Practical guide
DynamoDB index costs: budget write amplification explicitly
estimate DynamoDB writes with secondary indexes. Includes a worked example, calculator scope, and practical questions.
Updated · Sources checked
Index work can add to the base-table request budget
A database cost estimate should include the request units consumed by its indexes, not just the original application writes. How much extra work occurs depends on the index design, projected data, and whether an update changes indexed information.
Use measured consumed units when possible. Treat a simple multiplier as a clearly labelled scenario rather than assuming every index always costs exactly the same as the table.
Worked example: a deliberately simple index scenario
Suppose a month contains two million base-table writes. For this example, each base-table write consumes one write request unit, and each also causes one unit of write work in each of two global secondary indexes.
The total is two million base units plus four million index units: six million write units. Enter six in the calculator’s millions field, not two, when estimating this hypothetical combined request subtotal.
If a proposed design removes one of those index writes for every operation, usage becomes four million units. The difference is two million write units. This assumes the same operation count and per-operation units; larger projected index items or different update behavior require a new calculation.
Evaluate the access pattern you would lose
An index exists to serve an access pattern. Removing it can push queries toward less efficient reads or require an application redesign. Estimate that read impact before calling the avoided writes a net saving.
Record which attributes queries need and how frequently they run. A smaller projection may change index storage and write size, but it can also require additional table reads to retrieve omitted attributes. A useful cost comparison models those effects together.
The tool prices supplied Standard on-demand read and write units in US East. It excludes storage, backups, global tables, streams, transfer, and other services. Index storage therefore still needs a separate line, and a request-only saving is not the total database saving.
Frequently asked questions
Do all updates write every index? No. Use the actual index definitions and affected attributes; the worked example intentionally assumes every operation affects both indexes.
Can an index lower total cost? Potentially, if it enables more efficient reads or avoids other work. Compare the additional write and storage costs against the access pattern it supports.
Calculate your scenario
Use the dynamodb cost calculator. Keep the displayed region, pricing date, billing units, and exclusions alongside your result. The arithmetic examples above illustrate usage or explicitly hypothetical rates; they are not AWS quotes.
Source
Billing structure checked against official AWS documentation on September 6, 2026.