DynamoDB Capacity Calculator
Convert a DynamoDB read and write workload into RCUs and WCUs, then compare provisioned capacity against on-demand pricing.
Inputs
Provisioned Monthly Cost
$249.75
Read Capacity Units
500RCU
Write Capacity Units
400WCU
Storage
$12.50
On-demand Monthly Cost
$833.75
Cheaper Capacity Mode
Provisioned capacity is cheaper for this traffic shape
Step by step
Values used
Read operations = 500 reads/s; Average item size read = 6 KB; Read consistency = Eventually consistent — half an RCU per 4 KB; Write operations = 200 writes/s; Average item size written = 2 KB; Write type = Standard write — one WCU per 1 KB; Provisioned read capacity price = 0.0001 $/RCU-hour; Provisioned write capacity price = 0.0007 $/WCU-hour; Billed hours per month = 730 hours; Table storage = 50 GB; Storage price = 0.2500 $/GB-month; On-demand read price = 0.1250 $/million read units; On-demand write price = 0.6250 $/million write units
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).
Provisioned Monthly Cost
= 249.75
Read Capacity Units
= 500 RCU
Write Capacity Units
= 400 WCU
Storage
= 12.50
On-demand Monthly Cost
= 833.75
Cheaper Capacity Mode
= Provisioned capacity is cheaper for this traffic shape
How it works
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.
Formula
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
- One strongly consistent 4 KB read per second, or two eventually consistent
- WCU
- One 1 KB write per second
- consistency factor
- Halves the cost for eventual reads, doubles it for transactions
- 2,628,000
- Seconds in a 730-hour month, used to convert a rate into request units
Frequently Asked Questions
How is DynamoDB Capacity calculated?
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.
Why does DynamoDB Capacity matter?
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.
What values do I need to enter?
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.
Why do transactions cost double?
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.
Are these prices current?
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.