The margin requirement for an uncovered put is the greatest of the following calculations times the number of contracts times the multiplier (usually 100):

• 20% of the underlying price minus the out of money amount plus the option premium
• 10% of the strike price plus the option premium
• $2.50

The premium received from the sale of the short put may be applied to meet the initial margin requirement.
 

example

Sell to open 1 MAR 45 Put at .50 with the underlying stock at $47.50:

• [((.2 x 47.50) - 2.5) + 0.50] x 1 x 100 = $750
• [(.1 x 45) + 0.50] x 1 x 100 = $500
• $2.50 x 1 x 100 = $250

The requirement for this position would be $750. The $50 generated from the sale can be applied to the margin requirement, resulting in a buying power requirement of $700.

Please note that some underlyings may have elevated margin requirements. For example, naked options in cash-settled indices use 25%/15% instead of 20%/10%.