Create an Alarm from a Metric on a Graph
You can graph a metric and then create an alarm from the metric on the graph, which has the benefit of populating many of the alarm fields for you.
To create an alarm from a metric on a graph
Open the CloudWatch console at https://console.aws.amazon.com/cloudwatch/.
In the navigation pane, choose Metrics.
Select a metric namespace (for example, EC2) and then a metric dimension (for example, Per-Instance Metrics).
The All metrics tab displays all metrics for that dimension in that namespace. To graph a metric, select the check box next to the metric.
To create an alarm for the metric, choose the Graphed metrics tab. For Actions, choose the alarm icon.
Under Alarm Threshold, type a unique name for the alarm and a description of the alarm. For Whenever, specify a threshold and the number of periods.
Under Actions, select the type of action you want the alarm to perform when the alarm is triggered.
(Optional) For Period, choose a different value. For Statistic, choose Standard to specify one of the statistics in the list or choose Custom to specify a percentile (for example, p95.45).
Choose Create Alarm.