from official site: http://abc.go.com/primetime/dancing/scoring.html
The judges' scores and the public vote both carry equal weight in deciding the fate of our couples.
In the first two shows, we add together the judges' scores from both shows to give each couple a total out of 60 possible points. For every subsequent show, the judges' scores just come from the performances in that show and give each couple a total out of 30.
The public phone vote is gathered from phone calls to the voting lines that are open for one hour following the show. Sometimes we may rerun the show and reopen the lines to give more people the chance to vote. At present each phone line is limited to making a maximum of five calls although this limit may change. The other element of the public vote is the online vote via this site, ABC.com. Although it may appear that you can vote online many times, the site will only register one vote per email address. The online vote closes at midnight, Eastern Time, of the Tuesday preceeding each Wednesday night's show.
When both the phone and online votes have closed, we verify them and add them together to create a total public vote for each couple. We then have a confidential leaderboard of total public votes for each couple, with one couple having the most votes and descending in order of quantity. The couple at the top of this leaderboard are then given maximum RANKING POINTS. If there are six couples in the competition that week, the top-ranked couple will get six ranking points, the second-placed will get five, the third-placed four, etc. The last-placed couple will receive one ranking point. As the number of couples reduces, so does the maximum ranking points -- so when four couples are in the competition, the maximum points is four, when three are left, the maximum is three. These ranking points are kept secret as they will be combined with the judges' scores in the upcoming show to determine who is eliminated.
The judges score each couple's dance within the show, giving a total out of a possible 30 points. As the show progresses, a leaderboard is formed with the couples with the highest judges' points at top and the totals for each descending couple below. When the show has finished we will have all six couples ranked on the leaderboard. As with the public vote, each couple is then assigned a RANKING POINT based on where they stand on the leaderboard. The couple in top position get maximum ranking points (again the maximum points is the same as the number of couples competing that week) and everyone placed below gets one less ranking point. In the event that couples are tied on judges points, they will both recieve maximum ranking points available to them in that position. The couple (or couples) below this will revert to what their original ranking point position would have been irrespective of how many above them have been given higher points. An example of this is the ranking points that would have been assigned to the judges' scores at the end of the first show (if they were not being combined with the judges scores from Show Two):
Couple Judges' Score Ranking Points
Joey & Ashly 20 6
John & Charlotte 20 6
Rachel & Jonathan 20 6
Evander & Edyta 18 3
Trista & Louis 18 3
Kelly & Alec 13 1
Combining the Judges' Scores with the Public Vote
When the final judges' scores are in, and each couple has been assigned their ranking points, we then add on the ranking points each couple received from the previous week's public vote. Each couple then has a total number of ranking points. The couple that has the lowest total of ranking points from judges' scores and the public vote is eliminated from the competition. If one or more couples are tied on lowest ranking points, then the couple that has the lowest overall public vote between the tied couples is eliminated from the competition. In the very unlikely event that the one or more couples tied on the lowest ranking points are also tied on the number of public votes they have received, then our head judge, Len Goodman, will decide which of the couples he feels has been the weakest dancers in the competition so far, and they will be eliminated.