This is a tool that calculates the call and put prices of a perpetual American (style) option, i.e. an option with an infinite time to expiration.

For these option price calculations, it is necessary to supply the following information:

• The stock price.
• The strike price, or exercise price, which is the price at which the underlying stock can be purchased or sold upon exercise of the option.
• The annualised, continuously compounded, risk-free interest rate. This is usually expressed in percentage terms (unless the formatting is explicitly set to express it as a decimal).
• The annualised, continuously compounded, dividend yield. This is usually expressed in percentage terms (unless the formatting is explicitly set to express it as a decimal).
• The volatility. This is usually expressed in percentage terms (unless the formatting is explicitly set to express it as a decimal).

Associated tool link: http://www.coggit.com/tools/perpetual_option_pricing.html