Sibling Age Gap Calculator

A sibling age gap, also called birth spacing, is the time between the birth dates of two brothers or sisters. If your first child was born on 15 March 2018 and your second on 10 June 2021, the age gap is 3 years, 2 months and 26 days. This sibling age gap calculator takes two or more birth dates, puts them in order and works out the gap between each neighbouring pair, the average gap, the biggest and smallest gap, and the total number of days between births.

Parents use a birth spacing calculator to plan a family, to compare the gaps in their own family with friends and relatives, or just out of curiosity. Grandparents, genealogists and researchers use the same maths to build family timelines. The animated timeline under your result shows every sibling as a bar from the date of birth to today on one shared time axis, so you can see the gaps instead of only reading them.

1. Sort all the birth dates from the oldest sibling to the youngest.
2. For each neighbouring pair, subtract the years, then the months, then the days (younger sibling's date minus older sibling's date).
3. If the day difference is negative, borrow one month: subtract 1 from the months and add the number of days in the month before the younger sibling's birth month.
4. If the month difference is negative, borrow one year: subtract 1 from the years and add 12 to the months.
5. For the decimal gap, divide the number of days between the two births by 365.25 (the average length of a year, which allows for leap years).
6. The average gap is the sum of all gaps divided by the number of gaps. With n siblings there are n − 1 gaps.

Sibling A was born on 2018-03-15 and sibling B on 2021-06-10. Years: 2021 − 2018 = 3. Months: 6 − 3 = 3. Days: 10 − 15 = −5, which is negative, so borrow a month: months become 2 and days become −5 + 31 = 26 (May has 31 days). The gap is 3 years, 2 months and 26 days. In days: three years from 2018-03-15 to 2021-03-15 is 3 × 365 + 1 = 1,096 days (2020 is a leap year), plus 87 days from 15 March to 10 June (16 + 30 + 31 + 10), which gives 1,183 days. The decimal gap is 1,183 ÷ 365.25 = 3.24 years.

Birth dates 2010-09-01, 2013-01-20 and 2017-11-05. Gap 1 (first to second): 3 years, −8 months and 19 days. The months are negative, so borrow a year: 2 years, 4 months and 19 days, which is 872 days or 2.39 years. Gap 2 (second to third): 4 years, 10 months and −15 days. Borrow a month (October has 31 days): 4 years, 9 months and 16 days, which is 1,750 days or 4.79 years. The two gaps add up to 872 + 1,750 = 2,622 days (7.18 years) between the oldest and the youngest. The average gap is 7.18 ÷ 2 = 3.59 years, the largest gap is 4.79 years and the smallest is 2.39 years.

• Close gap: under 2 years. Medium gap: from 2 up to 4 years. Large gap: 4 years or more. In example 2, gap 1 (2.39 years) is medium and gap 2 (4.79 years) is large.
• Competition score = 10 − min(average gap, 10). A smaller average gap gives a higher number. For example 2: 10 − 3.59 = 6.41.
• Mentoring score = min(average gap × 1.5, 10). For example 2: 3.59 × 1.5 = 5.38.
• Parenting complexity score = 5 + min(number of gaps, 5). For example 2 there are 2 gaps, so 5 + 2 = 7.
• Overall score = (competition + mentoring + parenting) ÷ 3 = (6.41 + 5.38 + 7) ÷ 3 = 6.26 out of 10.

These scores are a rule of thumb that turns the numbers into plain-language talking points. They do not predict how your children will actually get along.

In the timeline each bar starts on the sibling's date of birth and ends today, so older siblings have longer bars and the empty space to the left of a bar is the gap before that sibling arrived. The gap table below it lists each neighbouring pair with the gap in years, months and days, the total number of days and the decimal years. Birth dates in the future are accepted too, which is useful when you are planning ahead with an expected due date.

• Planning: see what gap you would have if the next baby arrived on a given date.
• School planning: the gap between children shows how many school years apart they will be and when their milestones overlap.
• Family trees: record exact gaps between siblings, cousins or generations.
• Comparing families: line up your own gaps against those of friends or relatives.
• Curiosity and trivia: find the days between any two birthdays, including twins, who show a gap of 0 days.

• Twins and multiples have a gap of 0 days when they share a birth date, and they count as separate siblings in the average.
• The 365.25-day year makes the decimal gap very slightly different from the years, months and days breakdown, because months have unequal lengths.
• Adding a sibling with a date between two others splits one gap into two smaller ones, so the average gap goes down.
• Your birth dates are stored only in your own browser so the form is filled in next time. They are not sent anywhere by the calculation.