An essential patent or standard-essential patent (SEP) is a patent that claims an invention used to comply with a technical standard. The patent policy of the standards setting organization specifies a condition called FRAND as licensing terms and conditions for SEP. However, it has been difficult for companies that adopt standards to grasp the total royalties, because the specific royalties of FRAND are usually not defined in the patent policy of the standards setting organization. The potential royalties payment may thus represent an intellectual property risk to those companies that implement technology subject to standardization. In this paper, we derive a formula for calculating the intellectual property risk of the implementer of SEP by modeling SEP holders. In addition, the IPR risk of IEEE 802.11 is calculated and the result of the calculation is validated. Disclaimer. While the present paper has been prepared with HCC14, The authors are not liable for any direct, indirect, special or consequential losses or damage of any kind, or loss of profit, derived from using any of the information contained in the present paper.