McDonald TMRCA Calculator
October 2014 version

This program calculates the probability that two people have a certain number of generations between them, based on the standard infinite alleles formula of Walsh. It calculates both the probability of being at an exact number of generations back to the Most Recent Common Ancestor (MRCA) of a certain pair of people and the cumulative probability that the actual number of generations is less than a certain value.

Editor's note: This version yields different results than the 2006 version. Whether due to higher mutation rates or a different calculation algorithm, TMRCA appears closer.


Note that the convention using generations is changed from an earlier version of this calculator which used "transmission events". It can list both result types in a table or graph. In either case the horizontal axis stops at the point where the cumulative probability reaches 95% or 10 generations, whichever is longer, or an absolute max of 50,000. Beyond 90% the calculation becomes inaccurate.

Editor's note: Despite the above caution, the "governor" to restrict pcum 95% has been modified in this version. Results will be returned to pcum 99.99%; results 99.95% will be displayed as 1.000 due to rounding.
The original is available at

Suggested values for the average mutation rate per marker per generation are given below. These are estimates from all available sources by the author as of October, 2014 and except for the FTDNA 12 to 67 marker sets are still not very well determined. The SNP rates are particularly poorly known. For the Y chromosome these rates assume a 31 year generation.

This calculator can use billions of markers. For BigY we suggest 10 million markers, for Full Genomes Elite we suggest 20 million, using the mutation rate listed below, and counting all markers which differ between two people. However, if comparing many people and using only markers for which no more than 5% of those people have no-calls, use instead a value of 8.3 million markers for BigY.

FTDNA Y‑12 STR  0.0020     * mtDNA complete genome  3·10‑6  (type 3.e-6)
FTDNA Y‑25 STR 0.0026 mtDNA coding   2·10‑6  (type 2.e-6)
FTDNA Y‑37 STR 0.0042 mtDNA HVR   2·10‑5  (type 2.e-5)
FTDNA Y‑67 STR 0.0029 autosomal and X SNPs 1·10‑8  (type 1.e-8)
FTDNA Y‑111 STR     0.0026 Y SNPs (BigY etc.) 3.0·10‑8  (type 3.e-8)

This program was created by J. D. McDonald and is public domain.
modified to return pcum <= 0.9999


    Number of markers             Number of non-matching markers

   Mutation Rate                   Cumulative Probability