The margin requirements for stock purchases and short sales is 50%. This means that for stock purchases, you can borrow up to 50% of the total cost. For short sales, you must deposit an amount equal to 50% of the current market value of your short positions.
If you are starting with $100,000, you can get up to $100,000 of additional loans, making your total buying power equal to $200,000, if you are trading marginable securities.
If the stock is trading at less than $3.00 you may not use your loan balance to trade the stock.