The **baseline** of an EC2 burstable (T-family) instance is the CPU utilization level at which credits earned exactly equal credits spent — a **net credit balance of zero**. Below baseline the instance accrues credits; above it, the balance drains. Baseline is not set independently; it is *derived* from the instance size's credit earn rate. **Formula:** `Baseline % = (credits earned per hour / vCPUs) / 60 minutes` | Instance | Credits/hr | vCPUs | Baseline | |---|---|---|---| | t3.nano | 6 | 2 | 5% | | t3.micro | 12 | 2 | 10% | | t3.medium | 24 | 2 | 20% | | t3.large | 36 | 2 | 30% | | t3.xlarge | 96 | 4 | 40% | **Critical reading nuance:** baseline is expressed **per vCPU**, and CloudWatch's `CPUUtilization` is reported per instance (averaged across vCPUs) on the same scale — so the two are directly comparable. AWS's own example: a `t3.large` running at baseline shows **30%** in CloudWatch. This makes baseline a precise, knowable reference line per instance size (5%–40% across the T3 line), not a generic "low CPU" threshold. Draw it on the `CPUUtilization` graph: utilization sitting flat at the baseline % with a zero credit balance signals throttling, not idleness — see [[Burstable CPU Utilization Masks Saturation]]. The full earn/spend mechanics are in [[EC2 Burstable Instance Credit Model]]. --- *Source: [[AWS EC2 Burstable Key Concepts (AWS Docs)]] — https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/burstable-credits-baseline-concepts.html*