Amazon EC2 Reserved Instances and Other AWS Service Reservation Models
AWS Whitepaper

AWS Trusted Advisor

AWS Trusted Advisor is an online resource to help you reduce cost, increase performance, and improve security by optimizing your AWS environment. AWS Trusted Advisor provides real-time guidance to help you provision your resources following AWS best practices. To help you maximize utilization of Reserved Instances, AWS Trusted Advisor checks your Amazon EC2 computing-consumption history and calculates an optimal number of Partial Upfront Reserved Instances. Recommendations are based on the previous calendar month's hour-by-hour usage aggregated across all consolidated billing accounts. Note that Trusted Advisor does not provide size-flexible Reserved Instance recommendations.

For more information about how the recommendation is calculated, see Reserved Instance Optimization Check Questions in the Trusted Advisor FAQs.