See the exact age gap between children.
The age gap between siblings is worth expressing in months as well as years, because at the younger end a few months changes a great deal — a 39-month-old and a 45-month-old are in different developmental places entirely. The two derived figures are the ones that affect family logistics. Years overlapping in school determines how long you have two sets of school runs, uniforms and holidays to coordinate, and it is simply the length of schooling minus the gap: a 3.75-year gap over 13 years of schooling means 9.25 years of overlap. Years until the younger starts school is the childcare horizon, which for most families is the larger financial number.
Sibling Age Gap
Gap months = |older age - younger age|; gap years = gap months / 12; school overlap = years of schooling - gap years; years to school = (school start age - younger age) / 12
Under two years is generally called a close gap, two to four years a moderate one, and over four years a wide gap. Each has different implications for childcare cost and for how much equipment can be reused.
It is the period with the highest logistical load — two school runs, two sets of uniforms and activities — but also the cheapest for childcare, since neither child needs paid daytime care.