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.

Last verified: April 2026

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.

Real-World Scenario

Your team runs a user profile service on DynamoDB in on-demand mode because traffic was unpredictable at launch. Twelve months later, the table handles a steady 2,000 reads/sec and 500 writes/sec with predictable daily patterns. The calculator shows on-demand costs $1,830/month. Switching to provisioned mode with auto-scaling (base 2,000 RCU / 500 WCU, max 5,000 / 1,500 for peaks) drops the bill to $450/month. Adding 1-year reserved capacity for the baseline brings it to $210/month — an 89% reduction from on-demand.

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.

Pro Tips

TIP

On-demand mode costs roughly 6.5x more per request than provisioned mode at steady state. The breakeven point is when your traffic is unpredictable enough that you'd need to over-provision by more than 6.5x to handle peaks. For most production tables with established traffic patterns, provisioned mode with auto-scaling is significantly cheaper.

TIP

DynamoDB charges for the full item size on writes, even if you only update one attribute. A 10 KB item with a 100-byte attribute update still consumes 10 WCUs. If your items are large and updates are frequent on small attributes, consider splitting hot attributes into a separate table to reduce WCU consumption.

TIP

Reserved capacity for DynamoDB offers 53% savings on a 1-year term and 76% on a 3-year term — among the deepest discounts in all of AWS. If your table's baseline throughput is stable, reserved capacity is one of the highest-ROI commitments you can make. The calculator shows the exact savings to justify the purchase.

How It Works Under the Hood

The calculator converts item sizes and operation rates into capacity unit requirements using DynamoDB's sizing rules: 1 RCU per strongly consistent read (or 2 eventually consistent reads) per 4 KB, and 1 WCU per write per 1 KB. Item sizes are rounded up to the nearest KB boundary. Transactional operations double the unit consumption. Monthly cost is computed by multiplying provisioned units by per-unit hourly rates (or per-request rates for on-demand mode) and adding storage costs at the per-GB rate.

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

Featured In

DynamoDB Design Patterns: Single-Table, GSI Overloading, and When to Use What

Was this tool helpful?

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.