What this calculator does
This age difference calculator finds the exact gap between two people's dates of birth, however they are related: siblings, colleagues, partners, or any two people you want to compare. Enter both dates of birth and it returns the gap in years, months and days, plus the total number of days and weeks apart.
Calculating an age difference by hand runs into the same trap as any date arithmetic: months have different lengths, so you cannot just subtract the year and month numbers and expect a clean answer. This calculator handles the borrowing across months and years automatically, the same way it works out a single person's age from a date of birth.
The formula
The earlier of the two dates of birth is treated as the reference point. Years, months and days are counted forward from that date to the later date of birth, borrowing a month whenever the day-of-month figure would otherwise go negative, and borrowing a year whenever the month figure would otherwise go negative. This mirrors how age itself is normally counted, just applied to two dates of birth instead of one date of birth and today.
| Term | Meaning |
|---|---|
| Age difference | The gap between two dates of birth, expressed as whole years, then whole months, then remaining days. |
| Total days apart | The full gap converted into a single number of days, useful for comparing gaps of different sizes directly. |
| Older / younger | Whichever date of birth comes first in time belongs to the older of the two people being compared. |
The inputs explained
| Field | What to enter |
|---|---|
| First date of birth | The first date of birth. |
| Second date of birth | The second date of birth. It does not matter which date is entered first; the calculator works out which one is earlier. |
When to use it
Working out the gap between siblings
Parents comparing two children's birth dates get a precise years-months-days gap rather than a rough subtraction of birth years, which can be a year out either way depending on the months involved.
Comparing ages at work or in a social group
Age gaps between colleagues, teammates or friends are often estimated from birth year alone. Entering both exact dates of birth gives the real gap, which can differ from the year-only estimate by close to a full year.
Checking an age gap against a rule or eligibility cutoff
Some competitions, insurance policies and family arrangements set rules around a maximum or minimum age gap. This calculator gives the precise figure to check against that kind of cutoff.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the age difference grows against a fixed birth date
One date of birth held fixed at 15 June 1990, against a range of second dates of birth spanning several decades.
| Second date of birth | Age difference | Total days apart | In weeks |
|---|---|---|---|
| 1988-01-01 | 2 years, 5 months, 14 days | 896 | 128 |
| 2000-06-15 | 10 years, 0 months, 0 days | 3,653 | 522 |
| 2010-06-15 | 20 years, 0 months, 0 days | 7,305 | 1,044 |
| 2020-06-15 | 30 years, 0 months, 0 days | 10,958 | 1,565 |
Questions
Does it matter which date of birth I enter first?
No. The calculator compares both dates and works out which one is earlier before calculating the gap, so the result is identical either way round.
Is this the same as an age gap calculator?
Yes. "Age difference" and "age gap" describe the same calculation: the span of time between two people's dates of birth. This calculator is deliberately general-purpose, suited to siblings, colleagues, partners or any other pair of people.
Why is the age difference in years sometimes different from just subtracting birth years?
Subtracting birth years alone ignores the month and day. Two people born in the same year but in January and December are only a matter of months apart, while two people born a year apart in December and January can be barely more than a day apart, despite the birth years differing by one.
Can I use this to check my own age instead of comparing two people?
This tool is built specifically for the gap between two dates of birth. For a single person's current age in years, months and days, use the dedicated age calculator instead.
For one person's current age in years, months and days, use the age calculator. For the plain number of days between any two dates, see the days calculator.