The short version
The calculator no longer invents daily birthday percentages from monthly patterns. Its primary rarity ranking now comes from historical daily U.S. birth records covering 1994 through 2014.
For every month-and-day combination, the source records are added together across the non-overlapping 21-year period. The 366 dates are then sorted from the lowest recorded total to the highest. A date near the low end receives a high rarity score; a date near the high end receives a low rarity score.
Where the birthday data comes from
The source series combines two public daily-birth datasets:
- 1994–1999: U.S. National Center for Health Statistics (CDC/NCHS) daily birth records.
- 2000–2014: U.S. Social Security Administration daily birth records.
Both files are published in FiveThirtyEight’s public births dataset. The site deliberately uses 1994–1999 from the CDC/NCHS file and 2000–2014 from the SSA file so the overlapping years are not counted twice.
Total source births used: 85,712,738. The public FiveThirtyEight data repository makes its datasets available under the Creative Commons Attribution 4.0 International license unless otherwise noted.
View the FiveThirtyEight births source files · View the repository and license information
Step 1: combine the same calendar date across the source years
Each daily source record contains a year, month, day of month, weekday, and number of births. The site groups records by month and day. For example, every June 7 in the source period is added together to create one June 7 total.
The same process is repeated for every valid date from January 1 through December 31, including February 29.
Step 2: rank all 366 dates from rarest to most common
The 366 totals are sorted from lowest to highest. On this site, rarity rank #1 means the rarest date in the historical dataset. The date with the highest recorded total sits at the common end of the ranking.
That means the ranking is based on observed source counts. There are no hand-added holiday penalties, seasonal boosts, or artificial country adjustments in the main rarity score.
Step 3: convert the rank into a rarity score
The site converts the date’s position among 366 dates into a simple score from 100 to 1. The rarest-ranked date receives the highest score and dates become progressively lower-scoring as they move toward the common end.
The score is meant to be easy to read at a glance. The page also shows the underlying rank, recorded birth total, and share of the historical source births so the score is not a mystery number.
Why February 29 needs special explanation
February 29 appears only in leap years. There are only five Leap Days in the 1994–2014 source period, so its total is naturally much lower than dates that appear in all 21 years.
That makes February 29 the rarest calendar birthday in the ranking, but it is not a like-for-like comparison with an annually recurring date. For that reason, the Rarest Birthdays page separately identifies the rarest date that occurs every year.
What does the birth year do?
Your birth year does not change the main month-and-day rarity rank. It is used to validate the date, identify your weekday of birth, calculate your next birthday and age, and recognize calendar details such as a birthday that fell on a moving holiday.
This keeps the rarity question simple: “How rare is this birthday date?” rather than trying to estimate how many people were born on one exact date in one exact year.
What the ranking can and cannot tell you
The calculator can tell you how your month and day compare with other dates in this particular historical U.S. dataset. It can show the recorded total, share of source births, rarity rank, rarity score, and the broader seasonal pattern.
It cannot tell you how many living people currently share your birthday, guarantee that the same rank applies in another country, or prove why a specific date has more or fewer births. Birth patterns change over time and can be influenced by many factors, including medical scheduling, holidays, seasonality, and population changes.
Why the site now uses one primary dataset
Earlier versions of the site offered six country-specific daily tables built from broader patterns. They were clearly labeled as modeled estimates, but they looked more precise than the underlying data justified.
The current approach is deliberately narrower. The primary rarity calculator uses one large, transparent daily dataset with named sources. If equally defensible daily records are added for another country in the future, they can be introduced as a separate documented dataset rather than as an inferred daily ranking.
Methodology questions
How many births are in the dataset?
85,712,738 recorded U.S. births from 1994 through 2014 are used in the primary ranking.
Does rank #1 mean rarest or most common?
On How Rare Is My Birthday, rank #1 means the rarest date. The ranking then runs toward the most common date at the opposite end.
Are the data current?
No. They are historical records from 1994–2014. The period is stated throughout the site so the results are not presented as current national birth totals.
Why not show a different rank for every country?
Reliable daily birth data are not equally available for every country. The site now prefers one well-sourced ranking over multiple precise-looking daily estimates built from weaker information.