Space Calculation

Correct space calculation prevents failed inits and rent surprises. Sum discriminators, Borsh fields, or size_of::<Pod>(), then query Rent for lamports.

Recipe

pub const SPACE: usize = 8 + User::LEN;
let lamports = Rent::get()?.minimum_balance(SPACE);

When to reach for this:

• Creating accounts in program or client.
• Estimating protocol rent costs.
• Reviewing Anchor space = attributes.

Working Example

#[derive(BorshSerialize, BorshDeserialize)]
pub struct User { pub authority: Pubkey, pub points: u64 }
impl User { pub const LEN: usize = 32 + 8; }
pub const USER_SPACE: usize = 8 + User::LEN; // disc + body

What this demonstrates:

• 8-byte discriminator plus body.
• LEN constant documented.
• Rent from SPACE constant.

Deep Dive

Formula

SPACE = disc + sum(field sizes) + variable length caps.

Variable Vec

4 + max_entries * entry_size.

Rust Notes

// Host test: assert_eq!(serialized.len(), USER_SPACE - 8);

Gotchas

• Forgot discriminator - Off-by 8. Fix: Always add disc.
• String unbounded - Cannot precompute. Fix: Use max UTF-8 bytes.
• Option fields - 1 + inner size each. Fix: Include in sum.
• Wrong Pubkey size - Must be 32. Fix: Use constants.
• Padding in Pod - Use size_of not manual guess. Fix: Compile-time assert.

49454e4f4d5f434e4f5904434556494d4345191f1256181a181c1a1d

Alternatives

FAQs

Related

• Borsh Structs - Field sizes
• Rent & Rent Exemption - Economics
• Creating Accounts via CPI - Allocate

Stack versions: This page was written for Agave 4.1.1, Solana CLI 3.0.10, Anchor 0.32.1, anchor-lang 0.32.1, Rust 1.91.1, @solana/kit 7.0.0, Surfpool 0.12.0, and LiteSVM 0.6.x.