# burn_half_life (/docs/hyperparameters/burn-half-life)

Sets how quickly an elevated registration price cools off. After registrations
bump the burn cost up, it decays exponentially — this parameter is the number
of blocks for the price to halve. Owners tune it against
[`burn_increase_mult`](/docs/hyperparameters/burn-increase-mult) to pick the
registration rate their subnet sustains; miners watching an expensive subnet
can read it as "how long until the price is reasonable again."

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

Every block,
[`update_registration_prices_for_networks`](/code/pallets/subtensor/src/subnets/registration.rs#L473-L515)
(in `pallets/subtensor/src/subnets/registration.rs`, called from
[`block_step`](/code/pallets/subtensor/src/coinbase/block_step.rs#L7-L36))
multiplies each subnet's burn by a per-block factor `f` chosen so that
`f ^ burn_half_life = 1/2`, then clamps into
\[[`min_burn`](/docs/hyperparameters/min-burn),
[`max_burn`](/docs/hyperparameters/max-burn)]. The factor is computed in Q32
fixed-point by binary search
([`decay_factor_q32`](/code/pallets/subtensor/src/utils/misc.rs#L1043-L1067) in
`pallets/subtensor/src/utils/misc.rs`), giving continuous exponential decay:
after `n` blocks without registrations the price is `burn × 0.5^(n /
half_life)`, until it hits the `min_burn` floor.

The value is stored in
[`BurnHalfLife`](/code/pallets/subtensor/src/lib.rs#L2726) as a u16 block
count; the default is
**360 blocks** (\~72 minutes at 12s blocks). A stored value of 0 disables decay
entirely, but the owner-set extrinsic
([`sudo_set_burn_half_life`](/code/pallets/admin-utils/src/lib.rs#L2128-L2164)
in `pallets/admin-utils/src/lib.rs`) requires **1 to 36,100 blocks** (\~5 days) and
rejects the root subnet. Together with the multiplier, the break-even
registration cadence is one registration per `half_life × log2(mult)` blocks —
faster than that and the price climbs, slower and it falls.

<HyperparamBurnController focus="burn_half_life" />

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

```bash
btcli sudo get --netuid 12 --name burn_half_life
btcli sudo set --netuid 12 --name burn_half_life --value 720
```

Settable by the subnet owner or root; the value is a plain block count
(12-second blocks).

## Related [#related]

* [`burn_increase_mult`](/docs/hyperparameters/burn-increase-mult) — the bump
  this decay works against
* [`min_burn`](/docs/hyperparameters/min-burn) — where the decay comes to rest
* [`max_burn`](/docs/hyperparameters/max-burn) — the ceiling it decays down from