# Estimate Fees & Access Lists

## Overview

The [`Fee`](/api/Fee) module converts `eth_feeHistory` payloads and provides the fee math for
EIP-1559 transactions; [`AccessList`](/api/AccessList) converts access lists between their
object and serialized tuple forms. Both operate on plain data — pair them with an
[`RpcTransport`](/api/RpcTransport) when values come from a node.

## Recipes

### Compute the Effective Gas Price

Use [`Fee.effectiveGasPrice`](/api/Fee/effectiveGasPrice) to determine what an EIP-1559
transaction actually pays per gas:
`min(maxFeePerGas, baseFeePerGas + maxPriorityFeePerGas)`.

```ts twoslash
import { Fee, Value } from 'ox'

const effectiveGasPrice = Fee.effectiveGasPrice({
  baseFeePerGas: Value.fromGwei('20'),
  maxFeePerGas: Value.fromGwei('30'),
  maxPriorityFeePerGas: Value.fromGwei('2'),
})
// @log: 22000000000n (22 gwei = base fee + tip, under the 30 gwei cap)
```

`maxFeePerGas` is a cap, not a price — the difference between the cap and the effective price
is refunded to the sender.

### Estimate `maxFeePerGas` from Fee History

Fetch recent blocks with `eth_feeHistory`, convert the payload with
[`Fee.fromHistoryRpc`](/api/Fee/fromHistoryRpc), then derive a cap with
[`Fee.estimateMaxFeePerGas`](/api/Fee/estimateMaxFeePerGas). The default multiplier doubles the
base fee for headroom against base-fee bumps.

```ts twoslash
import { Fee, RpcTransport, Value } from 'ox'

const transport = RpcTransport.fromHttp('https://1.rpc.thirdweb.com')

// Fetch the last 5 blocks with the 20th-percentile priority fee.
const history = Fee.fromHistoryRpc(
  await transport.request({
    method: 'eth_feeHistory', // [!code hl]
    params: ['0x5', 'latest', [20]],
  }),
)

// `baseFeePerGas` includes the *next* block's base fee as its last element.
const baseFeePerGas = history.baseFeePerGas[history.baseFeePerGas.length - 1]!

// Tip at the sampled percentile of the most recent block.
const rewards = history.reward ?? []
const maxPriorityFeePerGas =
  rewards[rewards.length - 1]?.[0] ?? Value.fromGwei('1')

const maxFeePerGas = Fee.estimateMaxFeePerGas({
  baseFeePerGas,
  maxPriorityFeePerGas,
})
// @log: baseFeePerGas * 2n + maxPriorityFeePerGas
```

Tune the headroom with `multiplierNumerator`/`multiplierDenominator` (e.g. `3n`/`2n` for 1.5x).

### Build & Serialize Access Lists

Attach an access list of addresses and storage keys directly to an envelope — `serialize`
converts it to tuple form automatically. Use
[`AccessList.toTupleList`](/api/AccessList/toTupleList) and
[`AccessList.fromTupleList`](/api/AccessList/fromTupleList) when working at the RLP boundary
yourself.

```ts twoslash
import { AccessList, TxEnvelopeEip1559, Value } from 'ox'

const envelope = TxEnvelopeEip1559.from({
  // [!code hl:start]
  accessList: [
    {
      address: '0x1e0049783f008a0085193e00003d00cd54003c71',
      storageKeys: [
        '0x0000000000000000000000000000000000000000000000000000000000000001',
      ],
    },
  ],
  // [!code hl:end]
  chainId: 1,
  maxFeePerGas: Value.fromGwei('10'),
  maxPriorityFeePerGas: Value.fromGwei('1'),
  to: '0x70997970c51812dc3a010c7d01b50e0d17dc79c8',
})

// Object form <-> serialized tuple form.
// @log: [['0x1e0049783f008a0085193e00003d00cd54003c71', ['0x0000...0001']]]
const tuples = AccessList.toTupleList(envelope.accessList)
const accessList = AccessList.fromTupleList(tuples)
```

Both directions validate address shape and enforce 32-byte storage keys, throwing
`AccessList.InvalidStorageKeySizeError` on malformed input.

## Best Practices

### Give the Base Fee Headroom

The base fee can rise 12.5% per block, and a transaction may not be included immediately. The
2x default multiplier in `Fee.estimateMaxFeePerGas` absorbs roughly six consecutive full
blocks; lower it only when inclusion time is not critical.

### Access Lists Only Pay Off When Used

Each listed address costs 2,400 gas and each storage key 1,900 gas up front. The discount only
nets out if the transaction actually touches the listed slots — measure with real workloads
before shipping EIP-2930-style lists.

## See More

<Cards>
  <Card icon="lucide:send" title="Build, Sign & Send" description="Apply estimated fees when constructing an envelope." to="/guides/transactions/build-sign-send" />

  <Card icon="lucide:layers" title="Choose an Envelope Type" description="Compare fee fields across the five envelope types." to="/guides/transactions/envelope-types" />

  <Card icon="lucide:search" title="Parse & Inspect Transactions" description="Convert RPC transactions and receipts to typed objects." to="/guides/transactions/parse-inspect" />
</Cards>