How do I calculate a battery internal resistance?

You can measure it by observing the voltage drop when it is loaded with a known resistive load. First measure the unloaded voltage, then the loaded voltage. Let's say that the known load resistance is 100 Ohms, and the voltage reads 8.85V when the load is applied.

The internal resistance is then (9V - 8.85V)*(100 Ohms)/9V

Or you could go to the manufacturer's web site and look for battery technical data sheets. For example, the Duracell 9V spec sheet can be found at:

Rule of thumb is ~0.2R per alkaline cell of new battery; so 9V it is built from

6 cells that gives ~1.2 to 1.5R; Measure the best ; Datasheets from Duracell the next.;