How exactly do you calculate TMRCA?

[spruithean]

I've been interested in this lately.

Wondering how its done. I've read its not an exact science given all the variables but it is still interesting to find a calculation that places a relation at some point in history!

Good luck to all

[rucksack]

The tool in FTDNA works good for most purposes.

Just click on the icon in the Y DNA Matches page.

Here is a comparison between me and my 60/67 match.

In comparing 67 markers, the probability that Mr. X and Mr. Y shared a common ancestor within the last...
4 generations is .96%
8 generations is 15.48%
12 generations is 46.59%
16 generations is 74.48%
20 generations is 90.14%
24 generations is 96.75%

I use a range of 25 to 30 for calculating a generation.

They say:

Since each marker has a different mutation rate, identical Genetic Distances will not necessarily yield the same probabilities. In other words, even though Mr. Y has a Genetic Distance‡ of 7 from X, someone else with the same Genetic Distance may have different probabilities, because the distance of 7 was prompted by mutations in different markers, with different mutation rates.

The differences are not large, however.

[rucksack]

The above is regarding TMRCA between two individuals, not between two haplogroups.