Convert a DynamoDB read and write workload into RCUs and WCUs, then compare provisioned capacity against on-demand pricing.
DynamoDB rounds every operation up to a whole capacity unit before charging, so a 6 KB item costs two read units and a 1.2 KB item costs two write units — item size shaping is a real lever. Provisioned mode bills reserved units per hour whether used or not, while on-demand bills the units actually consumed, which is why the crossover sits at roughly 15–20% sustained utilisation of a provisioned table. All four prices are editable inputs with realistic us-east-1 defaults, so confirm current DynamoDB pricing with AWS for your region. The two modes can differ by an order of magnitude in either direction: on-demand wins on spiky or unknown traffic, provisioned with autoscaling wins on steady traffic, and the only way to know which is yours is to price both.
DynamoDB Capacity
RCU = reads/s × ceil(item KB ÷ 4) × consistency factor (0.5 eventual, 1 strong, 2 transactional); WCU = writes/s × ceil(item KB) × (2 if transactional).
RCU = reads/s × ceil(item KB ÷ 4) × consistency factor (0.5 eventual, 1 strong, 2 transactional); WCU = writes/s × ceil(item KB) × (2 if transactional). DynamoDB rounds every operation up to a whole capacity unit before charging, so a 6 KB item costs two read units and a 1.2 KB item costs two write units — item size shaping is a real lever. Provisioned mode bills reserved units per hour whether used or not, while on-demand bills the units actually consumed, which is why the crossover sits at roughly 15–20% sustained utilisation of a provisioned table. All four prices are editable inputs with realistic us-east-1 defaults, so confirm current DynamoDB pricing with AWS for your region.
The two modes can differ by an order of magnitude in either direction: on-demand wins on spiky or unknown traffic, provisioned with autoscaling wins on steady traffic, and the only way to know which is yours is to price both.
This calculator takes 13 inputs: Read operations, Average item size read, Read consistency, Write operations, Average item size written, Write type, Provisioned read capacity price, Provisioned write capacity price, Billed hours per month, Table storage, Storage price, On-demand read price, On-demand write price. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
A transactional read or write is executed as a two-phase operation — prepare then commit — so DynamoDB charges two capacity units for every unit of data. Reserve TransactGetItems and TransactWriteItems for the operations that genuinely need atomicity.
They are realistic us-east-1 standard-table defaults. Prices differ by region and for Standard-Infrequent Access tables, so confirm them on the DynamoDB pricing page before budgeting.