The U.S. birth records behind the results, what each number means, and how we account for the calendar.
85,712,738 recorded births from 1994–2014.
The checker, rankings, and month comparisons use recorded U.S. births from 1994 through 2014. FiveThirtyEight compiled these CDC/NCHS and Social Security Administration records. We combine them into 366 month/day totals.
The records cover January 1, 1994 through December 31, 2014. They contain 7,670 daily observations, with no missing dates. See the source files and coverage.
For a recent annual count and average, read how many U.S. babies are born per day . That article uses current NCHS annual reports rather than this historical daily series.
Your birthday rank
The checker ranks dates by total births. Rank #1 means the most births. Rank #366 means the fewest when every date has a different count. A higher rank number means a less common birthday.
Dates with equal counts share a rank. If two dates tie for first, the next date ranks third. On ranking pages, the births-per-occurrence option recalculates ranks using that measure.
Share of births and “1 in…”
A date’s share is its birth count divided by all births in the dataset. For example, 20 births out of 10,000 gives 0.2%, or 1 in 500. This is a mathematical example.
Displayed numbers are rounded. Calculations use unrounded values. Historical birth shares describe the counted records. They do not forecast a particular baby’s birth date.
What “rarer than” means
This percentage compares dates. “Rarer than 80% of other birthday dates” means 80% of the other dates have more births. It does not give an 80% probability or compare you with 80% of people.
Equal counts do not count as strictly more common. The selected date is excluded from its own comparison.
February 29 needs context
February 29 occurred 5 times in 1994–2014. Every other month/day occurred 21 times. Total-birth ranks include this calendar effect. A common year contributes no February 29 row because that date does not occur.
Births per occurrence divides a date’s total by its actual occurrence count. It answers how many births happened on an average occurrence.
Explore both measures on the rare birthdays page.
Putting months on equal footing
Monthly totals show births in each month. Daily means divide each total by actual covered days, including leap days. This lets you compare months of different lengths.
Compare monthly totals and daily averages.
Holiday and weekday research
The holiday comparison and weekday analysis use SSA records from 2000 through 2014 alone. Using one series avoids mixing administrative sources within those comparisons. Their coverage differs from the checker and main guides.
Weekday means divide births by actual days in each group. The holiday study compares four fixed dates with nearby dates on the same weekday. It reports coverage-edge exclusions and comparisons that exclude holiday controls. Each article gives its method and limits.
Download the research and guide comparison data. The file includes source coverage, counts, denominators, and alternative holiday comparisons.
Date guides use 1994–2014 totals and SSA 2000–2014 nearby same-weekday comparisons. Controls stay in the same year. Most use dates one and two weeks before and after. November 13 uses one week to avoid Thanksgiving Day and the Friday after it. Additional date studies exclude actual and standard observed federal holidays, Christmas Eve, December 26, and the day after Thanksgiving from controls. Each guide reports alternative windows and holiday overlap. The comparison divides summed target counts by summed annual control means. It does not estimate a causal effect. Download the date-guide data and comparison dates.
The Friday-the-13th study uses every actual SSA occurrence. It tests one-week and two-week windows separately and a predefined holiday-window exclusion. Download its events, annual comparisons and sensitivities.
The Memorial Day study uses each year’s actual last Monday in May. It compares births with nearby same-year Mondays, one and two weeks before and after. Holiday controls are excluded. One-week and two-week results are also shown separately. Download its dates and comparisons .
Annual weekday/weekend means divide group totals by actual covered days. Holidays retain their weekday classification. Leap-day and printable top-100 comparisons use the 1994–2014 baseline. Download these comparison data or the top-100 CSV .
Shared-birthday probability
The birthday paradox calculator compares two models for at least two people sharing a month and day. Both assume independent birthdays.
The classic model gives each of 365 dates equal probability and excludes February 29. The historical model uses 366 date shares from 85,712,738 recorded U.S. births in 1994–2014. It includes February 29. Both calculate the probability of all birthdays differing and subtract it from 1. Historical weights use total birth shares.
These are modeled probabilities. They do not measure matching in actual social groups. The comparison changes date weights and leap-day treatment together. Rounded values can appear certain. The calculator shows 100% only when the group exceeds the model’s possible date count.
Download the probability calculations and September ranking comparison. The article also explains someone matching your known birthday and links to the mathematical derivation.
For two specified people , the probability that they share any date is the sum of squared date probabilities. For three specified people , it is the sum of cubed probabilities. For a known birthday, one other person matches with that date’s probability. Two others both match with its square. These events differ from any match in a larger group.
For a group triple, calculate the probability that no date receives more than two people. Subtract it from 1. Multiply the coefficient of xⁿ in the product of (1 + pᵢx + pᵢ²x²/2) by n!. This gives the no-triple probability. Here, pᵢ is date i’s probability in the chosen model. The sibling article uses independent draws as a benchmark for separate births. Aggregate files do not identify families or measure actual sibling matching. Download the pair, triple, group and worked-example calculations.
Calendar calculators
The half birthday calculator offers six calendar months or the elapsed midpoint between chosen anniversaries. The month method uses the target month’s final day when necessary. The midpoint assumes equal-length days without a birth time. A 365-day interval has a half-day midpoint.
The golden birthday calculator adds the birth day number to the birth year. The golden age equals that day number. Both tools require a February 28 or March 1 choice for missing leap-day anniversaries. These are celebration conventions without legal-age authority. Download the worked examples .
Sources and attribution
The original daily files are published in FiveThirtyEight’s U.S. births dataset with source descriptions. CDC/NCHS means the Centers for Disease Control and Prevention’s National Center for Health Statistics. SSA means the Social Security Administration.
- CDC/NCHS daily birth counts Original file: 1994–2003. Years used here: 1994–1999.
- SSA daily birth counts Original file: 2000–2014. Years used here: 2000–2014.
The files overlap in 2000–2003. We use SSA alone for those years. This excludes overlapping CDC/NCHS rows and prevents double counting. We retain original files and calculate month/day and monthly summaries.
The files were retrieved on September 15, 2026 from revision 4c1ff5e3aef1 of the upstream repository. The retrieval date records when we obtained the files. The birth records end in 2014.
FiveThirtyEight makes its datasets available under Creative Commons Attribution 4.0 International (CC BY 4.0) unless otherwise noted. Its births documentation lists no separate exception. Attribution: U.S. birth counts from CDC/NCHS and SSA, compiled by FiveThirtyEight. Birthday Odds selects CDC/NCHS 1994–1999 and SSA 2000–2014. It calculates month/day and monthly summaries.
What these records can tell us
These results describe recorded U.S. births in 1994–2014. They do not describe worldwide birthdays, current birth rates, or everyone alive. Visitor selections retrieve historical records and never change rankings.
Administrative coverage differs between CDC/NCHS and SSA. Combining sources extends the study period. A difference between sources does not establish a population trend. Weekday patterns and selected years can affect comparisons.
A pattern on the calendar does not establish its cause or explain why an individual was born on a particular date.