AWS Reserved Instances: How They Work and When to Use Them

Definition

AWS Reserved Instances (RIs) are a pricing model in which you commit to using a specific instance configuration for a 1-year or 3-year term in exchange for a significant discount — up to 72% off On-Demand pricing. RIs are available for EC2, RDS, ElastiCache, Redshift, and OpenSearch (each service has its own RI program).

RIs were the original AWS commitment discount mechanism, introduced long before Savings Plans. For EC2, Savings Plans have largely superseded RIs due to their greater flexibility. However, RIs remain the only commitment option for RDS, ElastiCache, Redshift, and OpenSearch, making them still essential knowledge.

How It Works

When you purchase an RI, you commit to a specific instance type, Region (or AZ), platform (OS/engine), and tenancy for the term. AWS applies the RI discount automatically to matching running instances in your account (or across accounts in an AWS Organization).

If your running instances match the RI attributes, you pay the discounted rate. If no matching instances are running, you still pay for the RI — unused commitment is wasted money. If your usage exceeds the RI, the excess runs at On-Demand rates.

Payment options:

All Upfront: pay 100% at purchase — deepest discount (up to 72% off).
Partial Upfront: pay ~50% at purchase, the rest monthly — moderate discount.
No Upfront: pay nothing at purchase, monthly billing — smallest discount but lowest cash outlay.

Key Features and Limits

Standard RIs: locked to a specific instance family, size, Region, and platform. Can be sold on the RI Marketplace if no longer needed. Up to 72% discount.
Convertible RIs: can be exchanged for a different instance family, size, OS, or tenancy during the term (equal or greater value). Cannot be sold on the Marketplace. Up to ~54% discount.
Zonal RIs: reserved in a specific AZ — guarantees capacity reservation in that AZ.
Regional RIs: apply across all AZs in a Region — more flexible but no capacity guarantee. Automatically apply size flexibility within the instance family (e.g., a single m5.xlarge RI can cover 2x m5.large).
RI Marketplace: third-party marketplace where you can sell unused Standard RIs to other AWS customers. Convertible RIs cannot be listed.
Scope: EC2 RIs cover EC2 only. RDS RIs cover RDS. ElastiCache RIs cover ElastiCache. Each is purchased separately.
Cannot be cancelled: once purchased, you own the RI for the full term (Standard RIs can be sold on the Marketplace as an alternative).
Organization sharing: RIs apply across linked accounts in an Organization by default (can be disabled).

Common Use Cases

Steady-state RDS databases — RDS does not support Savings Plans, so RIs are the only commitment discount for RDS. A production RDS Multi-AZ instance running 24/7 is the ideal RI candidate.
ElastiCache and Redshift clusters — same situation as RDS: RIs are the only commitment option.
OpenSearch domains — Reserved Instances are the discount mechanism for Amazon OpenSearch Service.
Legacy EC2 commitments — organizations with existing Standard EC2 RIs continue to benefit from them. New EC2 commitments are better served by Savings Plans.
Capacity reservation — Zonal EC2 RIs guarantee that capacity is available in a specific AZ, important for regulated or latency-sensitive workloads.

Pricing Model

RI pricing varies by instance type, term, payment option, and service:

EC2 example: an m7g.large On-Demand in us-east-1 costs ~$0.0816/hour. A 3-year All Upfront Standard RI drops this to ~$0.023/hour — a 72% discount.
RDS example: a db.r6g.large Multi-AZ On-Demand costs ~$0.48/hour. A 1-year All Upfront RI might cost ~$0.30/hour — a ~37% discount. 3-year terms offer deeper discounts.
Unused RIs are still charged: if you buy an RI and no matching instance runs, you pay the full RI cost with zero benefit.
RI Marketplace fees: AWS charges a 12% service fee on RI Marketplace sales.
Billing priority: when both RIs and Savings Plans could apply, AWS applies the discount that saves the customer the most money first.

Use Cost Explorer RI Utilization and Coverage reports to monitor whether your RIs are being used and whether additional purchases would save money.

Pros and Cons

Pros

Up to 72% discount — the deepest commitment discount available for some services.
Only commitment option for RDS, ElastiCache, Redshift, and OpenSearch.
Zonal RIs provide capacity reservation guarantees.
Standard RIs can be resold on the RI Marketplace.
Regional RIs provide size flexibility within an instance family.
Organization-wide application across linked accounts.

Cons

Less flexible than Savings Plans for EC2 — locked to specific instance attributes.
Cannot be cancelled or modified (Standard can only be sold on Marketplace).
Convertible RIs offer lower discounts than Standard.
Complexity: managing RIs across multiple services, accounts, and terms requires tooling.
Unused RIs are pure waste — no refund, no credit.
For EC2, largely superseded by Savings Plans which are simpler and more flexible.

Comparison with Alternatives

| | Reserved Instances (Standard) | Reserved Instances (Convertible) | Savings Plans (Compute) | Savings Plans (EC2 Instance) | On-Demand | | --- | --- | --- | --- | --- | --- | | Max discount | 72% | ~54% | 66% | 72% | 0% | | Flexibility | Low (fixed instance type/Region) | Medium (exchangeable) | High (any EC2/Fargate/Lambda) | Medium (family + Region locked) | Full | | Resellable | Yes (RI Marketplace) | No | No | No | N/A | | Capacity reservation | Yes (Zonal) | No | No | No | No | | Covers RDS/ElastiCache/Redshift | Yes (service-specific RIs) | Yes | No | No | Yes | | Recommended for EC2 | Legacy only | Legacy only | Yes (default) | Yes (stable fleets) | Short-term |

Rule of thumb: use Savings Plans for EC2, Fargate, and Lambda. Use Reserved Instances for RDS, ElastiCache, Redshift, and OpenSearch where Savings Plans do not apply.

Exam Relevance

Cloud Practitioner (CLF-C02) — know that RIs offer discounts for 1- or 3-year commitments, understand All/Partial/No Upfront, and recognize that RIs are less flexible than Savings Plans.
Solutions Architect Associate (SAA-C03) — scenario questions: "How to reduce cost for a 24/7 RDS database?" → RDS Reserved Instance. Know Standard vs Convertible, Zonal vs Regional, and when RIs are the right choice vs Savings Plans.
Solutions Architect Professional (SAP-C02) — RI portfolio management across Organizations, RI Marketplace strategies, combining RIs with Savings Plans and Spot for optimal cost.

Exam trap: Savings Plans do not cover RDS, ElastiCache, Redshift, or OpenSearch. If the question involves reducing cost for these services, the answer is Reserved Instances.

Frequently Asked Questions

Q: Should I buy EC2 Reserved Instances or Savings Plans?

A: For new EC2 commitments, Savings Plans are almost always the better choice. Compute Savings Plans offer flexibility across instance families, Regions, and even compute services (Lambda, Fargate), while EC2 Instance Savings Plans match the RI discount level (72%) with slightly more flexibility. Use EC2 RIs only if you specifically need Zonal capacity reservation or plan to resell on the RI Marketplace.

Q: What happens if I no longer need my Reserved Instance?

A: For Standard EC2 RIs, you can list them for sale on the RI Marketplace — another AWS customer can buy the remaining term at a discount. Convertible RIs cannot be sold on the Marketplace, but you can exchange them for a different configuration of equal or greater value. In either case, you cannot cancel the RI and get a refund. For RDS/ElastiCache/Redshift RIs, there is no marketplace — you absorb the cost.

Q: Do Reserved Instances provide actual capacity reservations?

A: Only Zonal EC2 RIs (scoped to a specific Availability Zone) guarantee capacity reservation. Regional EC2 RIs, Convertible RIs, and all RDS/ElastiCache/Redshift RIs provide a billing discount but no capacity guarantee. If you need guaranteed capacity without a 1- or 3-year commitment, use On-Demand Capacity Reservations (ODCR), which can be combined with Savings Plans for both discount and capacity guarantee.

This article reflects AWS features and pricing as of 2026. AWS services evolve rapidly — always verify against the official AWS Reserved Instances documentation before making purchasing decisions.