Space Reference

This reference tells you how much space you should allocate for an account. This only applies to accounts that don't use zero-copy. zero-copy uses repr(C) with a pointer cast, so there the C layout applies.

In addition to the space for the account data, you have to add 8 to the space constraint for Anchor's internal discriminator (see the example).

TypesSpace in bytesDetails/Example
bool1would only require 1 bit but still uses 1 byte
u8/i81
u16/i162
u32/i324
u64/i648
u128/i12816
[T;amount]space(T) * amounte.g. space([u16;32]) = 2 * 32 = 64
Pubkey32
Vec<T>4 + (space(T) * amount)Account size is fixed so account should be initialized with sufficient space from the beginning
String4 + length of string in bytesAccount size is fixed so account should be initialized with sufficient space from the beginning
Option<T>1 + (space(T))
Enum1 + Largest Variant Sizee.g. Enum { A, B { val: u8 }, C { val: u16 } } -> 1 + space(u16) = 3
f324serialization will fail for NaN
f648serialization will fail for NaN

Example

#[account]
pub struct MyData {
    pub val: u16,
    pub state: GameState,
    pub players: Vec<Pubkey> // we want to support up to 10 players
}

impl MyData {
    pub const MAX_SIZE: usize = 2 + (1 + 32) + (4 + 10 * 32);
}

#[derive(AnchorSerialize, AnchorDeserialize, Clone, PartialEq, Eq)]
pub enum GameState {
    Active,
    Tie,
    Won { winner: Pubkey },
}

#[derive(Accounts)]
pub struct InitializeMyData<'info> {
    // Note that we have to add 8 to the space for the internal anchor
    #[account(init, payer = signer, space = 8 + MyData::MAX_SIZE)]
    pub acc: Account<'info, MyData>,
    pub signer: Signer<'info>,
    pub system_program: Program<'info, System>
}