What the Age Difference Calculator does
This calculator gives the exact age gap between two people from their dates of birth - in years, months and days, and as total days and weeks - and says who is older. It also answers the follow-up questions people usually have: how old the older person was when the other was born, when the older one was the age the younger one is today, and the day one was exactly twice the other's age.
The gap between two birthdays is fixed for life. Saying "three years apart" is a rounded version of it; the calendar answer, such as 3 years, 8 months and 19 days, is what you need for a family tree, a school-year question, a visa or pension form, or simply settling an argument. Everything is worked out in your browser from the dates you type; nothing is sent anywhere.
How to use it
- Enter the first person's date of birth and, if you like, a name. Names only make the answer easier to read.
- Enter the second person's date of birth. The order does not matter - the calculator works out who was born first.
- Read the headline gap, then the totals in days, weeks and months underneath.
- Open "Ages on a different date" if you want both people's ages on a date other than today, for example on a wedding day or at the start of a school year.
- Copy the answer as a sentence, or print the page.
Reading the results
The headline is a calendar difference, counted the way people count birthdays: whole years first, then whole months, then the days left over. A month here is a calendar month, not 30 days, so the same number of days can read slightly differently depending on which months it spans.
"Was X old when the other was born" is the gap itself. If Ben is 3 years, 8 months and 19 days older than Asha, Ben was exactly that age on the day Asha was born, and Asha arrived that long after Ben.
The "twice as old" date is counted in days: before it the older person was more than twice the younger's age, after it less. The ratio keeps falling towards one while the gap in years never changes - which is why a 10-year gap feels huge at 5 and 15 and unremarkable at 60 and 70.
Worked example: Ben, born 15 May 1990, and Asha, born 3 February 1994
From 15 May 1990 to 15 May 1993 is 3 whole years. From 15 May 1993 to 15 January 1994 is 8 more whole months. From 15 January to 3 February is 16 days to the end of January plus 3, so 19 days. Ben is 3 years, 8 months and 19 days older.
In days: 15 May 1990 to 15 May 1994 is four years including one 29 February (1992), so 1,461 days. 3 February to 15 May 1994 is 28 + 31 + 30 + 12 = 101 days. The gap is 1,461 - 101 = 1,360 days, which is 194 weeks and 2 days.
On 20 October 2026 Ben is 36 years, 5 months and 5 days old and Asha is 32 years, 8 months and 17 days. Ben was Asha's current age on 1 February 2023: 32 years and 8 months after 15 May 1990 is 15 January 2023, plus 17 days.
Formulas and scoring rules
- Whole months between two dates
m = the largest whole number with add_months(older_dob, m) <= younger_dobadd_months keeps the day of the month and clamps to the last day of a shorter month: 31 January + 1 month = 28 or 29 February.- Years, months and days
years = floor(m / 12); months = m mod 12; days = younger_dob - add_months(older_dob, m)- Totals
total_days = younger_dob - older_dob; weeks = floor(total_days / 7), remainder days = total_days mod 7- Gap in decimal years
years = total_days / 365.2425365.2425 is the average length of a Gregorian calendar year. Shown to two decimal places.- Older person was the younger one's current age on
older_dob + younger's age (years, months, days)- Exactly twice as old on
day = 2 x younger_dob - older_dob (in day numbers)Solves (day - older_dob) = 2 x (day - younger_dob).
How this differs from the Age Calculator
The Age Calculator answers "how old am I?" for one person. This page compares two people: the gap between them, who is older, and how old each was relative to the other. Use the Age Calculator for a single exact age and this one when the question is about the difference.
Birthdays on 29 February and at the end of a month
Someone born on 29 February has no birthday in three years out of four. Calendar arithmetic, and this calculator, treats 28 February as the anniversary in those years, so a person born on 29 February 2000 is counted as one year old on 28 February 2001. Some legal systems use 1 March instead; if a form you are filling in specifies a rule, follow the form.
The same clamping applies to the 31st: one month after 31 January is the last day of February. That is why the days part of a gap can look one or two days different from what you might get by assuming every month has 30 days.
Limitations: what the result does not prove
- It works from calendar dates only. Two people born on the same date are the same age here even if one was born at 00:05 and the other at 23:55.
- All dates use the Gregorian calendar. Dates before a country adopted it (1752 in Britain and its colonies, later elsewhere) may be recorded differently in old documents.
- The 29 February convention (28 February in common years) is the usual one but not universal; a legal deadline or eligibility rule may define it differently.
- It does not judge whether an age gap is appropriate for anything - it only measures it.
Privacy: where your data goes
Everything you paste, type or drop is processed in this browser tab. It is not uploaded, logged, stored or sent to analytics. Session recording and tag-manager scripts are switched off on this page.
Standards and sources
- ISO 8601 - Date and time format (calendar dates YYYY-MM-DD)
- Gregorian calendar - leap year rule and history
Frequently asked questions
How do I calculate the age difference between two people?
Take the two dates of birth, count whole years from the earlier one, then whole months, then the days left over. Here you only enter both dates: the calculator finds who was born first and gives years, months and days plus the totals in days and weeks.
Does the order I enter the birth dates in matter?
No. The calculator always measures from the earlier birthday to the later one and tells you who is older, so entering the younger person first gives the same gap. There is also a Swap button if you want the names to appear the other way round.
How many days older is my sibling than me?
Enter both dates of birth and read Total days. It counts every calendar day between the two birthdays, including any 29 February in between, so it is exact rather than an estimate based on 365 days a year.
How old was my mother when I was born?
Exactly the age gap between you. Enter her date of birth and yours: the headline - for example 27 years, 4 months and 2 days - is her age on the day you were born, and the first line underneath says so in words.
When was one person exactly twice as old as the other?
On the date that is as far after the younger person's birth as the age gap itself. With a 10-year gap the older person was twice the younger's age when the younger turned 10 and the older turned 20. The calculator gives the exact date, counted in days.
Why does the age gap in months not equal the years times twelve plus the months shown?
It does - Months shows the whole months, years x 12 plus months, and then the extra days. If it seems off, it is usually because the days part spans a short month such as February, which the calendar counts as it really is rather than as 30 days.
How is a 29 February birthday handled in the age gap?
In years without 29 February, the calculator treats 28 February as the anniversary, the convention most calendar software uses. The total in days is unaffected, because it simply counts days.
Last reviewed by the A2Z.Tools team against the sources listed above.
