DynamoDB Capacity Calculator

About the DynamoDB Capacity Calculator

Amazon DynamoDB offers two capacity modes: on-demand (pay-per-request) and provisioned (pre-allocated throughput). Choosing the wrong mode can mean paying 5–10x more than necessary. This calculator helps you compare both modes side by side and find the break-even point for your workload.

On-demand mode charges per read and write request with no capacity planning required. It's ideal for unpredictable workloads, new tables, or spiky traffic. Provisioned mode requires you to specify read and write capacity units (RCUs and WCUs) in advance, offering lower per-request costs for steady workloads.

By entering your expected read and write volumes, item sizes, and consistency requirements, this calculator shows you exactly which mode is cheaper and by how much. It also factors in DynamoDB storage costs and optional features like auto-scaling and reserved capacity.

When This Page Helps

DynamoDB capacity mode selection is one of the most impactful cost decisions for NoSQL workloads on AWS. On-demand mode can cost 6.5x more per request than provisioned capacity. This calculator quantifies the difference for your specific usage pattern, helping you avoid significant waste on a service that often runs 24/7.

How to Use the Inputs

1. Enter the average number of read requests per second your application needs.
2. Enter the average number of write requests per second.
3. Set the average item size in KB (affects capacity unit consumption).
4. Choose between eventually consistent and strongly consistent reads.
5. Enter storage volume in GB for your table data.
6. Review the on-demand vs provisioned cost comparison.
7. Check the break-even analysis to determine which mode to use.

Example Calculation

Tips & Best Practices

• Start with on-demand mode for new tables until traffic patterns stabilize.
• Switch to provisioned mode once you have 2+ weeks of steady usage data.
• Enable auto-scaling on provisioned tables to handle traffic spikes without over-provisioning.
• DynamoDB Reserved Capacity offers further discounts for provisioned mode (1 or 3-year terms).
• Use DAX (DynamoDB Accelerator) to cache reads and reduce RCU consumption.
• Strongly consistent reads consume 2x the capacity of eventually consistent reads.
• Design partition keys for even distribution to avoid hot partitions and throttling.

On-Demand vs Provisioned Capacity Deep Dive

On-demand pricing is $1.25 per million write request units and $0.25 per million read request units. Provisioned pricing is $0.00065 per WCU per hour and $0.00013 per RCU per hour. The math clearly shows provisioned is cheaper per-unit, but the key question is utilization: if you provision for peak but average 20% utilization, on-demand may still win.

Auto-Scaling Best Practices

DynamoDB auto-scaling adjusts provisioned capacity based on utilization targets (typically 70%). It responds within minutes, making it suitable for gradual traffic changes but not sudden spikes. Set minimum capacity high enough to handle baseline traffic without throttling, and use a target utilization of 50–70% for a good balance of cost and headroom.

Reserved Capacity for Maximum Savings

For tables with predictable, steady throughput, DynamoDB Reserved Capacity offers significant discounts. A 1-year commitment provides up to 53% savings, while a 3-year commitment saves up to 76%. Reserved capacity applies to provisioned mode only and can cover base throughput while auto-scaling handles peaks.

Frequently Asked Questions

• What is a Read Capacity Unit (RCU)?

  One RCU provides one strongly consistent read per second (or two eventually consistent reads) for items up to 4 KB. Larger items consume additional RCUs proportionally.

• What is a Write Capacity Unit (WCU)?

  One WCU provides one write per second for items up to 1 KB. A 3 KB item consumes 3 WCUs per write. Transactional writes consume 2 WCUs per item.

• When is on-demand mode cheaper?

  On-demand is cheaper when your workload is highly variable or spiky with long idle periods, or when average utilization is below about 15–18% of what you would provision. For steady workloads running consistently, provisioned mode is usually 5–6x cheaper.

• Can I switch between modes?

  Yes, you can switch between on-demand and provisioned mode once every 24 hours. The change takes effect immediately. Some teams use on-demand for weekends and provisioned for weekdays.

• How does DynamoDB storage pricing work?

  DynamoDB charges $0.25 per GB per month for storage, measured as the total size of all items including attribute names. Local secondary index data is also charged at the same rate.

• What about Global Secondary Index costs?

  Each GSI provisions its own RCUs and WCUs independently of the base table. In on-demand mode, GSI reads and writes are billed at the same per-request rates. GSI storage is billed at the same $0.25/GB rate.