do you have any cheat sheet in order how the variables should be ordered to achieve best possible packing and to be more gas efficient

For example I know bool takes 8 bits, address takes 20 bytes but what is the correct order for all do you have any idea? Thanks in advance!

1 Answer 1


The EVM operates on a 256 bits basis, so you need to order variables to fit in 256 bits.

Unpacked variables:

uint128 a; // 128 bits => fills one slot => 256 bits
uint256 b;
uint128 c; // 128 bits => fills one slot => 256 bits

Packed variables:

uint128 a;
uint128 c; // 128 + 128 = 256 bits
uint256 b;

The first example is not gas efficient since 3 slots will be used to store the data, while the second will only need 2 slots.

Source: https://www.maxgrok.com/posts/saving-gas-with-packing-in-solidity

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