Developer tools · Capacity
DynamoDB capacity calculator.
Read and write capacity units from item size and request rate, with on-demand against provisioned cost.
Inputs
Capacity needed
Enter the values.
How do you calculate DynamoDB RCU and WCU?
One read capacity unit is one strongly consistent read per second of an item up to 4 KB; an eventually consistent read costs half, a transactional read costs double. One write capacity unit is one write per second of an item up to 1 KB; a transactional write costs double. Item sizes round up to the next 4 KB for reads and 1 KB for writes, so a 3.5 KB item costs the same to read as a 4 KB one.
Worked example. A 6 KB item rounds to 8 KB for reads, which is 2 units per strongly consistent read, so 100 reads per second need 200 RCU. It rounds to 6 KB for writes, so 50 writes per second need 300 WCU. Eventually consistent reads would need 100 RCU.
Cost uses the prices you enter. The defaults are the published US East (N. Virginia) standard-table prices at the time of checking: $0.625 per million write request units and $0.125 per million read request units on demand, and $0.00065 per WCU-hour and $0.00013 per RCU-hour provisioned. Prices differ by region and table class and change over time, so confirm them on the AWS pricing page. The estimate assumes a steady rate all month and ignores storage, indexes, reserved capacity, auto scaling and free tier.
Frequently asked questions
How many RCU do I need for 100 reads per second?
For items up to 4 KB, 100 strongly consistent reads per second need 100 RCU, 50 RCU if eventually consistent and 200 RCU if transactional. Larger items need one more unit per extra 4 KB.
How is a DynamoDB write capacity unit calculated?
One WCU covers one write per second of an item up to 1 KB. A 2.5 KB item rounds up to 3 KB and uses 3 write units; transactional writes use twice as many.
Is on-demand or provisioned cheaper for DynamoDB?
At a steady, well-predicted rate provisioned is usually lower; on-demand wins for spiky or unpredictable traffic because you pay per request and need no capacity planning. Enter your own rate and prices to compare.
Do eventually consistent reads cost less?
Yes. AWS documents an eventually consistent read of up to 4 KB as one-half read unit, a strongly consistent one as one, and a transactional one as two.
Sources
Checked
Unit definitions are from the AWS documentation. Price defaults are the US East (N. Virginia) standard-table figures AWS published when checked; they are inputs, not guarantees. Runs entirely in your browser. Nothing you type is sent or stored.
Scope
Planning estimates, not guarantees. Real systems add overhead this model leaves out and behave differently under load. Measure on your own data and load-test before committing to a design or a bill.