Western History & Genealogy Blog

Tombstone Math

Here's a shortcut for calculating a birth date when the death date is given on the tombstone or in an obituary, but only the age in years, months and days.

It is called the 8870 formula.

Here's how to use the "magic" 8870 formula.

Here's an example: Ancestor died May 6, 1889 at the age of 71 years, 7 months and 9 days, convert those to numbers

18890506 (year 1889, month 05, day 06) subtract the age

-710709  (71 years, 7 months, 9 days)

equals= 18179797 then subtract the magic number


giving 18170927 or year 1817, month 09, day 27 or 27 September 1817

Note that when you have more than 30 days in the "days" column, subtract 30 from the days and add 1 to months.

Likewise when you have more than 12 in the "months" column, subtract 12 and add one to the "years" column