DOB calculator
The age calculator run backwards: enter an age and the date it was true, and get the range of possible birth dates for forms, records and family history.
Guide
How to use it
- Enter the age in completed years.
- Set the date that age was true: today by default, or a census night, a record's date, a form's deadline.
- Read the two tiles: the earliest and latest possible birth dates, inclusive.
Examples
Worked examples
Great-grandfather listed as 34 on 19 June 1921: born between 20 June 1886 and 19 June 1887. Order birth records for both years.
A file says "aged 42" and is dated 3 March 2020. The window is 4 March 1977 to 3 March 1978, enough to match against other documents.
Know they had already had this year's birthday when the record was made? Then the true date sits in the later part of the window, the note under the tiles explains which end.
Method
How it works
Someone aged N on date D has had their Nth birthday but not their (N+1)th. The latest possible birth date is D minus N years (birthday exactly on D); the earliest is D minus N+1 years, plus one day. Everything between is possible, and nothing narrower is honest without more information.
Runs in your browser; ages and dates are never transmitted.
FAQ
Frequently asked questions
Can I work out a date of birth from an age?
To a range, yes. Someone aged 30 on 8 August 2026 was born between 9 August 1995 and 8 August 1996 inclusive. A single age pins the birth date to a one-year window, never one day.
Why is the answer a range and not a date?
Because an age stays the same for a whole year. Only extra information, a birthday month, a star sign, a school year, narrows the window further.
How do genealogists use this?
Census records give an age on census night. Enter that age and the census date, and the range tells you which years of birth records to search.
What does "aged 30 on their last birthday" mean on old records?
Exactly what this calculator assumes: completed years. Some historical records rounded or estimated ages, so treat the window as a starting point, not gospel.
Which end of the range is more likely?
Neither, without more information. If the record was made shortly after a birthday season you may have a hunch, but the maths alone gives every day in the window equal footing.
Is anything I enter stored?
No. The calculation runs in your browser and nothing is transmitted.
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