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Caucus Math: An NBC News Primer

So, how does a caucus really work? Here's what you need to know before Monday's contest in Iowa.
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On Monday night, thousands of Iowans will gather in community centers, fire stations, libraries and town halls to have the nation's first say in who the Republican and Democratic presidential nominees should be.

But the famed Iowa caucuses don't work the same way as the typical Election Day ritual you probably know.

Rather than cast ballots in voting booths throughout the whole day, voters will attend evening community meetings where speaking out and trying to convince your neighbors to vote for your candidate isn't just allowed -- it's the name of the game.

Here's our in-depth primer on how the caucuses work -- and the differences between how Republicans and Democrats approach the process.


If you're caucusing as a Democrat, you might get a little bit of exercise in the process! Democrats move around the caucus site itself to show their support for a given candidate.

Hillary Clinton supporters, for instance, will gather in one corner; Bernie Sanders backers in another. At most Democratic caucus locations, a candidate must get support from at least 15 percent of attendees to achieve what's called "viability." If that threshold isn’t met, a candidate’s supporters must realign to a different candidate or to a category called “uncommitted.”

In other words, if at a caucus of 100 participants, only 10 support Candidate A, they're out of luck, and they have to pick another favorite.

After everyone's standing in their candidate's corner, the number of delegates awarded at each caucus site is then determined by a mathematical formula.

It works like this:



And wait, there's more.

The activity on caucus night doesn't directly elect delegates to the Democratic National Convention. The results actually elect delegates -- in each of Iowa’s 1,681 precincts -- to the county convention.

So, the person who we call the "winner" on Caucus Night is the candidate who accrues the most state delegate equivalents, a number calculated using a ratio of state-to-county convention delegates.

The Iowa Democratic Party says there will be a total of 1,406 state delegates. So, if you see for example that Hillary Clinton gets 55% on caucus night, that means she won 55% (or 773) of the state delegate equivalents.

Make sense?


By contrast, Republicans select their candidate via a simple, secret-ballot vote.

There is no shuffling from one corner of the caucus site to the other. There is no 15 percent viability or realignment. And there’s no mathematical formula to determine delegates awarded at each caucus site.

For example, say there are four candidates (Candidates A, B, C, D), 100 total attendees, and the site awards seven delegates.

Let's assume Candidate A gets support from 35 caucus-goers, B gets 20, C gets 30, and D gets 15.

Under that scenario:

Candidate A (35 x 7/100) = 2.45…rounded up to 3

Candidate B (20 x 7/100) = 1.4… rounded to 1

Candidate C (30 x 7/100) = 2.1… rounded to 2

Candidate D (15 x 7/100) = 1.05 … rounded to 1

NOTE: Candidate A’s 2.45 gets rounded up to 3 because the additional delegate gets awarded to candidate with the highest decimal point below 0.5.