Storage mass

Storage mass in brief

Kaspa charges for the UTXOs a transaction creates through storage mass (KIP-9). Small outputs cost a lot of mass; large outputs cost little. For an output of value v (in sompi), the output’s term is:

C × p² / v        with C = 10¹²  (SOMPI_PER_KASPA × 10 000)

p is the output’s plurality: how many 100-byte storage units its UTXO occupies. The transaction’s storage mass is the sum of its outputs’ terms, less a term for its inputs. Before Toccata, a transaction above 100 000 grams of mass is not relayed (see After Toccata).

Why covenant UTXOs count double

Plurality is computed from the UTXO’s size:

p = ceil((63 + script_length) / 100)

A P2SH script is 35 bytes, so a plain P2SH UTXO has p = 1. KIP-20 treats a covenant UTXO as carrying 32 more bytes, for its covenant ID. That makes 63 + 35 + 32 = 130 bytes, so p = 2. Because the term grows with p², a covenant output costs four times the storage mass of a plain output of the same value.

What this means for token states

The output term for different KAS values:

KAS in the output	Plain output (`p = 1`)	Covenant output (`p = 2`)
0.5 KAS	20 000	80 000
1 KAS	10 000	40 000
1.5 KAS	6 667	26 667
2 KAS	5 000	20 000
3 KAS	3 333	13 333

A mint creates the BridgeMint successor and its token states, all covenant outputs. With too little KAS per state, a mint would cost far more storage mass, and so more fee. The test bridge/kaspa/tests/standard.rs runs the real builders at their largest shapes, with 1.5 KAS per token state and a 5 KAS minter:

Transaction	Storage mass
Mint to 3 recipients (the maximum)	87 993
KCC-20 transfer, 3 inputs to 3 outputs (the maximum)	79 957
Redemption: one 0.3 KAS burn and a 1.5 KAS change token	159 994

A burn is never spent, so its KAS is gone for good. It carries 0.3 KAS, not 1.5: the KAS saved outweighs the extra storage mass it pays for. Measured with the same builder:

KAS in the burn	Storage mass	Fee	The redemption costs
1.5	53 327	0.053 KAS	1.553 KAS
0.5	106 661	0.107 KAS	0.607 KAS
0.3	159 994	0.160 KAS	0.460 KAS
0.2	226 661	0.227 KAS	0.427 KAS
0.15	293 327	0.293 KAS	0.443 KAS

Below 0.3 KAS a burn saves almost nothing more and soon costs more in fee. Mints and transfers stay under the old 100 000 limit; a redemption with a 0.3 KAS burn is over it, which is fine after Toccata (below). Run bash tools/rust.sh bridge/kaspa cargo test --test standard -- --nocapture to print the masses.

After Toccata

The 100 000 per-transaction standard cap applies only before Toccata. It is lifted for good 30 minutes before Toccata activates. After that only the block limits apply: 500 000 compute and storage mass, and 1 000 000 transient mass. Covenant transactions only exist after Toccata, so the old cap never meets them.

The kUSDT builders cap a transaction’s compute and storage mass at 300 000 (MAX_STANDARD_MASS, under the 500 000 block limit). Redemptions use that room: two 0.3 KAS burns in one transaction come to about 293 000. Pool transactions carry more transient mass: a sell is about 108 000, an add about 114 000. tests/standard.rs allows up to 250 000 transient mass, a quarter of the block limit.

So each kUSDT state carries 1.5 KAS by default. This is not a fee:

the KAS sits inside the token UTXO and belongs to its owner;
when the owner spends the token, that KAS funds the new outputs or returns as change;
whoever creates a token output supplies its KAS. For a mint, that is the coordinator’s Kaspa funding, paid out of the deposit fee.

The one exception is a burn: a burn state can never be spent, so its KAS is gone for good. Burn outputs therefore carry the deployment’s smaller burn value, 0.3 KAS, not the 1.5 KAS token value. A smaller output costs more storage mass, so the redemption pays a higher network fee: about 0.3 KAS in the burn plus about 0.16 KAS of fee, some 0.46 KAS per redemption, against about 1.55 KAS with a 1.5 KAS burn (SPEC §9).

Other covenant UTXOs

UTXO	KAS in the simnet run
BridgeMint	5 KAS (changeable by `set_value`, 4 of 5)
TokenSeed	1 KAS
kUSDT state	1.5 KAS
KasPool Position	2 KAS
Pool	A 1 KAS base deposit, plus its reserve and the protocol’s KAS

Measuring it

The transaction builders compute the exact mass with Kaspa’s own mass calculator and refuse a transaction above the standard limit. The simnet run prints the compute, storage and transient mass of every transaction. Set PROBE_MASS=1 to make it also print the storage mass of a mint at token values from 0.1 to 3 KAS.