Reg T basics

• Initial margin on stock: you can borrow up to 50% of the purchase price under Regulation T.
• Maintenance: FINRA's minimum is 25% equity; many brokers use 30% or more.
• Margin call price = loan ÷ (shares × (1 − maintenance %)).
• Short put or call (standard): premium + the greater of 20% of the stock price minus the out-of-the-money amount, or 10% of the strike (puts) / stock price (calls).
• Defined-risk spreads: requirement is the width × 100; the credit you receive offsets it, so buying power used = (width − credit) × 100.
• Portfolio margin uses risk-based stress tests and can be much lower or higher.