Daily AncestorTombstone age calculator
To get a birth date from a stone that reads "aged 64 y. 3 m. 12 d.," subtract the days, then the months, then the years from the death date, borrowing from the month before when needed.
Many nineteenth century gravestones skip the birth date and give the age at death instead: "died May 20, 1878, aged 64 years, 3 months, 12 days." Death notices and burial registers often do the same. The birth date is hidden in that arithmetic, and this calculator works it out.
Enter the death date and the age in years, months, and days. The calculator counts back on the real calendar, so it knows that February is short and that some months have 31 days.
How to use it
- Type the death date exactly as carved or printed: day, month, and year.
- Enter the age at death: years, months (0 to 11), and days (0 to 30). Leave a part at 0 if the stone does not give it.
- Read the birth date. It updates as you type.
- Compare the result with a baptism, a family Bible, or the census ages before you record it as fact.
Worked examples
Died May 20, 1878, aged 64 years, 3 months, 12 days
Born February 8, 1814.
Twelve days before May 20 is May 8. Three months before that is February 8. Sixty-four years earlier gives 1814.
Died May 6, 1889, aged 71 years, 7 months, 9 days
Born September 27, 1817.
This is the worked example the Denver Public Library uses for the 8870 shortcut, and the calendar count lands on the same day.
Died March 2, 1870, aged 40 years, 0 months, 3 days
Born February 27, 1830.
Three days back from March 2 crosses into a 28-day February. A method that assumes every month has 30 days would give February 29, 1830, a date that never existed.
How the subtraction works
Work from the smallest unit up. Take the days off the death date first. If that runs past the start of the month, borrow the previous month and count back through its real length. Then take off the months, borrowing a year if you pass January. Finally take off the years.
Whoever figured the age in the first place, often a relative, minister, or undertaker, was doing the same subtraction in reverse, and not always with a calendar at hand. That is why the carved age is best treated as careful evidence rather than proof.
The 8870 shortcut
Some genealogists use a number trick. Write the death date as one eight-digit number (year, month, day), write the age as a six-digit number (years, months, days), subtract the age, and then subtract 8870. The Denver Public Library's example: 18890506 minus 710709 minus 8870 gives 18170927, or September 27, 1817.
The trick quietly treats every month as 30 days long. When the days in the age are more than the day of the month of death, the count crosses back into the month before, and if that month has 31 days, or is February, the result can be a day or two off from the calendar. It can even produce a date that never existed, such as February 29, 1830, which you then have to adjust by hand.
Why the result can still be a day off
Ages were figured by hand. Whoever did it could count the day of birth or the day of death differently than you would, or use a shortcut like the one above. A one or two day difference between this result and a birth record is normal and usually means both point to the same person. A baptism usually came days or weeks after the birth, so expect it to fall a little later.
A gap of a year or more is a different signal. It can mean a misread carving, a stone set decades later from family memory, or a different person with the same name. Look for the birth in a church register, a delayed birth certificate, or the ages given in each census.
Questions people ask
How do I calculate a birth date from a tombstone?
Subtract the days, then the months, then the years from the death date, borrowing from the previous month or year when you run short. The calculator above does this on the real calendar.
What is the 8870 rule in genealogy?
It is a shortcut: death date as YYYYMMDD minus age as YYMMDD minus 8870 gives the birth date, after you carry any day over 30 or month over 12. It treats every month as 30 days, so check the result when the count crosses back over a 31-day month or February.
What does "aged 64 y. 3 m. 12 d." mean on a gravestone?
The person was 64 years, 3 months, and 12 days old on the day they died. Older stones may write it as "AE 64" or "aet. 64," from the Latin aetatis, meaning aged.
Why does my calculated birth date differ from the birth record?
Small differences usually come from how the age was counted. Large differences suggest a carving or memory error. Trust the record made closest to the birth, and note both dates in your tree.
How to cite this page
Daily Ancestor. "Tombstone age calculator." Tools. https://dailyancestor.com/tools/tombstone-age-calculator. Last checked 2026-10-15.