Kai Ryssdal: The rest of us have a good two and a half months 'til tax time. For Mitt Romney, it came last night.

The Republican presidential hopeful did what you can only call a midnight data dump. He gave his 2010 and 2011 returns to a couple of media organizations, then the candidate held a conference call this morning. The details you've probably heard: \$45 million or so in income the past two years, taxed at something less than 15 percent.

The U.S. tax code being a cumbersome beast, we've called an expert for help in figuring out what all the numbers really amount to. Tiffany Couch is a forensic accountant in Portland, Ore. Nice to talk to you again.

Tiffany Couch: Nice to talk to you.

Ryssdal: So Mr. Romney and this 13.9 percent effective tax rate that we've been reading about all day. What exactly does that mean?

Couch: That is a simple tax calculation where you take the total amount of tax Mr. Romney paid and you divide that by the total income that he earned.

Ryssdal: Simple math, right? Simple math.

Couch: It's simple math. In 2011, he earned approximately \$20 million. He paid a total amount of tax of approximately \$3 million. When you divide that \$3 million by the \$20 million, you get an effective tax rate for 2011 of about 15 percent.

Ryssdal: OK but I thought we have what we call a marginal tax rate system in this country -- that is, you have tiers and then the more you make, the more you pay on that extra money.

Couch: That's correct. We do, we have tax tables. If you look at the back of the booklet you get with your 1040 form, there are all those tax brackets that you see. If you make a certain amount of money, you have a certain amount of tax that you pay. And the more that you make, the more that you have to pay into the IRS.

Ryssdal: OK, but what happened here? Because Mitt Romney is like off the tax table, he's way up there.

Couch: Yes, he's way up there. But if you actually look at the types of income that he earned, it mostly came from capital gains, interest and dividend income, and all of that is taxed at a much lower or a capital gains rate for about 15 percent.

Ryssdal: All of this, though, we should point out, is entirely legal. I mean, he got some good accountants and said, 'listen, help me do what I've got to do.'

Couch: Well, you know, it's a couple of things. He's used his money to earn money and invest that money -- that is a great tax strategy. And not only is he growing his money obviously because he had gains, but his tax on those gains is much lower than had he earned that in wages.

Ryssdal: Everything we've been talking about, Tiffany, are federal tax rates, right? What are we missing here? There's state, there's local, there's Social Security, there's payroll taxes. I mean, there's a whole bunch of other stuff that we're not seeing in this Romney story today.

Couch: Well sure, it becomes quite complicated.

Ryssdal: Nuh-uh, tax codes? Complicated?

Couch: Oh, you know, maybe just slightly. There's state taxes, there are local taxes, we haven't even talked about Social Security and the payroll types of taxes. So there are lots of other taxes that Mr. Romney may be paying that we aren't privy to or really isn't part of this equation today.

Ryssdal: Tiffany Couch, a forensic accountant at the Acuity Group in Portland, Ore. Tiffany, thanks a lot.

Couch: Thank you.

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Follow Kai Ryssdal at @kairyssdal