Under the Affordable Care Act, Box 12 and a two-letter code, DD, signify that the number there will tell you exactly what you and your employer spent on premiums for health coverage.

“It’s going to be an eye-opener for a lot of people,” said Jerry Nebbia, a health benefits expert at Mercer, a private benefits exchange. “A lot of people have no idea what the true cost is.”

The W-2 reporting requirement for insurance will expand next year to include employers with fewer than 250 on the payroll.

The health benefit amount isn’t taxable as personal income — for now, anyway. But it is insight into your employer’s total cost of your compensation.

It also is a close reflection of what you would pay if you lost your employer subsidy and wanted to continue the coverage under COBRA.

In the workplace at large, the cost of employer-paid benefits equals nearly 31 percent of total employment costs, according to the U.S. Bureau of Labor Statistics. Of that, health insurance costs account for 7.7 percent of employer costs in private industry and 11.7 percent in state and local government.

For some workers, employer-sponsored insurance is a hefty benefit amounting to $10,000 or even $20,000 a year.

Last year, according to the Kaiser Family Foundation’s survey, employer-sponsored health insurance cost an average of $5,615 for individuals and $15,745 for families.

The requirement to include the full cost on W-2s was conceived partly to make employees more aware of the cost of their coverage. Often, employees with subsidized coverage are paying only one-fourth of the full premium cost.

Knowing the real price is important because workers are being asked to be smarter consumers of health care, to make more coverage choices, or to shoulder more of the cost.

Knowing the full cost of health benefits also might help explain why pay raises are smaller than employees would like; sometimes, employers are putting more money into health insurance instead of direct wages and salaries.

If you see the Box 12 information, you might have to do some extra work to figure out what your employer’s cost was. You might need to look at your final pay stub from 2012 and subtract what you paid for your health insurance from the new DD amount. The difference is your employer’s share.

Of note: This box shows only premium costs. It does not reflect anything you spent on out-of-pocket health care expenses, your co-payments or deductibles. It also doesn’t include money in health savings accounts, and it isn’t required to include amounts spent on separate dental or vision plans.

Though your employer’s health care contribution isn’t counted as taxable income now, the Box 12, DD information lays the groundwork for future tax law changes if Congress decides the value of that employee benefit should be taxed.

The tax break for employer-paid health benefits equals about $180 billion a year in potential tax revenue. Analysts say that is worth 80 percent more than the tax break for home mortgage interest deductions.