Anchor init & Account Space

How Anchor's

Anchor framework concept

What it is

When an Anchor account is created with #[account(init, payer = …, space = N)], N is the exact byte size to allocate. That size must equal 8 (the discriminator) plus the Borsh-serialized size of every field. Get it right and the account is rent-exempt and correctly sized; get it wrong and you either waste rent or can’t fit the data.

Why it exists

Solana accounts have a fixed size set at creation, and you pre-pay rent for that size to be rent-exempt. Anchor doesn’t know your struct’s serialized size automatically in older versions, so you declare it. The byte math is the bridge between your Rust types and the on-chain allocation.

Byte layout

The space budget is a sum of byte sizes:

Component	Bytes	Notes
discriminator	8	Always — every Anchor account starts with it.
`bool`	1
`u8` / `i8`	1
`u16` / `i16`	2
`u32` / `i32`	4
`u64` / `i64`	8
`Pubkey`	32
`Option<T>`	1 + sizeof(T)	1-byte Borsh tag + the inner value when present.
`Vec<T>`	4 + len × sizeof(T)	4-byte length prefix; you must budget for the max length.
`String`	4 + max_byte_len	4-byte length prefix + UTF-8 bytes; budget the max.

So a struct { owner: Pubkey, amount: u64, label: String(≤16) } needs 8 + 32 + 8 + (4 + 16) = 68 bytes.

Where you see it

Every init constraint in an Anchor program, and the rent calculation behind every account-creation transaction. The allocated size shows up as the account’s space/data length on-chain.

Common gotchas

Always add 8. The discriminator is part of the account; forgetting it under-allocates by exactly 8 bytes and your last field gets truncated. The single most common Anchor space bug.
Vec and String need a max budget. They’re variable-length but the account is fixed-size — you must reserve space for the largest value you’ll ever store, plus the 4-byte length prefix.
Option<T> is 1 + T, not T. The Borsh tag byte is always present even when the value is None. Don’t size it as just sizeof(T).
Over-allocating wastes rent; under-allocating bricks writes. Size is locked at creation (absent realloc). Compute it exactly. Recent Anchor offers InitSpace derive to compute it for you — prefer it over hand-counting.

Sources

Anchor docs — space reference