Getting a health insurance through the public marketplace? Know How to avoid Tax surprise.

About 12.9M people enrolled in health insurance through the public exchange are eligible for subsidies that reduce their monthly premiums.

At tax time, you have to calculate whether those advance tax credits were accurate, or whether you should have received a different amount.

Any shortfall would decrease your bill or boost your refund, while Any excess received gets tacked on to what you owe, or it reduces your refund.

Kristin Esposito, Director for Tax policy said, "You really should go into and take the steps to change your estimate so they can revise the subsidies ASAP"

Esposito said a drop in income should also be reported — which could result in you getting bigger monthly subsidies.

Make sure your account reflects other life changes, too, including marriage or a new member of your household.

Changing your information generally involves calling the exchange or going to your online account and updating your application.