect on group-term life insurance coverage over $50,000. It only appears for former employees.
Does Code M affect my taxes?
You owe this tax, and it was not withheld because you were no longer on the payroll.
What to do about it
Report it with your return as the Form 1040 instructions direct. See also Code N, which is the Medicare half of the same situation.
Where to find Code M on your W-2
Look at box 12, in the middle-right of the form. It has up to four lines, labelled 12a, 12b, 12c and 12d. Those letters are only line labels. The code you want is the small letter or pair of letters printed next to the amount, and it will read M. If you have more than four entries, your employer issues a second W-2 to carry the rest.
Not the code you were looking for? The full box 12 code list covers every letter, and the W-2 box decoder explains every box on the form.