What Is the Most Common Birthday in the United States: September 9 Leads, With the Full Date-by-Date Pattern

September 9 is the most common birthday in the United States, based on the widely cited birth-date rankings compiled from Social Security Administration birth records and analyzed in the Harvard Data Science Review's "How Common Is Your Birthday?" dataset. Roughly 12,000 to 12,300 babies are born on that calendar date in a typical year, depending on the year window used. The answer comes with a condition: September 9 leads by a narrow margin over September 19 and September 12, and the ranking shifts slightly when you separate single-year counts from multi-year averages.

How the Most Common Birthday Is Defined and Measured

The figure behind this ranking isn't a survey of preferences or a hospital sample. It's a count of actual birth certificates: the Social Security Administration publishes counts of live births by date of birth for everyone issued a Social Security number, and those counts have been analyzed across multi-year windows to smooth out one-off spikes.

The Harvard Data Science Review analysis by data scientist Matt Stiles, built on SSA data covering 1994 through 2014, ranks September 9 first with an average of about 12,301 births per date, followed by September 19 at about 12,229 and September 12 at about 12,229 or slightly lower depending on the exact window. These are averages per calendar date, not single-year totals, which is why the figure lands near 12,000 rather than the roughly 11,000 daily average across the whole year.

Definition matters here. If you count by "most common birthday" as a date across all years pooled, September dates dominate because of a conception-timing effect explained below. If you count single years, individual dates can spike or dip due to scheduled inductions, holidays, and weather events.

A third definition is "most common birth month," and that answer is August, which has held the top spot in CDC natality data for most of the past two decades. August wins on monthly totals even though no single August date cracks the top five, because August births cluster across many days rather than concentrating on one.

The Top 10 Most Common Birthdays, Ranked With Approximate Counts

Here's the ranking that answers the query directly. Counts are averages per date from the SSA-derived dataset covering 1994–2014, rounded to the nearest hundred. Treat them as ranked order, not exact truth for any single year.

Notice the pattern: every date in the top 10 falls between September 9 and September 22. That's not a coincidence, and it's not a data artifact. It reflects a real conception-timing pattern in the U.S. population.

The gap between rank 1 and rank 10 is about 500 births per date, or roughly 4%. That's small enough that the ranking can flip between adjacent dates depending on which years you include. If you're citing a single date, September 9 is the safe pick; if you're citing a top-five list, all five are defensible.

Why September Dates Dominate: The Conception-Timing Explanation

The September peak traces back to conception. A baby born September 9 was conceived around mid-December, which lands in the window between Thanksgiving and Christmas and just after the winter holidays begin. The most commonly cited explanation is a mix of holiday-season conception patterns, colder weather keeping couples indoors, and a slight seasonal fertility shift.

There's a second explanation that gets less attention: scheduled deliveries. Late-August and early-September inductions and cesarean sections cluster around the Labor Day weekend and the start of the school year, which pulls some births forward from mid-September and pushes others into the first half of the month. That's why September 1–8 ranks lower than September 9–22: the early-September dates absorb scheduled deliveries that would otherwise have landed a week or two later.

The Labor Day effect is visible in the data. Births on September 1–7 run noticeably below the September peak, and births on August 30–31 fall sharply. Hospitals schedule fewer elective deliveries on weekends and holidays, so the dates immediately around Labor Day underperform.

A conception-timing story also explains why late September and early October dates stay high while November and December dates fall. Nine months after the winter holiday conception window is late September through early October, so the peak extends past September 22 rather than dropping off a cliff.

The Least Common Birthdays: Holidays and the Dates That Beat Them

The bottom of the ranking is just as instructive. The least common birthday is December 25, followed closely by December 24, January 1, and July 4. These are dates when hospitals schedule fewer elective inductions and cesareans, and when spontaneous labor is statistically less likely to coincide because there's no scheduled pull toward those days.

December 25 averages roughly 6,500 births per date, about half the September peak. The dates immediately around it — December 23, 24, 26, and January 1 — sit in the 6,500 to 7,500 range, then rebuild through January. July 4 sits near 7,500, and Thanksgiving Day (the fourth Thursday of November) also underperforms, though it moves year to year.

Leap Day, February 29, is the outlier that breaks the ranking entirely. Because it occurs only in leap years, the SSA count for February 29 is roughly one-quarter of what a typical date would show in a four-year window, around 3,000 per leap year averaged across four years. If you count only actual leap years, February 29 births run closer to 11,000–12,000, which puts it in the middle of the pack rather than at the bottom.

The holiday effect compounds with the weekend effect. Births on Saturdays and Sundays run about 10–15% below weekday births because scheduled deliveries cluster on weekdays. That weekend dip is why any ranking built on pooled dates slightly understates the true popularity of weekend-heavy dates.

How the Ranking Changes by State, Region, and Year

The national ranking is a useful anchor, but it doesn't hold perfectly at the state level. States with different holiday schedules, school-start dates, and climate patterns show their own peaks. Southern states, where school starts earlier in August, sometimes show a September peak shifted a week later than the national pattern.

The year window matters more than geography. A single year's data can be distorted by major weather events, hospital closures, or policy changes. Hurricane Katrina displaced hundreds of thousands of people from Louisiana and Mississippi in 2005, and births in the affected region that year show unusual date clustering. Pooling years, which is what the SSA-derived rankings do, smooths those distortions out.

If you need a state-level answer, the CDC's WONDER natality database lets you query births by date of birth, state, and year. The national ranking won't necessarily hold in your state, and the difference can be a full week.

A worked example makes the year-to-year variation concrete. Suppose the national September 9 average is 12,300 births per date, but a single year's count for that date lands at 13,100, about 6.5% above the pooled average. That spike doesn't mean September 9 became more popular; it means that year's conception window and scheduled-delivery pattern lined up differently. When you cite the "most common birthday," cite the pooled figure and name the window.

Common Birthdays Compared With Common Birth Months

Month-level rankings and date-level rankings answer different questions, and confusing them is the most common mistake people make. The most common birth month is August, which has held the top spot in CDC natality data since the early 2000s, with July and September close behind.

The reason a September date can win while August wins the month is distribution. August births are spread across 31 days with no single date dominating; September births concentrate heavily in the September 9–22 window, so one date can outrank any August date while August's monthly total still leads.

The day-of-week pattern adds another layer. Tuesday, Wednesday, and Thursday carry most scheduled deliveries, so if you're asking "what's the most common day to be born," the answer is a weekday, not a date. That's a different question with a different dataset, and mixing them up produces wrong answers.

Worked Example: Computing Your Odds of Sharing a Birthday

The ranking is more useful when you turn it into a probability. If September 9 averages about 12,300 births out of roughly 3.9 million births per year, that's about 0.32% of all births landing on that date. Any single date in the top 10 sits between 0.30% and 0.32%; the least common dates sit near 0.17%.

Here's the computation: 12,300 ÷ 3,900,000 = 0.00315, or 0.315%. Multiply by 100 to express as a percentage: 0.315%. Compare that to December 25 at roughly 6,500 ÷ 3,900,000 = 0.167%. The most common date is nearly twice as likely as the least common one.

The birthday-paradox calculation is different and gets misapplied constantly. In a room of 23 people, the chance two share any birthday is about 50.7%, but that's about any match, not a match on September 9 specifically. The chance that a specific person shares your September 9 birthday is roughly 1 in 317, based on 0.315% per random person. The 23-person figure comes from comparing every pair, not from the popularity of any single date.

If you want to compute your own odds of sharing a birthday with a random American, divide that date's average count by the annual birth total and invert. For September 9: 1 ÷ 0.00315 = 317. For December 25: 1 ÷ 0.00167 = 600. The math is simple; the input is the pooled count and the annual birth total, both of which change slowly enough that a two-year-old figure is still useful.

Hospital Scheduling and the Delivery-Date Effect on the Data

The date ranking isn't just about when babies are conceived. It's about when they're delivered, and that's increasingly a scheduling decision. Elective inductions and scheduled cesareans now account for a meaningful share of U.S. births, and those procedures cluster on weekdays, avoid holidays, and sometimes get pushed just past a weekend.

That scheduling pattern produces visible data signatures. Births on Saturdays and Sundays run 10–15% below weekday counts. Births on December 24, 25, and 26 fall sharply. Births in the first week of September dip around Labor Day. The September 9 peak sits safely after Labor Day weekend, which is part of why it outranks September 5 and September 6.

Hospital policy also shapes the ranking at the margins. Some hospitals restrict elective inductions before 39 weeks, which shifts some deliveries later in the month. Others have loosened those rules, which pulls deliveries toward earlier dates. The net effect on the national ranking is small but real, and it varies by state and hospital system.

For anyone using the ranking to answer a trivia question, none of this matters much. For anyone using it to plan a birthday party, a marketing campaign, or a data analysis, the scheduling effect is the piece that explains why the ranking looks the way it does and why a single year's data can look different.

How to Verify the Ranking Yourself With Public Data

You don't have to take the ranking on faith. The SSA publishes birth-date counts by year, and the CDC's WONDER natality database lets you query by date of birth, state, and year. Both are free and public. The SSA data supports year-by-year counts; the CDC data supports state and county breakdowns.

Start with the SSA's "Popular Baby Names" page, which links to the birth-date count files. Then pull the CDC WONDER natality dataset and query "Births by date of birth" for your state and year. Compare the two. The SSA counts everyone issued a Social Security number, which covers virtually all U.S. births; the CDC counts birth certificates, which is the same population but reported differently.

Expect small discrepancies. The SSA and CDC datasets occasionally disagree by a few hundred births per date because of reporting lags and corrections. If your goal is a citation for an article or a class project, cite the ranking and name both the source and the year window.

If you need the ranking for a specific use case, say a school project on your own birthday, query your birth year and your state. The pooled ranking is a national average; your actual birth-date count might be higher or lower depending on the year you were born.

FAQ

What is the most common birthday in the United States? September 9, with an average of roughly 12,300 births per date across the SSA-derived dataset covering 1994–2014. September 19 and September 12 are effectively tied for second at about 12,200. The margin over the tenth-ranked date, September 22, is only about 4%, so any ranking based on a shorter year window can shift the order slightly.

What is the least common birthday? December 25, averaging about 6,500 births per date, roughly half the September peak. December 24, January 1, and July 4 follow. The reason is scheduling: hospitals perform fewer elective inductions and cesareans on major holidays, and spontaneous labor doesn't cluster on those dates the way it does in mid-September.

Is September 9 more common than September 19? Barely. The two dates sit within about 100 births per date of each other in the pooled dataset, a margin under 1%. That's within the noise of year-to-year variation, so calling September 9 "the" most common date is defensible but not decisive. Treat September 9 through September 19 as a cluster rather than a strict ordering.

Why are so many top birthdays in September? Because of conception timing. A September birth means a December conception, which falls in the holiday-season window when conception rates rise. The Labor Day weekend also pushes scheduled deliveries away from the first week of September, concentrating births in the September 9–22 window.

What is the most common birth month? August, which has led CDC natality data for most of the past two decades. July and September follow. August wins on monthly totals even though no single August date cracks the top five, because August births spread across many days rather than concentrating on one.

Does the most common birthday change by state? Yes. States with earlier school-start dates and different holiday schedules show slightly shifted peaks, sometimes a week later than the national pattern. The CDC WONDER natality database lets you query births by date of birth and state to check your own state's ranking.

How is February 29 counted in these rankings? February 29 is a special case. In a four-year window, the SSA count for that date averages around 3,000 because it only occurs once every four years. If you count only actual leap years, February 29 births run closer to 11,000–12,000, which puts the date in the middle of the ranking rather than at the bottom.

How many people share the most common birthday? Roughly 12,300 babies are born on September 9 in an average year, so about 0.315% of annual U.S. births land on that date. That translates to roughly 1 in 317 odds that a random American shares a September 9 birthday. The birthday-paradox figure of 23 people for a 50% match is about any shared birthday, not this specific date.