A gap that stays still and shrinks at once
Siblings, couples and friends with an age difference often notice that it feels huge when they are young and hardly matters later. That feeling has a simple explanation. The gap measured in days never changes, but the share of life it represents keeps falling. When a younger sibling is born, the older one may have lived twice, three times or ten times as long. Decades later they are almost the same age in proportion. The curve above shows that ratio, starting high and gliding down towards one.
Move the slider to any age of the younger person and the page tells you how old the older one was at that moment. It is a neat way to picture what life looked like for each of you at the same point.
How we calculate it
The gap is the number of calendar days between the two birth dates, and we also show it in years, months and days. The ratio at any moment is the older person’s age divided by the younger person’s. The older one is exactly twice the younger’s age on the day the younger has lived as many days as the gap, so we add the gap to the younger birth date. For the yearly quirk, we check every day of the current year and count the days when the whole-year difference is one bigger than usual.
| Younger person’s age | With a 3-year gap | With a 10-year gap |
|---|---|---|
| 3 | 2 times as old | 4.3 times as old |
| 10 | 1.3 times | 2 times |
| 30 | 1.1 times | 1.33 times |
| 60 | 1.05 times | 1.17 times |
Worked example. Someone born on 12 April 1990 with an older sibling born on 2 November 1984 has an age gap of 1,987 days, or 5 years, 5 months and 10 days. The older sibling was exactly twice the younger’s age on 20 September 1995. By 30 October 2026 the older one was only about 15% older in proportion, and when the younger turns 80 that will fall to about 7%.
Why it feels the way it does
Our sense of time is relative. A year is a fifth of a five-year-old’s life but a fiftieth of a fifty-year-old’s, which is one reason childhood gaps feel enormous and adult ones fade. Knowing the twice-as-old date is a fun fact to share with a sibling: it is the day the gap stopped being bigger than the younger one’s whole life.
Questions people ask
Does an age gap ever change?
In days it never changes: the older person is always the same number of days ahead. What changes is the proportion. A five-year gap is half a lifetime when one of you is five, but less than a tenth of a lifetime at sixty.
When was the older person twice as old?
Exactly when the younger person was as old as the gap. With a five-year gap, the older one was twice the younger’s age on the day the younger turned five. The page works out the precise date.
Why is the gap sometimes one year bigger?
For part of every year the older person has had their birthday and the younger has not, so the difference in whole years is one more than usual. The page counts how many days a year that happens for you.
Is anything saved?
Your own birthday is remembered on this device. The other date stays only while the page is open.
Sources. Gregorian calendar. Calculations run in your browser; nothing you enter is sent anywhere.