During open enrollment most people look at the monthly premium first, and the plan with the smallest number wins. The premium is only one piece of what you'll pay. Total cost for the year is the premium plus what you pay when you use care, and that second piece depends on how much care you use.

Take two plans. Plan A costs $200 a month, which is $2,400 a year, with a $5,000 deductible. Plan B costs $380 a month, which is $4,560 a year, with a $1,000 deductible. A deductible is what you pay before the plan starts sharing costs. After it, both plans use 20% coinsurance, meaning you pay 20% of each bill and the plan pays 80%, up to the out-of-pocket maximum. Those figures are made up for the example, so swap in your own.

In a year with $2,000 of care, Plan A costs you $2,400 plus $2,000, or $4,400. Plan B costs $4,560, plus the $1,000 deductible, plus 20% of the other $1,000, or $5,760. The cheaper premium wins by $1,360.

Now a year with $10,000 of care. Plan A is $2,400 plus the $5,000 deductible plus 20% of the next $5,000, which is $1,000. That's $8,400. Plan B is $4,560 plus $1,000 plus 20% of $9,000, which is $1,800. That's $7,360. The higher premium wins by $1,040.

A bad year looks different again. Say the care is bad enough to reach each plan's out-of-pocket maximum. If Plan A's is $8,000 and Plan B's is $4,500, Plan A costs $2,400 plus $8,000, or $10,400. Plan B costs $4,560 plus $4,500, or $9,060. The higher premium still comes out $1,340 ahead. Plan A also asks you to have up to $8,000 ready in a bad year, so check your emergency money using how big your emergency fund should be.

The break-even sits at about $3,700 of care. Below that, Plan A is cheaper, and above it Plan B is. So the question to ask is whether you expect more than $3,700 in care next year. Last year's explanation-of-benefits statements are the best evidence you have. A planned surgery, a pregnancy, a long-term prescription, or a kid who plays contact sports all point toward the richer plan.

Two things can change the math. If Plan A is a high-deductible plan that qualifies for a health savings account, you can save for the deductible with pre-tax money, as covered in the health account that gets three tax breaks, and your employer may add money to it. And check the network. If your doctor or your prescription isn't covered, the cheaper plan costs more. The terms are defined in deductible, copay, coinsurance, and out-of-pocket max.

I'm a licensed insurance broker, not a health plan advisor. Before the enrollment window closes, pull your explanation-of-benefits statements for the year and add up what care actually cost, since that total is the first number the comparison needs.