Delegates Needed to Win the 2020 Democratic Nomination
Summary: The math behind how the Democratic National Committee calculates the number of delegates a candidate needs to win.
A candidate will need 1,991 of the 3,979 pledged delegates to win the Democratic nomination on the first ballot. Per the Democratic National Committee, a candidate needs a majority of those eligible to vote on the ballot. Most importantly for the calculation, the candidate needs "a whole unit of delegate above half."
Half of 3,979 is 1,989.5. As there are no delegates in this round with a half vote, a whole unit of delegate is one. Therefore, the requirement is 1,990.5 (1,989.5 + 1) delegates, which is rounded to 1,991.
If no candidate wins on the first ballot, all delegates become unpledged. There are 4,750 delegate votes on the second - and any subsequent - ballot. This total is comprised of the 3,979 formerly-pledged delegates from the first ballot as well as 767 automatic delegates with a full vote and 8 automatic delegates with a half vote.1 This means there are 775 automatic delegates with a total of 771 votes, with 4,750 equal to 3,979 + 771.
Since there are delegates with a half vote, a half vote is considered a whole unit of delegate for any ballot after the first round. Half of 4,750 is 2,375. Therefore, the requirement is 2,375.5 delegates to win the nomination when all delegates are voting.
Note that since automatic delegates are specific people or positions, the number can vary slightly - up or down - over time. For example, all Democratic members of the U.S. House are automatic delegates. If there was to be a new vacancy that remained unfilled at the time of the convention, there would be one less delegate in this category.
A Fine Point
If a candidate earns pledged delegates greater than a majority of all delegate votes (i.e., 2,375.5 or greater) during the primary and caucus contests, that person's nomination will be a foregone conclusion. In this scenario, all delegates will be able to vote on the first ballot. This outcome seems unlikely given the size of the field and the party's proportional allocation of delegates in each contest.