Short version: We analyze historical birth frequency data for every day of the year, normalize it against total annual births, and classify each date into one of five rarity tiers. Every result is a statistical estimate, not an exact count. Full details below.
Data Sources
Our birthday frequency data is derived from multiple aggregated, publicly available sources:
- CDC National Vital Statistics System — US birth certificate data providing daily birth counts going back several decades
- Harvard T.H. Chan School of Public Health — Published research on birthday frequency distributions across the US population
- Social Security Administration records — Supplementary data on birth date distribution patterns
- International demographic databases — Cross-referenced to ensure global applicability, not just US-centric estimates
- Academic peer-reviewed research — Published studies on seasonal birth patterns, holiday birth avoidance, and Leap Day statistics
We do not use a single dataset. Cross-referencing multiple sources allows us to identify and correct for anomalies in any one dataset, improving the reliability of our estimates.
Calculation Steps
Raw Frequency Collection
We collect the estimated number of births occurring on each of the 366 possible calendar dates (including February 29) across our source datasets, normalized to a common base year.
Multi-Source Averaging
Birth frequencies from each source are weighted and averaged. Sources with broader population coverage receive higher weighting. This reduces the impact of regional biases in any single dataset.
Leap Day Adjustment
February 29 is handled separately. Because it only occurs in leap years (approximately every 4 years), its effective frequency is divided by 4.25 (accounting for the occasional skipped leap year in century years) relative to other dates.
Relative Frequency Scoring
Each date's frequency is expressed as a percentage of total annual births. We then calculate a rarity score (0–100) using a logarithmic scale, so that meaningful differences across the rarity spectrum are visible rather than compressed at the extremes.
Rarity Classification
Scores are mapped to five classification tiers based on their position in the birth frequency distribution. Thresholds are set at statistically meaningful breakpoints, not arbitrary round numbers.
Global Population Extrapolation
The "number of people who share your birthday" estimate is calculated by applying the date's frequency percentage to the current estimated world population. This is an approximation — birth patterns vary by country and region.
Rarity Tier Thresholds
The five classification tiers are defined as follows, based on a date's share of total annual births:
| Tier | Birth Share | Approx. 1-in-X | Example Dates |
|---|---|---|---|
| Ultra Rare | < 0.08% | 1 in 1,250+ | February 29 |
| Rare | 0.08% – 0.22% | 1 in 455–1,250 | Dec 25, Jan 1, Jul 4 |
| Uncommon | 0.22% – 0.27% | 1 in 370–455 | Various winter dates |
| Common | 0.27% – 0.32% | 1 in 312–370 | Various summer dates |
| Very Common | > 0.32% | 1 in 312 or less | September dates |
Known Limitations
- Geographic variation: Our primary data is weighted toward US birth patterns. Birth frequency distributions differ meaningfully in other countries and regions. A date that is rare in the US may be common in another country with different holiday calendars.
- Historical shift: Birth patterns have changed over time due to advances in planned delivery medicine. More recent birth data is weighted more heavily, but our estimates represent a blended historical average rather than a single-year snapshot.
- Rounding and estimation: All figures are statistical estimates. The margin of error is smaller for very common dates (where large data samples exist) and larger for very rare dates.
- Year of birth: The year you enter allows us to show contextual information but does not significantly change your rarity classification, which is based on the calendar date alone.
- Name vs. exact count: When we say "X million people share your birthday," this is a global estimate, not a verified headcount. No public database tracks every living person by birth date in real time.
Accuracy Commitment
We are committed to transparent, honest communication about what our tool can and cannot do. We will never overstate the precision of our estimates. If you identify an error or have a question about a specific result, please contact us through the Contact page and we will investigate.