Why is September 9 the most common birthday?
September 9 is first by total births in these historical U.S. records. It accounts for 259,228 births, or 0.3024% of all births recorded from 1994 through 2014.
The lead is close. September 9 has 1,247 more births than runner-up September 19, a margin of 0.48%. September 9 occurred 21 times during the study and averaged 12,344.2 births per occurrence.
| Birthday | Rank | Recorded births | Births per occurrence |
|---|---|---|---|
| September 9 | 1 | 259,228 | 12,344.2 |
| September 19 | 2 | 257,981 | 12,284.8 |
| September 12 | 3 | 257,913 | 12,281.6 |
| September 17 | 4 | 256,231 | 12,201.5 |
| September 10 | 5 | 255,991 | 12,190.0 |
The counts tell us which date leads and by how much. They cannot tell us why it leads or what happened in any individual pregnancy or delivery.
Why do September dates fill so many of the top spots?
September accounts for 9 of the top 10 birthday dates and 13 of the top 20 in this series. September 9 is part of a cluster, but the counts alone do not explain why the cluster occurs.
Birth patterns vary with the season, but the peak is not the same everywhere. A study of monthly births across U.S. states from 1931 through 2008 found regional differences in both timing and strength. Its authors said the mechanisms remain poorly understood. Their study helps put the September cluster in context; it cannot identify why September 9 tops this particular ranking.
Is September also the most common birth month?
By total births, August is first. September is first by average births per calendar day. August has an extra day each year, which matters when comparing totals. The birth-months guide shows both measures.
Try another birthday question
Use the birthday checker to find another date's rank and share. The birthday paradox calculator answers a different question: how likely is it that any pair in a room shares a birthday, compared with someone matching your own date?
Sources and limits
The ranking and table come from FiveThirtyEight's historical U.S. birth files: CDC/NCHS for 1994 through 1999, then SSA for 2000 through 2014. The methodology explains how we handle their overlapping years.
The figures describe births recorded in that period. They cannot give a current or worldwide birthday ranking or count how many living people share September 9.