The quick way people calculate a sibling age gap is subtracting one birth year from another. It’s usually close, but it’s often wrong by close to a year, and the reason is simple once you see it.
Why subtracting birth years goes wrong
Say one sibling was born in December 2015 and the other in January 2018. Subtract the years: 2018 minus 2015 is 3. But count the actual months between December 2015 and January 2018, and it’s only 2 years and 1 month, not 3. The birth month and day matter just as much as the year, and ignoring them can throw the answer off by nearly a full year in either direction depending on which sibling was born earlier in the year.
The accurate way is to subtract full dates, not just years: take the exact birth date of each sibling and count the years, months, and days between them the same way you’d count anyone’s age.
What counts as a close, moderate, or wide gap
There’s no official standard here, but a common informal way people group sibling age gaps is:
- Close gap: under 2 years apart.
- Moderate gap: 2 to 4 years apart.
- Wide gap: more than 4 years apart.
These aren’t scientific categories, just a common-sense way people talk about it. A 13-month gap and a 23-month gap are both “close” by this grouping even though one is almost twice the other, so if the exact number matters to you (for school-year planning, for example), the precise years/months/days figure is more useful than the category label.
A worked example
Sibling A was born March 10, 2012. Sibling B was born September 22, 2016.
- Years: 2016 minus 2012 = 4, but September is after March, so the full 4 years haven’t quite passed yet at the year boundary; counting properly gives 4 years and about 6 months.
- That’s a wide gap by the grouping above, even though a quick year-only subtraction might round it to “4 years” and undercount the extra 6 months.
Our Age Gap Calculator does the full date-to-date subtraction automatically, so you get the exact years, months, and days between any two people, siblings included, without doing the year-only shortcut that can be off by months.
Frequently Asked Questions
How do you calculate the age gap between siblings?
Count the years, months and days between the two full birth dates rather than subtracting birth years. Year-only subtraction can be off by almost a full year.
What is considered a close sibling age gap?
There’s no official definition, but a gap under 2 years is commonly called close, 2 to 4 years moderate, and more than 4 years wide.
What birth spacing do health organisations recommend?
The World Health Organization recommends waiting at least 24 months after a live birth before trying to conceive again, to reduce health risks for mother and baby.