We are going to provide an example of valuing American options. We’re going to use the Barone-Andesi-Whaley approximation. The Barone-Adesi and Whaley Model has the advantages of being fast, accurate and inexpensive to use. It is most accurate for options that will expire in less than one year.

http://tech.harbourfronts.com/derivatives/valuing-american-option-derivative-pricing-excel/