DynamoDB Item Size Calculator
Paste a DynamoDB item as JSON to see its exact size in bytes and the read and write capacity units it consumes — then share the result with a link.
Estimate per AWS sizing rules. Need cost too? Pricing calculator
Why item size matters
In DynamoDB you don’t pay per row — you pay for the capacity each operation consumes, and that capacity is a function of item size. A read of an item up to 4 KB costs one read unit; a write up to 1 KB costs one write unit. Cross a boundary by a single byte and the operation rounds up to the next unit, so an item that creeps from 4 KB to 4.1 KB doubles its read cost.
Item size also caps what DynamoDB will store: a single item, including every attribute name and value, must stay under 400 KB. Large blobs belong in S3 with a pointer in the item. Knowing the byte size before you design your access patterns helps you keep hot items small, batch efficiently, and avoid surprise throttling.
This calculator applies AWS’s documented sizing rules to plain JSON: UTF-8 byte length for names and strings, significant-digit encoding for numbers, and the per-element overhead for nested lists and maps. The capacity grid then shows the units for every read consistency mode and for standard versus transactional writes.
Every byte is an attribute value — DynamoDB data types explains how each type is encoded and sized.
Frequently asked questions
How does DynamoDB calculate item size?
Item size is the sum, over every attribute, of the UTF-8 byte length of the attribute name plus the size of its value. Strings count their UTF-8 bytes, numbers are encoded by significant digits, booleans and null take 1 byte, and lists and maps add 3 bytes of overhead plus 1 byte per element. This calculator applies those exact rules to your pasted JSON.
How are capacity units derived from item size?
A read capacity unit covers a strongly-consistent read of an item up to 4 KB; eventually-consistent reads cost half a unit and transactional reads cost two. A write capacity unit covers a write up to 1 KB, and transactional writes cost two. Item size always rounds up to the next 4 KB (reads) or 1 KB (writes) boundary, with a minimum of one unit.
What is the maximum DynamoDB item size?
A single DynamoDB item — the item plus all its attribute names and values — cannot exceed 400 KB. If your item is larger, split it across multiple items or store the large attributes in Amazon S3 and keep a reference in the item. The calculator flags items that cross the limit.
Why can’t I enter Sets or Binary values?
Plain JSON has no way to express a DynamoDB String Set, Number Set, Binary Set, or Binary value — a JSON array always maps to a List (L) and a JSON object to a Map (M). For sizing those types, model them as their byte length manually, or use the DynamoDB JSON converter to see the attribute-value form.