Calculating Birth Dates from Death Date Information

What day was that ancestor born? It seems like such a simple question, and yet finding the answer can be surprisingly complex, even when you have the numbers in front of you. Exact dates are often found in death certificates and frequently on tombstones. The problem is that these are often written as death dates followed by the person’s age at death.

Here is a common example:

Here lies the body of John Smith,

Died August 3, 1904,

Aged 79 years, 9 months, 29 days

How do you tell John Smith’s date of birth?

You obviously need to subtract 79 years and 9 months and 29 days from the date of death. Simple, right? Well, not as simple as it first appears.

First, was it a 31-day month or a 30-day month? Or was it 28 days or 29 days in the case of February?

Next, you have to calculate in Leap Years. Everyone knows that Leap Years occur every four years; but did you know that there is an exception every 100 years? And, just to complicate matters more, there is an exception to that exception every 400 years? Then there is the issue of which calendar was in use. Was it the Julian calendar or the Gregorian calendar?

There are easy solutions to the question.

The simplest answer is to let a computer make the calculation for you. However, if you are in the cemetery or in the courthouse without your computer and an Internet connection, you can use a simple method of determining birth dates by using a handheld calculator. And… if you don’t have a calculator but do remember your grammar school math, you can even make the same calculation with paper and pencil. I’ll describe methods you can use for these calculations wherever you are.

For now, let’s assume that the calendar used is not an issue. If you are calculating 19th, 20th, and 21st century dates in North America, the calendar in use is Gregorian and is not a factor. I will describe the Julian and Gregorian calendars and the conversion dates and locations in a future article.

All of the better genealogy programs of today have the capability to do these calculations for you. If you have a genealogy program installed on your computer and it is readily available, use that.

If you do not have a genealogy program or your program is limited with no date calculator, you can download a small Windows program that will perform the same calculations. The RJT Date Calculator is such a program, described as an aid for family historians. It is a free program although the author does suggest you pay a small donation if you find it useful. You can download the RJT Date Calculator at

NOTE: The RJT Date Calculator obviously has not been updated in a long time. It is listed as working on “Windows 95/98/NT/2000/ME/XP.” I haven’t tried Windows 8 or 10.

Many of us will find it easier to go to the Internet to find a calculator. The advantages of online apps is that there is no software to install in your computer plus they work on Windows, Macintosh, Linux, Android, Apple iOS, and other operating systems. There are many online birth date calendars available; here are a few that I found:

Birth Date Calculator:

Birth Date calculator:

Birth Date Calculator:

Age Calculator:

Birth Date Calculator:

Tombstone Birthdate Calculator with Tips:

This is great when you’re sitting in front of a computer; but what do you do if you are in the cemetery with no computer available? You pull out your handheld calculator or paper and pencil and use “Formula 8870.”

NOTE: You do carry a calculator with you at all times, right? I do since a calculator is included as part of my cell phone. Most modern cell phones include built-in calculators. If you have your cell phone in your pocket or purse, you probably have an available calculator, too.

The following is a nifty math trick. You only have to remember a four-digit number and a bit of process.

Using a calculator or a piece of paper, enter the death date as one long series of numbers, starting with the year followed by month and then the day of the month. In other words, write it as yyyymmdd.

For instance, if a person died August 3, 1904, enter the date as: 19040803 (make sure you enter all days and months as 2-digit numbers. Use “03” instead of “3”.

Next, subtract the age at death with the number written in the same manner, using two digits for the years (yymmdd). For a person aged 79 years, 9 months, 29 days, you would write that as: -790929

Now subtract still another number, the constant of 8870

The result is the date of birth, written as: yyyymmdd.

Here is how it looks on paper, using the earlier example of John Smith:

Died August 3, 1904,

Aged 79 years, 9 months, 29 days

19040803 Year, month,and day of death (yyyymmdd)

-790929 Subtract age at death (yymmdd)



-8870 Subtract the constant of 8870


18241004 John Smith was born October 4, 1824 (yyyymmdd)

The drawback is that “Formula 8870” always assumes that every month is 30-days long. The results may be off by as much as two or three days. When recording dates, always refer to them as “about” or “circa.” In the above example, I would record the results as “John Smith was born on or about October 4, 1824.”

You can perform these calculations on paper, on a calculator, or on your cell phone’s built-in calculator. I also keep a text note on my cell phone with the above formula as a reminder when I need it.

Of course, the above programs, web sites, and the “Formula 8870” all assume that the person who recorded the information on the tombstone or on the death record knew the above information and used it accurately. Sadly, you cannot count on that. Clerks and tombstone carvers of a century or more ago did not have access to computers or calculators and may not have known about “Formula 8870.” Even if they did know the formula, mistakes were still common.

I know that the date shown on one of my great-great-grandfather’s tombstone is off by one year from all the census records. I assume that the census takers usually talked directly with great-great-granddad and that the information he provided every ten years probably was accurate. I do know his information was consistent in every census record so I am guessing it was correct. Grieving relatives probably provided the information on his tombstone after his death, and they may have erred in the calculation. There is also a possibility that the tombstone engraver himself made the error. It is theoretically possible that the tombstone is correct and that all the census records were wrong. That seems unlikely, however.

I’d suggest that you first look for a birth record on the exact date calculated by the program/web site/formula. If not found, start looking at records for several days before and after the calculated date. You might also look for the same dates but in both the year before and the year after the calculated date, such as the example of my great-great-grandfather’s tombstone.

In short, use software tools when available. If software is not available at your fingertips at the time of need, use “Formula 8870.” Even then, you must consider the result to be approximate because those who recorded the information years ago may have erred.

May the records you encounter always be accurate.


I’ve notices that, in multiple cases where the DOB is available from other sources, the DOB as calculated from the DOD and reported age at death is off by a day. I’m guessing that they weren’t counting the day they died as one of the days they lived?


    It depends on the calculation method used by the person determining their age at the time. Let’s say someone died on April 15, 1820, and had been born Dec. 31, 1760. Most ages at death didn’t include the birth date. One person at the time might have calculated by subtraction, listing the year, then the month as a number, and then the day.
    1820 —4—-15
    To subtract 29 from 15, they would “borrow a month” [always 30 days in calculating this way] and change the days to 45 and subtract 29 from 45 to get 16 days. Since they borrowed from the month column, that is now a 3. They would then “borrow a year” [12 months] and make the 3 months now 15 months and subtract to get 3 months. Having borrowed a year, they subtract 1760 from 1819 and get 59 years, for a result of 59 years,, 3 months, and 16 days.

    Another method would be to say they were 59 when they died. They then count from their birthday for 3 months, which gets them to March 29. There would be still 2 days in March, plus the 15 in April, getting a result of 59 years, 3 months, and 17 days.

    I have seen county death records in Ohio from the 1870s and 1880s that list the death date and the their age, which differ by one day from the age (with no birth listed) on their tombstone, because each apparently used one of the two above different ways.

    That’s why in determining the birth date of a person from an age at death, I always list their birth date as “about”, since we don’t know what method they used. I’ve also seen tombstones that list the birth, death, and age, that the age doesn’t agree with the dates listed, in being a month off. Math was hard for some then, just as now.


I can confirm that the RJT Date Calculator works on Windows 10.


“Formula” 8870 is misleading. This is a conversion factor because of the magnitude of the numbers in this case. Had Mr. Smith lived between exactly 79 years and 79 years 7 months and 2 days no conversion factor would be necessary. In some instances, a different conversion factor would be needed. In this case one cannot subtract 9 months from 8, nor can one subtract 29 days from 3. To be able to perform this subtraction, one converts 1 year into 11 months and 30 days. Minus one year (-10000) plus 11 months (+1100) and plus 30 days (30) yields the -8870 constant. If Mr. Smith had lived 79 years, 9 months, and 1 or 2 days the necessary constant would have been -8800 (minus 1 year plus 12 months). If Mr. Smith had lived 79 years, 1 – 7 months, and 29 days the necessary constant would have been -70 (minus 1 month plus 30 days).


Interesting that when I tested this “formula” with my mother’s birth and death, her date of birth appeared as 19200641 which converts to Jun 41, 1920. It took me a bit but I finally recognized that that was equal to July 11, 1920…right on the nose.


Another significant and often confusing phrase in older records (and tombstones) is “in the 79th (or whatever) year of her age.” This appears in both American and German records, but it does not mean the person is what we would consider 79 years old. “In their 79th year” means that the person has passed their 78th birthday and is heading toward their 79th.


Phyllis W Bickley May 27, 2020 at 10:22 am

This is the link to Birth Date Calculator which I use.


Just ask Google or Alexa.


I use FTM and they have a date calculator built into the facts area.


it’s also possible that Grandmother had fibbed about her age for decades (maybe marrying ‘too young’ or was actually older than her fiance? not really a ‘done thing’ back then) and the tombstone is correct because the relatives had to register the death which could mean they found her birth certificate and revealed the deception. There are so many stories! but the key is document, document, document so you can find those interesting irregularities.


My thought is that they must have had the birthdate to begin with or how did they calculate days, months and years of age at death? Why not just put the birth date instead?


One of the cautions that I always tell folks at genealogy talks is “Just because it is carved in stone doesn’t mean it is ‘carved in stone’.” I have multiple examples of incorrect gravestones in my family tree.


exception every 100 years for leap years? Not correct. it is 400. 1600 and 2000 were leap years. Not with 1700, 1800 and 1900 because they are not divid-able by 4 or 400.


As stated above the computation is only as accurate as the info it is based on.
Going through English census data for someone who seemed to be born early 1800s, I could not find his birth in the relevant parish registers – until I realised that he only aged 8-years between 1841 and 1851. There was his birth record in 1797 – for decades he had been understating his age by increasing amounts.


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