DynamoDB Write Capacity Calculator
Calculate WCUs for a write workload including global secondary index amplification, transactions and on-demand comparison.
About DynamoDB Write Capacity Calculator
Writes round up to whole 1 KB units, transactions double the charge, and — the part that surprises people — every global secondary index whose projected attributes change consumes a full set of write units of its own. Three GSIs therefore make a write cost four times the base table figure, which is why projecting only the attributes an index query actually needs is a direct cost control. The WCU and on-demand write prices are editable inputs seeded with realistic us-east-1 defaults, so confirm current DynamoDB pricing with AWS for your region. GSI write amplification is the single most common cause of a DynamoDB bill that is several times the estimate, and it is invisible unless you count the indexes explicitly.
Formula
DynamoDB Write Capacity
units per write = ceil(item KB) × (2 if transactional); total WCU = peak writes/s × units per write × (1 + number of indexes updated).
- 1 KB
- Write unit block size — every write rounds up to a whole kilobyte
- index amplification
- Each GSI touched by a write consumes its own write units
- peak to average
- Ratio of busiest second to the daily average
Worked example
With the default values below, DynamoDB Write Capacity Calculator returns Total WCUs to Provision of 3,600. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Average write operationsaccepts 0–5,000,000 writes/s
- 300 writes/s
- Average item sizeaccepts 0.1–400 KB
- 1.5 KB
- Write typeone of Standard write — one WCU per 1 KB, Transactional write — two WCUs per 1 KB
- Standard write — one WCU per 1 KB
- Global secondary indexes updated per writeaccepts 0–20 indexes
- 2 indexes
- Peak to average ratioaccepts 1–20 x
- 2 x
- Provisioned write capacity priceminimum 0 $/WCU-hour
- 0.00065 $/WCU-hour
- Billed hours per monthaccepts 0–744 hours
- 730 hours
- On-demand write priceminimum 0 $/million write units
- 0.625 $/million write units
Results
- Total WCUs to Provision
- 3,600 WCU
- Base Table WCUs
- 1,200 WCU
- Index WCUs
- 2,400 WCU
- Write Units per Operation
- 2
- Provisioned Monthly Cost
- $1,708.20
- On-demand Monthly Cost
- $2,956.50
Step by step
- 1Values used
- 2DynamoDB Write Capacity
- 3Total WCUs to Provision= 3,600 WCU
- 4Base Table WCUs= 1,200 WCU
- 5Index WCUs= 2,400 WCU
- 6Write Units per Operation= 2
- 7Provisioned Monthly Cost= 1,708.20
- 8On-demand Monthly Cost= 2,956.50
Assumptions and limits
- The result is derived only from the 8 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is DynamoDB Write Capacity calculated?
units per write = ceil(item KB) × (2 if transactional); total WCU = peak writes/s × units per write × (1 + number of indexes updated). Writes round up to whole 1 KB units, transactions double the charge, and — the part that surprises people — every global secondary index whose projected attributes change consumes a full set of write units of its own. Three GSIs therefore make a write cost four times the base table figure, which is why projecting only the attributes an index query actually needs is a direct cost control. The WCU and on-demand write prices are editable inputs seeded with realistic us-east-1 defaults, so confirm current DynamoDB pricing with AWS for your region.
Why does DynamoDB Write Capacity matter?
GSI write amplification is the single most common cause of a DynamoDB bill that is several times the estimate, and it is invisible unless you count the indexes explicitly.
What values do I need to enter?
This calculator takes 8 inputs: Average write operations, Average item size, Write type, Global secondary indexes updated per write, Peak to average ratio, Provisioned write capacity price, Billed hours per month, 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.
Does every write hit every index?
No — a GSI only consumes write units when the write changes its key or one of its projected attributes. Narrowing projections from ALL to KEYS_ONLY or a short INCLUDE list removes most of the amplification.