DynamoDB Read/Write Capacity and On-Demand Mode

DynamoDB's pricing and performance are governed by capacity modes. Understanding the difference between provisioned and on-demand capacity — and when to use each — is essential for controlling costs and avoiding throttling.

---

Capacity Units Explained

DynamoDB measures throughput in Read Capacity Units (RCUs) and Write Capacity Units (WCUs).

Read Capacity Units (RCU)

Read Type	Item Size	RCUs Consumed
Strongly consistent	≤ 4 KB	1 RCU
Eventually consistent	≤ 4 KB	0.5 RCU
Transactional	≤ 4 KB	2 RCUs

For items larger than 4 KB, DynamoDB rounds up to the next 4 KB boundary. A 6 KB strongly consistent read consumes 2 RCUs (rounded to 8 KB ÷ 4 KB = 2).

Write Capacity Units (WCU)

Write Type	Item Size	WCUs Consumed
Standard	≤ 1 KB	1 WCU
Transactional	≤ 1 KB	2 WCUs

A 2.5 KB write consumes 3 WCUs (rounded up to 3 KB).

Calculation Examples

Scenario: Your application performs 500 strongly consistent reads per second on items averaging 3 KB.

Each read: ⌈3 KB / 4 KB⌉ = 1 RCU
Total: 500 × 1 = 500 RCUs

Scenario: 200 eventually consistent reads per second on 9 KB items.

Each read: ⌈9 KB / 4 KB⌉ = 3 RCUs (strongly consistent)
Eventually consistent: 3 × 0.5 = 1.5 RCUs per read
Total: 200 × 1.5 = 300 RCUs

Scenario: 100 writes per second on 2 KB items.

Each write: ⌈2 KB / 1 KB⌉ = 2 WCUs
Total: 100 × 2 = 200 WCUs

---

Provisioned Capacity Mode

In provisioned mode, you specify the number of RCUs and WCUs for your table. DynamoDB reserves that capacity, and you pay for it whether you use it or not.

Auto Scaling

You can enable Application Auto Scaling to adjust provisioned capacity automatically:

• Set a target utilisation (e.g., 70%)
• Define minimum and maximum capacity
• DynamoDB scales up when utilisation exceeds the target and scales down when it drops below

Target: 70%
Min RCUs: 100
Max RCUs: 5,000

If current utilisation = 90% → Auto Scaling increases RCUs
If current utilisation = 40% → Auto Scaling decreases RCUs

Reserved Capacity

For steady-state workloads, you can purchase reserved capacity with a 1- or 3-year commitment:

Commitment	Approximate Discount
1-year, no upfront	~20%
1-year, partial upfront	~25%
3-year, all upfront	~50%

Reserved capacity is purchased in blocks of 100 WCUs or 100 RCUs.

Burst Capacity

DynamoDB retains up to 5 minutes of unused capacity as burst credits. If your application suddenly needs more throughput, burst credits are consumed first. Once burst credits are exhausted, requests are throttled (HTTP 400 ProvisionedThroughputExceededException).

---

On-Demand Capacity Mode

In on-demand mode, DynamoDB instantly accommodates your workload as it ramps up or down. You pay per request — no capacity planning required.

Pricing (us-east-1, approximate)

Operation	Cost
Write request unit	$1.25 per million
Read request unit	$0.25 per million
Storage	$0.25 per GB-month

When to Choose On-Demand