Understanding the Size of Solidity Variables in Bytes


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Understanding the Size of Solidity Variables in Bytes

As a Smart Contract Engineer, I often encounter questions regarding the size of various data types in Solidity, the programming language primarily used for writing smart contracts on the Ethereum blockchain. Understanding these sizes is crucial for optimizing gas costs and ensuring efficient contract execution. In this blog post, I'll discuss the sizes of different Solidity variables in bytes and explain some key concepts related to Solidity data types.

Basic Solidity Data Types

1. Booleans

  • Size: 1 byte.

  • Description: Represents true or false.

2. Integers

  • Sizes: int and uint vary in size from 8 bits (int8, uint8) to 256 bits (int256, uint256) in 8-bit increments.

  • Description: Used for numerical values. int represents signed integers, and uint represents unsigned integers. For instance, uint8 occupies 1 byte, while uint256 occupies 32 bytes.

3. Address

  • Size: 20 bytes.

  • Description: Used to store Ethereum addresses.

4. Bytes

  • Sizes: bytes1 to bytes32 (1 to 32 bytes).

  • Description: Fixed-size byte arrays. bytes1 is 1 byte, and bytes32 is 32 bytes.

5. Strings

  • Size: Dynamically sized.

  • Description: Used for arbitrary-length string (UTF-8) data.

Complex Data Types

1. Arrays

  • Size: Depends on the element type and array length.

  • Description: Can be fixed-size or dynamically-sized. For example, an array of uint256[10] will occupy 320 bytes (10 elements * 32 bytes each).

2. Structs

  • Size: Sum of the sizes of individual elements.

  • Description: Custom-defined types that can group several variables.

3. Mappings

  • Size: Not stored in a straightforward manner like other types.

  • Description: Key-value pairs where the key is not stored in the contract, but the value is. The size depends on the data type of the value.

Storage and Memory Considerations

In Solidity, variables can be stored in three locations: storage, memory, and stack. The choice of storage location significantly impacts gas costs and efficiency:

  • Storage: Persistent storage of contract state. Expensive in terms of gas cost.

  • Memory: Temporary storage during function execution. Cheaper than storage but more expensive than the stack.

  • Stack: Very limited size, used for small local variables.

Best Practices

  1. Optimize Data Types: Use the smallest data types that can safely store your data.

  2. Pack Variables: Solidity allows for tight packing of small-sized variables in a single 32-byte word.

  3. Memory vs. Storage: Use memory for temporary variables to save gas costs.


Understanding the sizes of Solidity variables is key to writing efficient and cost-effective smart contracts. By choosing appropriate data types and storage locations, developers can optimize their contracts for better performance on the Ethereum blockchain.