How are read and write capacity units calculated?

One read capacity unit (RCU) handles one strongly consistent read per second for items up to 4 KB, or two eventually consistent reads. One write capacity unit (WCU) handles one write per second for items up to 1 KB. Larger items consume proportionally more units, rounded up to the next whole unit.

When should I use on-demand vs. provisioned mode?

On-demand mode is ideal for unpredictable or spiky workloads since you pay per request with no capacity planning. Provisioned mode is more cost-effective for steady-state workloads where you can predict throughput requirements and optionally purchase reserved capacity for additional savings.

Do DynamoDB transactions cost more?

Yes. Transactional reads consume twice the RCUs of a strongly consistent read, and transactional writes consume twice the WCUs of a standard write. This is because DynamoDB performs additional work to ensure ACID transaction guarantees across multiple items.

DynamoDB Capacity Calculator

ComputeAWS

Calculate DynamoDB read/write capacity units and estimated costs for on-demand and provisioned modes.

DynamoDB Configuration

Capacity Mode

Avg Item Size (KB)

Reads Per Second

Writes Per Second

Strongly Consistent ReadsTransactional

Storage (GB)

Raw Output

Output will appear here...

About This Tool

The DynamoDB Capacity Calculator helps you estimate read and write capacity unit requirements and monthly costs for Amazon DynamoDB tables. It models both on-demand and provisioned capacity modes, accounting for item size, read consistency (eventual vs. strong), and transaction overhead. The calculator helps you choose between capacity modes and right-size provisioned throughput to balance performance and cost.

When to Use This Tool

•Estimating required write capacity units for an event ingestion pipeline processing a known volume of writes per second.
•Comparing on-demand vs. provisioned pricing for a table with predictable traffic patterns.
•Calculating how item size affects capacity consumption to optimize DynamoDB schema design.
•Planning reserved capacity purchases based on projected steady-state read/write throughput.

Frequently Asked Questions

How are read and write capacity units calculated?: One read capacity unit (RCU) handles one strongly consistent read per second for items up to 4 KB, or two eventually consistent reads. One write capacity unit (WCU) handles one write per second for items up to 1 KB. Larger items consume proportionally more units, rounded up to the next whole unit.
When should I use on-demand vs. provisioned mode?: On-demand mode is ideal for unpredictable or spiky workloads since you pay per request with no capacity planning. Provisioned mode is more cost-effective for steady-state workloads where you can predict throughput requirements and optionally purchase reserved capacity for additional savings.
Do DynamoDB transactions cost more?: Yes. Transactional reads consume twice the RCUs of a strongly consistent read, and transactional writes consume twice the WCUs of a standard write. This is because DynamoDB performs additional work to ensure ACID transaction guarantees across multiple items.

Related Learning Guides

DynamoDB Data Modeling28 min read
Cost Optimization Strategies26 min read

Disclaimer: This tool runs entirely in your browser. No data is sent to our servers. Always verify outputs before using them in production. AWS, Azure, and GCP are trademarks of their respective owners.