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system
what margin is required to initiate and maintain a short sale
user
depends on the stock involved, but for the most part brokerages allow you gain entry at 50%, meaning you can short twice the cash on hand you have.   going forward, you need to maintain 30%, so on a $10,000 short, you'd have to maintain $3000 in your account. example, an account with $5000 cash - you can short $10,000 securities.  let say 100 shares of xyz at $100 per share. after trade settles, you won't receive a margin call until your balance falls to $3000, probably right around the time xyz rises to $120 per share.  riskier stocks will have higher margin maintenance requirements - leveraged vehicles like fas/faz (triple leveraged) require 90% margin (3x30%) if they are allowed to be 'shorted' at all.
assistant