How is buying power calculated?

It is clear something is wrong with my buying power.

With around 410K cash, my buying power should be at least 820K (my current position is marginable), but C2 shows my buying power is 425K.

Is there some FAQ we can read about how are margin and buying power calculated?

Model Account StatusStarted $100,000

Buy Power $424,919

Cash $425,169

Equity ($250)

Cumulative $ $341,669

Total System Equity $441,669

Margined $0

Open P/L ($250)

When you have cash of $1000, your Buying Power will say "$1000"

But if you buy 100 shares of stock at $10 per share, your Buying Power will be reduced by only $500, not $1000.

In other words, for every $1 of cash you have, you will be able to buy $2 worth of stocks.


What’s the margin for shorting spx option? For almost same cash, I can short 12 SPX straddle on June 16th, but I can only short less than 10 spx straddle on June 22th. Is there any place on C2 we can find the margin requirement?