There are many definitions of market cap and it is likely that depending on specific views and goals of different platforms or investment teams, one or several might get used.

Here are some variants I have been exposed to : 1) Definition given by Fenix. This is the most intuitive. A few questions asked frequently around that : a) What if a class is dual listed and has different prices ? b) What if a class is not listed ? c) Do you want the full market cap or the free float ? d) Also for some countries you might ask what part of this market cap is available to foreign investors.

2) Market Cap equivalent for a specific security... will be useful to have a Price to Earnings metrics for example. So each security has an equivalent market cap which is then used for ratios computations. Be aware that this number might need to be different depending on share structure (sometimes preferred have different attribution of earnings, so PE would need to be adjusted while BM wouldn't)

So in conclusion, I would ask myself : WHY do I want to use this market cap for ? And from there build my own formula and accept that there can be variations to what others think,compute and report.

Good luck !