# max_burn (/docs/hyperparameters/max-burn)

Caps how expensive a burned registration can get, no matter how many
registrations pile in. Miners care because it bounds their worst-case entry
cost during a registration rush; owners tune it to decide how hard a rush
should be throttled by price.

## How it works [#how-it-works]

The burn price moves in two ways, and both are clamped into
`[min_burn, max_burn]` in `pallets/subtensor/src/subnets/registration.rs`:

* Each successful registration multiplies the price by
  [`burn_increase_mult`](/docs/hyperparameters/burn-increase-mult)
  (`bump_registration_price_after_registration`). Once the product exceeds
  `max_burn`, the price is pinned at `max_burn` — further registrations in the
  same burst cost exactly `max_burn` each.
* Each block, the price decays exponentially with half-life
  [`burn_half_life`](/docs/hyperparameters/burn-half-life)
  (`update_registration_prices_for_networks`), pulling it back down from the
  ceiling toward [`min_burn`](/docs/hyperparameters/min-burn).

The value lives in `MaxBurn` storage as a rao amount (1 TAO = 1e9 rao); the
chain default is 100 TAO. The owner-set extrinsic (`sudo_set_max_burn` in
`pallets/admin-utils/src/lib.rs`) requires the new value to be **above 0.1 TAO**
(`MaxBurnLowerBound`) and **strictly greater than the subnet's current
`min_burn`**. A low ceiling makes registration cheap but lets bursts through; a
high ceiling makes sustained bursts increasingly expensive.

<HyperparamBurnController focus="max_burn" />

## Reading and setting [#reading-and-setting]

```bash
btcli sudo get --netuid 12 --name max_burn
btcli sudo set --netuid 12 --name max_burn --value 50.0
```

Settable by the subnet owner or root. A value with a decimal point is a TAO
amount (`50.0` = τ50); a plain integer is raw rao.

## Related [#related]

* [`min_burn`](/docs/hyperparameters/min-burn) — the matching floor
* [`burn_increase_mult`](/docs/hyperparameters/burn-increase-mult) — how fast a
  rush drives the price toward this cap
* [`burn_half_life`](/docs/hyperparameters/burn-half-life) — how fast it comes
  back down
* [`max_regs_per_block`](/docs/hyperparameters/max-regs-per-block) — the
  non-price rate limit