# Birthday Odds > Birthday Odds lets people look up a month-and-day birthday, compare dates, and explore historical U.S. birth records compiled by FiveThirtyEight. The main checker and rankings combine CDC/NCHS records from 1994–1999 with SSA records from 2000–2014, without counting overlapping source years twice. The main dataset contains 85,712,738 recorded births across 7,670 calendar days and all 366 possible month-and-day birthdays. Rank #1 is the most common date by total recorded births; larger rank numbers mean fewer births. A date's share of births is different from the percentage of other dates it is rarer than. February 29 occurred five times in this period, compared with 21 occurrences of other dates, so total-birth and per-occurrence comparisons answer different questions. These historical U.S. records do not describe worldwide births, current birth rates, or the birthdays of everyone alive today. Calendar patterns do not establish why a person was born on a date. Some research pages use only the SSA 2000–2014 series or a probability model; check each page's stated method before comparing its numbers with the main rankings. ## Start here - [Birthday rarity checker](https://birthdayodds.com/): Look up a date, compare with a friend, and see its rank and share of recorded births. - [Sources and methodology](https://birthdayodds.com/methodology/): Source coverage, overlap handling, rank definitions, leap-day treatment, models, attribution, and limitations. - [Guides and research hub](https://birthdayodds.com/guides/): Browse the site's completed guides and studies. - [Most common birthdays](https://birthdayodds.com/most-common-birthdays/): All 366 dates, with ranking measures and a closer look at September. - [Rarest birthdays](https://birthdayodds.com/rarest-birthdays/): Full ranking, leap-day shares versus per-occurrence means, and a printable top-100 list with CSV. - [Birth months](https://birthdayodds.com/birth-months/): Compare month totals, shares, and averages per actual calendar day. ## Birthdays within a month - [January birthdays](https://birthdayodds.com/birthdays/january/): All 31 January dates and their month and overall ranks. - [February birthdays](https://birthdayodds.com/birthdays/february/): All 29 February dates, including leap-day context. - [March birthdays](https://birthdayodds.com/birthdays/march/): All 31 March dates, including March 17 against nearby same-weekday dates. - [April birthdays](https://birthdayodds.com/birthdays/april/): All 30 April dates and their month and overall ranks. - [May birthdays](https://birthdayodds.com/birthdays/may/): All 31 May dates, with a qualified Memorial Day overlap check for May 26. - [June birthdays](https://birthdayodds.com/birthdays/june/): All 30 June dates and a same-weekday check for June 8. - [July birthdays](https://birthdayodds.com/birthdays/july/): All 31 July dates, including the July 4 to July 7 gap. - [August birthdays](https://birthdayodds.com/birthdays/august/): All 31 August dates and the month-total versus daily-average distinction. - [September birthdays](https://birthdayodds.com/birthdays/september/): All 30 September dates and a comparison of its first and second weeks. - [October birthdays](https://birthdayodds.com/birthdays/october/): All 31 October dates and a same-weekday Halloween comparison. - [November birthdays](https://birthdayodds.com/birthdays/november/): All 30 November dates and context for dates around Thanksgiving. - [December birthdays](https://birthdayodds.com/birthdays/december/): All 31 December dates and the late-December pattern. ## Individual dates - [February 3 birthdays](https://birthdayodds.com/birthdays/february/3/): Test the rarest-birthday claim against its overall rank and nearby same-weekday births. - [December 31 birthdays](https://birthdayodds.com/birthdays/december/31/): Overall rarity and a year-end comparison whose result changes with Christmas Eve controls. - [December 25 birthdays](https://birthdayodds.com/birthdays/december/25/): Christmas Day rank, birth share, and 15 annual nearby-weekday comparisons. - [January 1 birthdays](https://birthdayodds.com/birthdays/january/1/): New Year’s Day rank and nearby-weekday comparisons, with same-year boundary limits. - [October 31 birthdays](https://birthdayodds.com/birthdays/october/31/): Halloween rank and 15 annual nearby-weekday comparisons. - [November 13 birthdays](https://birthdayodds.com/birthdays/november/13/): Fixed-date rarity, its sole Friday-the-13th event, and holiday-sensitive comparison windows. - [November 26 birthdays](https://birthdayodds.com/birthdays/november/26/): Fixed-date rarity versus actual Thanksgiving years and adjacent holiday-week dates. - [July 4 birthdays](https://birthdayodds.com/birthdays/july/4/): Independence Day rank and 15 annual same-weekday comparisons, including the exception. - [September 11 birthdays](https://birthdayodds.com/birthdays/september/11/): Birth share and nearby-weekday comparisons; no causal claim about the 2001 attacks. - [February 14 birthdays](https://birthdayodds.com/birthdays/february/14/): Valentine’s Day rank and nearby-weekday comparisons, separated from an external adjusted study. - [April 1 birthdays](https://birthdayodds.com/birthdays/april/1/): April Fools’ Day rarity and 15 annual nearby-weekday comparisons. - [January 8 birthdays](https://birthdayodds.com/birthdays/january/8/): Overall rarity and sensitivity to including New Year’s Day among controls. ## Research - [Birthday paradox](https://birthdayodds.com/research/birthday-paradox/): Any-pair calculator, worked no-match formula, and expected-pairs distinction; classic and historical models assume independent draws. - [Are birthdays evenly distributed?](https://birthdayodds.com/research/are-birthdays-evenly-distributed/): Compare 365 recurring dates with equal-daily-birth and weekday references in historical U.S. records. - [Birthday rankings by month](https://birthdayodds.com/research/birthday-rankings-by-month/): The most and fewest recorded births on a date in each month, with leap-day context. - [Birthday rankings over time](https://birthdayodds.com/research/birthday-rankings-over-time/): Compare full date rankings across three SSA five-year windows and a weekday sensitivity measure. - [Birthdays by day of month](https://birthdayodds.com/research/birthdays-by-day-of-month/): Compare the 1st through 31st by births per actual date in SSA records from 2000–2014. - [Holidays and birthdays](https://birthdayodds.com/research/holidays-and-birthdays/): Fixed-date comparisons using SSA daily records from 2000–2014. - [Most common birthday by year](https://birthdayodds.com/research/most-common-birthday-by-year/): Annual leaders and sensitivity checks using SSA daily records from 2000–2014. - [Thanksgiving births](https://birthdayodds.com/research/thanksgiving-births/): Moving-holiday comparisons using actual Thanksgiving dates in SSA 2000–2014 records. - [U.S. births per day](https://birthdayodds.com/research/us-births-per-day/): Current annual average from NCHS 2025 provisional and 2024 final birth totals, distinct from the site's historical date rankings. - [Weekday and weekend births](https://birthdayodds.com/research/weekday-weekend-births/): Day-of-week comparisons and annual weekday/weekend gaps, using SSA daily records from 2000–2014. - [Two people sharing a birthday](https://birthdayodds.com/research/two-people-same-birthday/): Specified-pair probability versus matching a known date or finding any pair in a group. - [Three people sharing a birthday](https://birthdayodds.com/research/three-people-same-birthday/): Fixed-triple probability and group thresholds, with equal-date and historical independent-draw models. - [Siblings sharing a birthday](https://birthdayodds.com/research/siblings-same-birthday/): Independent non-twin benchmarks, with explicit limits on applying aggregate records to families. - [Friday-the-13th births](https://birthdayodds.com/research/friday-the-13th-births/): All 25 actual occurrences in SSA 2000–2014 records, annual comparisons and holiday-control sensitivities. ## Optional - [About Birthday Odds](https://birthdayodds.com/about/): The purpose and scope of the site. - [Privacy](https://birthdayodds.com/privacy/): How the site handles birthday selections and other visitor data. - [Public analysis downloads](https://birthdayodds.com/methodology/#research): Source-derived research data and calculation files linked from the methodology page. - [Original daily birth files](https://github.com/fivethirtyeight/data/tree/4c1ff5e3aef1816ae04af63218015066e186c147/births): The pinned FiveThirtyEight source; the methodology explains which years are used and how the overlapping records are handled. - [Sitemap](https://birthdayodds.com/sitemap.xml): All current canonical pages.