DynamoDB Capacity Units Calculator: RCU, WCU & Cost Guide

DynamoDB's capacity model — Read Capacity Units and Write Capacity Units — determines both performance and cost. Understanding how item sizes map to capacity consumption is essential to avoid throttling and bill surprises.

RCU and WCU Calculation Rules

On-Demand vs Provisioned Pricing

Provisioned Capacity with Auto Scaling

# Terraform: DynamoDB with auto-scaling
resource "aws_dynamodb_table" "users" {
  name           = "users"
  billing_mode   = "PROVISIONED"
  read_capacity  = 5     # Minimum provisioned
  write_capacity = 5

  hash_key  = "id"

  attribute {
    name = "id"
    type = "S"
  }

  ttl {
    attribute_name = "expiresAt"  # TTL = free deletes
    enabled        = true
  }
}

# Auto-scaling policy (scales between 5-100 WCU)
resource "aws_appautoscaling_target" "dynamodb_write" {
  max_capacity       = 100
  min_capacity       = 5
  resource_id        = "table/${aws_dynamodb_table.users.name}"
  scalable_dimension = "dynamodb:table:WriteCapacityUnits"
  service_namespace  = "dynamodb"
}

resource "aws_appautoscaling_policy" "dynamodb_write_policy" {
  name               = "DynamoDBWriteCapacityUtilization"
  policy_type        = "TargetTrackingScaling"
  resource_id        = aws_appautoscaling_target.dynamodb_write.resource_id
  scalable_dimension = aws_appautoscaling_target.dynamodb_write.scalable_dimension
  service_namespace  = aws_appautoscaling_target.dynamodb_write.service_namespace

  target_tracking_scaling_policy_configuration {
    predefined_metric_specification {
      predefined_metric_type = "DynamoDBWriteCapacityUtilization"
    }
    target_value = 70.0  # Scale when 70% utilized
  }
}

Cost Optimization Strategies

// 1. Use eventually consistent reads (half the RCU cost)
const result = await dynamoDB.get({
  TableName: 'users',
  Key: { id: { S: userId } },
  ConsistentRead: false  // Eventually consistent = 0.5 RCU (default)
  // ConsistentRead: true = 1 RCU (use only when freshness required)
}).promise();

// 2. Project only needed attributes
const result = await dynamoDB.query({
  TableName: 'orders',
  KeyConditionExpression: 'userId = :uid',
  ExpressionAttributeValues: { ':uid': { S: userId } },
  // Only fetch what you need — reduces data transfer and effective RCU
  ProjectionExpression: 'orderId, #s, amount',
  ExpressionAttributeNames: { '#s': 'status' },
}).promise();

// 3. Use TTL for auto-cleanup (0 WCU for deletes)
const item = {
  id: { S: sessionId },
  userId: { S: userId },
  data: { S: JSON.stringify(sessionData) },
  // TTL: Unix timestamp — DynamoDB deletes automatically (free!)
  expiresAt: { N: String(Math.floor(Date.now() / 1000) + 86400) }
};