For siblings, twins, studies & matched comparisons
Age Difference Calculator
The exact gap between two birthdays.
In years, months and days.
Enter two dates of birth to see who is older and by exactly how much. Useful for siblings, matching students or research participants by age, or just settling a family argument.
- Who is older, and by how much
- Years, months and days
- Total months, weeks and days
- Names are optional
How far apart are they?
Enter two dates of birth to see the gap between them.
- Gap (Y;M.D)
- Older
- In weeks
Show calculation steps
Three steps
How to use the age difference calculator
Order doesn’t matter. The calculator works out who is older.
- Enter the first date of birth. Add a name if you like.
- Enter the second date of birth.
- Press Compare ages. You get the gap in years, months and days, total months and total days.
Worked example
Two siblings
Liam was born on September 28, 2016. Ava was born on June 15, 2018.
Line up the dates
From September 28, 2016 to September 28, 2017 is 1 year.
From there to May 28, 2018 is 8 months.
Count the rest
From May 28 to June 15, 2018 is 18 days.
So Liam is older by 1 year, 8 months and 18 days (1;8.18).
Where it helps
Who uses an age difference calculator?
Anywhere two ages need comparing exactly, not roughly.
- Researchers: checking that participants in two groups are matched by age.
- Clinicians and teachers: comparing a student with a sibling or a classmate.
- Parents: working out the exact gap between siblings.
- Couples and friends: answering “how much older are you, exactly?”
Still unsure?
Questions people ask
Tap a question to open it.
Does the order of the dates matter?
No. Enter them in any order. The calculator works out which date is earlier and says who is older.
How is the age difference calculated?
It counts whole years from the earlier birth date, then whole months, then the days left over, the same way chronological age is counted.
What if both people were born on the same day?
The difference is zero, and the calculator says they share a birthday. For twins born either side of midnight, the gap is 1 day.
Does the difference change over time?
No. The gap between two birth dates stays the same for life, even though both ages keep going up.
Are the names and dates saved?
No. Everything is calculated in your browser and nothing is sent or stored.