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Possibly—but 20–30% is not a verified or guaranteed reduction in every customer’s total EC2 bill. AWS materials describe rightsizing as a way to find savings, but the figures they publish apply to particular workloads or product claims, not a universal result. Your outcome depends on which instances can safely be resized, their share of your bill, utilization data, and existing discounts.
The key distinction is whether a percentage applies to the instances you change or to your whole EC2 bill. An estimate for a subset of instances cannot be treated as the same percentage off the account total.
What EC2 rightsizing changes—and what it does not
Rightsizing means matching an EC2 instance’s type and size to the workload’s actual resource demand. AWS recommendations may identify instances to downsize or terminate. Downsizing reduces the capacity you run; terminating an instance removes its cost only when it is genuinely unnecessary.
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Rightsizing is different from Savings Plans. A Savings Plan lowers the unit price for eligible usage, while rightsizing reduces the capacity being run. They can complement each other, but a lower rate is not evidence that an instance was right-sized.
What the published savings figures really mean
The figures from AWS are useful context, not a forecast for your account. AWS Prescriptive Guidance gives a possible 10–35% production savings range in its Microsoft-workload rightsizing discussion, but cautions that estimates are workload-specific and do not imply the same reduction across an entire account. AWS also says Compute Optimizer can reduce costs by up to 25%; that is a product-level claim, not a promised result for an individual fleet.
Neither figure establishes that rightsizing alone generally cuts 20–30% from a customer’s total EC2 bill. Treat that percentage as a possibility to test against your own baseline, not as a dependable expectation.
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How AWS estimates rightsizing savings
AWS Cost Explorer’s documented rightsizing calculation uses the prior 14 days of utilization and billing information. It considers applicable On-Demand usage, Reserved Instance (RI) and Savings Plans coverage, and rate differences when estimating savings. The calculation does not include every second-order effect from reallocating RI hours, so its estimate may not match the eventual bill change.
Compute Optimizer recommendations also depend on observed demand and recommendation settings. AWS’s June 11, 2026 guidance describes a 14-day default lookback, with longer periods available for monthly or seasonal patterns. A 93-day lookback requires paid Enhanced Infrastructure Metrics. A short observation window may miss unusual or seasonal demand; settings should reflect the workload’s actual operating pattern.
Estimate the effect on your own bill
Start by defining the denominator. Are you measuring the costs of the instances you plan to change, a particular workload, or all EC2 usage? Then compare the estimated costs of those resources with your current costs, taking applicable discounts into account. If the affected instances make up only a portion of your EC2 spend, savings on those instances translate into a smaller percentage of the total bill.
Keep the estimate separate from realized savings. A recommendation is a projection based on usage and pricing inputs; it is not an audited reduction. After changes, compare actual costs for the affected resources against a suitable baseline, and distinguish rightsizing from other changes such as new commitments, idle-resource cleanup, architecture changes, or processor migration.
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Choose a recommendation view and check its inputs
AWS Cost Explorer provides EC2 rightsizing recommendations and points customers to Cost Optimization Hub for a broader view of cost optimization opportunities. Compute Optimizer covers additional resource types and offers configurable recommendation preferences. The right view depends on whether you need focused EC2 suggestions or broader optimization opportunities, and on the usage data available to each recommendation.
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Before acting, review recommendation inputs and workload context:
- Use a lookback period long enough to capture ordinary peaks, monthly cycles, and seasonal demand relevant to the workload.
- Check whether memory metrics are available where memory pressure could limit safe downsizing.
- Set utilization headroom to match the application’s performance and availability risk.
- Have the workload owner review changes that could affect latency, throughput, or resilience.
Apply changes safely and verify the result
- Identify candidates. Review EC2 rightsizing recommendations in Cost Explorer, or examine Compute Optimizer recommendations. Prioritize candidates with clear, sustained underutilization rather than treating every suggestion as an automatic action.
- Validate workload fit. Check the recommendation’s lookback, utilization and memory data, and headroom against the application’s normal and peak patterns. Confirm with the workload owner that a smaller instance or termination is operationally acceptable.
- Change incrementally. Apply a limited set of reviewed changes first. Monitor application performance and service health so that a regression can be caught and the change reversed or adjusted.
- Measure realized costs. Use Cost Explorer to compare post-change costs with the baseline for the affected resources. Account for billing coverage and other simultaneous changes; do not report the recommendation’s estimated monthly savings as a measured bill reduction.
Rightsize before buying new commitments
AWS recommends rightsizing before purchasing Savings Plans so commitments are not based on oversized capacity. Its June 2026 report says larger customers using rightsizing and commitments together tend to improve their median Cost Efficiency score four times faster than customers using Savings Plans alone. That statistic concerns improvement in AWS’s Cost Efficiency score, not four times more bill savings.
If you already have commitments, include their coverage in your estimate: the price effect of changing an instance may differ from the effect on an uncovered On-Demand instance. Rightsizing and commitments address different levers—capacity and unit price—so assess each separately and then evaluate their combined impact.
Quick Recap
Sources
- AWS Cost Management: Optimizing your cost with rightsizing recommendations
- AWS Cost Management: Understanding rightsizing recommendations calculations
- AWS Compute Blog: Optimize EC2 costs with AWS Compute Optimizer right sizing
- AWS Compute Optimizer User Guide: What is AWS Compute Optimizer?
- AWS Cloud Financial Management: The AWS State of Cost Efficiency Report
- AWS Prescriptive Guidance: Right size Windows workloads
- AWS Prescriptive Guidance: Top recommendations for optimizing costs
- AWS: Amazon EC2 Cost and Capacity Optimization
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