How is capital outflow calculated? I would like to understand the meaning of the following phrase:


  Capital flight from Russia in 2014 was 151 billion dollars.


First, I would like to know how such a calculation can be done. Secondly, I would like to know what the underlying mechanism of capital flight from Russia is. For example, does this mean approximatively the following: Russian companies sell Russian gas and oil and then instead of investing this money domestically, they invest it outside?
The Central Bank of Russia (CBR) gives tons of statistics. Take a look at "Net Inflows/Outflows of Capital by Private Sector" and “Balance of Payment” in different representations. I warn you ahead that it’s not that clear from the data what moves where. 

There are several reasons for capital flight:


Dollarization and flight to safety. That is related to high inflation and bank runs over the past 25 years, after the collapse of the USSR. Russians do not fully trust the ruble or the local financial system. A lot of Russians hold foreign cash, mainly dollars and euros. 
Sanctions. Because of the Ukraine issues, the West put sanctions and effectively closed its financial markets. So, Russian banks and enterprises cannot refinance and have no way but to pay debts. Paying debts is like capital flight. 
NX = S – I. Russia has chronic CA surplus, which means capital outflow. It lasted for years. See this. If you have insufficient investment demand inside country for its savings that means capital outflow. That is related to point 1 also.