Wednesday, June 23, 2010

Wisdom Wednesday - 8870 Formula Every Time

Exact Dates with Subtraction

Basic math can help even genealogist.  Let’s take a walk through the cemetery armed with only a birth date, and without your beloved calculator. And VOILA, you finally find Mrs. Ancestor’s tombstone!  Mrs. Ancestor’s grave marker (or death record) reveals important information.  She died at 83y, 11m,29d on 28 March 1901 (I’ve found my share of these types of etchings).[1]
Very basic math would place Mrs. Ancestor birth year as 1818 (1901 minus 83years).  But we have more information on Mrs. Ancestor now.  Ignoring the reason her loved ones declared it important to put 83y 11m 29d on her tombstone; this added information has helped me break a few cases. Armed with the 8870 formula, I can determine an exact birth date.

Let’s use the 8870 formula on the spot, at the tombstone, without a calculator to quickly derive at the answer.  There’s no need to count backwards. This method can also be used to verify the accuracy of your birth data, or may create questions if the birth year you have the one calculated by the 8870 formula is correct.

Apply the Formula*
Tombstone Facts on Mrs. Ancestor
Died:                                       March 28, 1901 
Age on Tombstone:                 83y 11m 29d
Birth date:                               unknown

Death              19010328        March 28, 1901
Age                 -  831129         83y 11m 29d
                      -       8870         Constant – 8870 Formula 
Birth date        18170329         March 29, 1817

There are times that you will get answers that don't work out perfectly.  For example you may get 13 months or 58 days.  If this is the case, base the answer on 12 months, 30 day  (i.e. 13 = 1 more year; whereas 58 days needs one more month and 28 days (58-30 =28 days).

More Information
If you do not wish to do this by hand, there are handy dandy calculators on the computer.  Try the Tombstone Calculator at Ancestor Search.  The Board for Certification of Genealogists webpage provides many examples and an explanation of the nuances.                 

This method has been used by genealogists for years.  Now you have the secret formula.

Accurate, Accessible Answers

[1] This is an example: I filled in the days with 29, to avoid the complication of nuances.  Tombstone does not read 29d.  Read the BCG nuances to learn how to calculate a birthdate without the days. 
*This example has been simplified. 


  1. This is really interesting take on the concept. I never thought of it that way. I came across this site recently which I think it will be a great use of new ideas and informations.
    Grave Markers

  2. Wow, I never heard of this one before, but now I'll never forget. Its much easier than looking up dates on line all the time.