How do you define the market cap of an ETF? I'm hesitating between these 2 solutions:


The conventional calculation:


MCAP = #shares * SharePrice


Perform the weighted average of the MCAP of each stock in the ETF


MCAP = sum(iStockWeight * iStockMCAP)

Thanks for your help.
An ETF’s assets will fluctuate based on both changes in the value of
  the underlying securities and the creation of new shares or redemption
  of existing shares. It is worth noting that there may be a difference
  between an ETF’s market capitalization and the net asset value (NAV)
  of its underlying securities. The difference results in either a
  discount or a premium in the trading price of the ETF.


ETF DB: Largest ETFs: Top 100 ETFs By Assets 

The market capitalization at time $t$ is therefore  the number of shares at $t$ times the price of those shares at $t$. The net assets are the total value of all the assets in the fund at 't':
$$ Net\ Assets_t = U_{t} \cdot NAV_t = U_{t} \cdot \sum_{i} (S_{i,t} \cdot weight_{i,t})$$
where $S_{i,t} $ is the value of a share of stock $i$ at time $t$, $weight_{i,t}$ is the fraction of the index invested in stock $i$ at time $t$, and $U_{t}$ is the number of shares of the index.
Or equivalently:

$$ Net\ Assets_t = \cdot \sum_{i} (S_{i,t} \cdot q_{i,t}) $$
Where $q_{i,t}$ is the number of shares of stock $i$ at time $t$ held by the ETF. This sounds simpler but it is often easier to find index weights than exact share holdings measured in number of shares. 

But for open-ended funds that allow creation and redemption or have minimal expenses these NAV and market price based approaches will give similar answers.