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Possibly for a particular workload, but 20–30% is not an established or guaranteed reduction in every customer’s total EC2 bill. AWS guidance supports rightsizing as a way to find savings, but its published figures are scoped to specific workloads or product claims. Your result depends on which instances you change, their usage patterns, and the discounts and commitments already applied to your account.
What rightsizing changes—and what the percentage should mean
Rightsizing matches an EC2 instance’s type and size to the workload’s observed resource demand. A recommendation may be to move to a smaller instance or terminate an underused one. AWS Cost Explorer identifies EC2 rightsizing opportunities; AWS also points customers to Cost Optimization Hub for a broader view of optimization opportunities. AWS explains its rightsizing recommendations.
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Before treating a percentage as a result, define its denominator. A 25% reduction on a set of recommended instances is not necessarily a 25% reduction on the whole EC2 bill. If the targeted instances account for only part of EC2 spend, the account-wide effect will be smaller; unchanged instances and existing charges remain in the total.
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The sources reviewed do not verify 20–30% as a general outcome from rightsizing alone. AWS Prescriptive Guidance gives a possible 10–35% production-savings range in its Microsoft-workload guidance, while cautioning that the estimate applies to a technical domain rather than all AWS spending. Separately, AWS says Compute Optimizer can reduce costs by up to 25%; that is a product-level claim, not a promise for an individual account. Neither figure establishes a typical reduction in every customer’s total EC2 bill. AWS’s Windows-workload rightsizing guidance and its EC2 Cost and Capacity page provide those qualifications.
How AWS estimates rightsizing savings
A recommendation is a projection, not proof that a bill has fallen. Cost Explorer’s documented calculation uses recent utilization and billing information. For the relevant calculation, it examines the previous 14 days, considers instance usage and applicable Reserved Instance or Savings Plans coverage, and estimates savings using eligible On-Demand hours and rate differences. The estimate does not include every second-order effect from reallocating Reserved Instance hours. AWS documents the calculation method and its limits.
That means two estimates can differ even when they concern similar instances: usage history, coverage by existing discounts, and the resources included in the calculation all matter. To judge a proposed 20–30% figure, check whether it refers to the recommended instances, a defined workload, or the entire EC2 bill—and whether it is estimated or realized.
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Choose recommendations that represent real demand
Use a lookback period that fits the workload
A short observation window can miss monthly peaks, seasonal demand, or unusual operating periods. AWS Compute Optimizer uses a 14-day lookback by default, with longer periods available for monthly or seasonal patterns. Its 93-day lookback requires paid Enhanced Infrastructure Metrics. Select a period that captures normal and peak behavior rather than treating the default as universally sufficient. AWS’s June 11, 2026 guidance describes the available lookback and recommendation preferences.
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CPU data alone may not describe a workload’s constraints. Compute Optimizer can ingest external EC2 memory-utilization metrics from supported observability products, which can enable recommendations for instances that otherwise may not receive one. In its June 9, 2026 State of Cost Efficiency report, AWS said enabling EC2 memory metrics was associated with 8–30 percentage points higher savings per recommendation, and that 17.7% of eligible customers had the feature enabled. This is an association reported by AWS—not a forecast that your fleet or total bill will save that amount. Read the AWS report.
Set headroom and involve the workload owner
A technically smaller instance can still be the wrong choice if it leaves too little capacity for bursts or affects application performance. Review recommendation headroom and the workload’s peaks, memory use, and application behavior with its owner. AWS Compute Optimizer supports configurable recommendation preferences, but the right setting depends on the workload and its tolerance for risk. AWS Compute Optimizer documentation describes the service and its recommendation inputs.
Compare AWS’s tools by the question you need answered
| Tool | Useful for | What to check |
|---|---|---|
| Cost Explorer rightsizing recommendations | Identifying EC2 downsizing or termination opportunities and reviewing estimated savings. | The calculation uses recent usage and billing data, including applicable On-Demand usage and RI or Savings Plans coverage; it does not model every second-order effect of RI-hour reallocation. |
| AWS Compute Optimizer | Getting resource recommendations based on observed demand, with configurable lookback and headroom preferences. | Whether the lookback captures the workload’s patterns and whether memory metrics are available for the instances being assessed. |
| Cost Optimization Hub | Viewing a broader set of AWS cost-optimization opportunities. | Use it for broader opportunity discovery; validate the savings for the specific resources and changes you plan to make. |
These services answer related but different questions: where EC2 rightsizing may help, what resource configuration fits observed demand, and what other optimization opportunities exist. AWS’s Cost Explorer guidance, Compute Optimizer documentation, and Compute Optimizer guidance describe their respective scopes.
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Rightsizing and Savings Plans address different costs
Rightsizing reduces the capacity you run; Savings Plans lower the unit price for eligible usage in exchange for a commitment. AWS recommends rightsizing before purchasing commitments so you do not commit against over-provisioned capacity. Existing commitments also affect how much a proposed change translates into savings, so account for them when evaluating a recommendation. AWS’s cost-optimization guidance discusses the sequence.
AWS’s 2026 report says larger customers combining rightsizing with commitments tend to improve their median Cost Efficiency score four times faster than customers using Savings Plans alone. That statistic concerns the score—not four times the bill savings—and does not establish the result for an individual account. The report explains its comparison.
How to verify savings after a change
- Set a baseline. Record the EC2 resources and spend included in the claim, and state whether the comparison is for a workload or the whole EC2 bill.
- Review recommendations with the workload owner. Check the observation period, headroom, memory visibility, peak demand, and performance requirements before choosing a new instance size or terminating an instance.
- Change a manageable group. Apply approved recommendations incrementally so that any performance issue can be traced to the relevant change.
- Monitor service performance. Check the workload after each change against its operational requirements, not just its utilization graph.
- Compare realized costs. Use Cost Explorer after implementation to compare like-for-like periods and resources, accounting for applicable discounts and commitments. Report the measured change separately from the recommendation’s estimated monthly savings.
This distinction is essential: a recommendation’s projected savings are not an audited reduction in your bill. AWS recommends tracking realized savings through Cost Explorer. Its implementation guidance covers recommendation preferences and validation.
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