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

Maximizing Utilization with Size Flexibility in Regional Reserved Instances

For additional flexibility, all Regional Linux Reserved Instances with shared tenancy apply to all sizes of instances within an instance family and an AWS Region, even if you are using them across multiple accounts via Consolidated Billing. The only attributes that must be matched are the instance type (e.g., m4), tenancy (must be default), and platform (must be Linux). All new and existing Reserved Instances are sized according to a normalization factor based on instance size, as follows.

Instance Size Normalization Factor
nano 0.25
micro 0.5
small 1
medium 2
large 4
xlarge 8
2xlarge 16
4xlarge 32
8xlarge 64
10xlarge 80
16xlarge 128
32xlarge 256

For example, if you have a Reserved Instance for a c4.8xlarge, it applies to any usage of a Linux c4 instance with shared tenancy in the AWS Region, such as:

  • One c4.8xlarge instance

  • Two c4.4xlarge instances

  • Four c4.2xlarge instances

  • Sixteen c4.large instances

It also includes combinations of c4 instances, such as one c4.4xlarge and eight c4.large instances. If you own a Reserved Instance that is smaller than the instance you are running, you are charged the prorated, On-Demand price for the excess.