Calculating Required RCU And WCU For Your DynamoDB Table

Core Concepts

• Write Capacity Unit (WCU): Represents one write operation per second for an item up to 1 KB in size.
• Read Capacity Unit (RCU): Represents read operations per second for an item up to 4 KB in size. The number of reads per RCU depends on the consistency model.
• The Rounding Up Rule: For any calculation, DynamoDB always rounds the item size up to the next whole kilobyte. A 1.2 KB item is treated as 2 KB for writes and 4 KB for reads.

1. Write Capacity Unit (WCU) Calculation

The Rule

• 1 WCU can perform 1 standard write per second for an item up to 1 KB.

Step-by-Step Calculation

1. Determine Item Size: Find the size of your item in KB.
2. Calculate WCUs per Item: Round the item size up to the nearest whole KB.
   • WCUs_per_item = CEILING(item_size_in_KB / 1_KB)
3. Calculate Total WCUs: Multiply the WCUs per item by the number of writes per second.
   • Total_WCUs = WCUs_per_item * number_of_writes_per_second

Example

You need to perform 10 writes per second for an item that is 2.5 KB in size.

1. Item size is 2.5 KB.
2. Round up to the nearest KB: 3 KB. This means each write requires 3 WCUs.
3. Total WCUs = 3 * 10 = 30 WCUs.

2. Read Capacity Unit (RCU) Calculation

Read calculations depend on the consistency model you choose.

The Rule

• 1 RCU can perform 1 strongly consistent read per second for an item up to 4 KB.
• 1 RCU can perform 2 eventually consistent reads per second for an item up to 4 KB.

Step-by-Step Calculation

Step A: Calculate the "Read Unit Size" of your item.

1. Determine Item Size: Find the size of your item in KB.
2. Calculate Read Units: Round the item size up to the nearest multiple of 4 KB, then divide by 4.
   • Read_Units_per_item = CEILING(item_size_in_KB / 4_KB)

Step B: Use the result from Step A to find your total RCUs.

For Eventually Consistent Reads (Default)

1. Calculate Total Read Units per Second:
   • Total_Read_Units = Read_Units_per_item * number_of_reads_per_second
2. Calculate Total RCUs: Divide the total by 2 and round up to the nearest whole number.
   • Total_RCUs = CEILING(Total_Read_Units / 2)

Example: 100 eventually consistent reads per second for a 9 KB item.

1. Step A (Read Unit Size):
   • CEILING(9 KB / 4 KB) = CEILING(2.25) = 3. Each read requires 3 "read units".
2. Step B (Total RCUs):
   • Total Read Units = 3 * 100 = 300.
   • Total RCUs = CEILING(300 / 2) = 150 RCUs.

For Strongly Consistent Reads

1. Calculate Total RCUs: Multiply the "Read Unit Size" by the number of reads per second.
   • Total_RCUs = Read_Units_per_item * number_of_reads_per_second

Example: 100 strongly consistent reads per second for a 9 KB item.

1. Step A (Read Unit Size):
   • CEILING(9 KB / 4 KB) = 3. Each read requires 3 "read units".
2. Step B (Total RCUs):
   • Total RCUs = 3 * 100 = 300 RCUs.

3. Transactional Operation Calculation

Transactional operations consume double the capacity units of standard operations.

• Transactional Write:

  • Rule: Requires 2 WCUs per write for an item up to 1 KB.
  • Calculation: Calculate the WCUs for a standard write, then multiply by 2.

• Transactional Read:

  • Rule: Requires 2 RCUs per read for an item up to 4 KB.
  • Calculation: Calculate the RCUs for a strongly consistent read, then multiply by 2.

Example

You need to perform 10 transactional writes per second for a 2.5 KB item.

1. Calculate standard WCUs: CEILING(2.5 KB / 1 KB) * 10 = 3 * 10 = 30 WCUs.
2. Apply transactional multiplier: 30 * 2 = 60 WCUs.

Quick Reference Summary