> ## Documentation Index
> Fetch the complete documentation index at: https://apidocs.finrock.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Gas Tanks: Pre-Fund Transaction Fees for EVM Chains

> Gas Tanks let you pre-fund transaction fees on EVM and other fee-token blockchains so your wallet addresses never stall waiting for gas.

Every transaction on a blockchain network requires a fee — commonly called "gas" — paid to validators for processing your transaction. On EVM-compatible chains like Ethereum, these fees are denominated in the network's native token (ETH, BNB, MATIC, etc.) and must be available in the sending address before a transaction can be submitted. Gas Tanks solve the operational problem of ensuring that gas is always present when you need it, without requiring manual top-ups before every transaction.

## What Is Gas?

Gas is the unit of computation cost on a blockchain network. When you send tokens, interact with a contract, or sweep funds, the network charges a gas fee calculated as:

```
Total Fee = Gas Units × Gas Price (in native token)
```

For example, a standard ETH transfer on Ethereum costs 21,000 gas units. At a gas price of 10 gwei, that equals 0.00021 ETH. More complex operations — such as ERC-20 token transfers — cost more gas. The fee is always paid in the **native token** of the chain (ETH on Ethereum, BNB on BSC, AVAX on Avalanche, and so on).

## What Is a Gas Tank?

A Gas Tank is a pre-funded reserve managed by Finrock that ensures your wallet addresses always have sufficient native token to cover transaction fees. Instead of manually topping up each address with small amounts of native token before every operation, you fund a single Gas Tank for a given chain, and Finrock automatically routes gas to your addresses when they need it.

<Info>
  Gas Tanks are specific to a blockchain network. You maintain a separate Gas Tank balance for each chain you operate on (e.g., one for Ethereum, one for BSC, one for Polygon).
</Info>

## Why You Need a Gas Tank

Without a Gas Tank, every address that needs to send tokens must also hold a small balance of the chain's native token. Managing this at scale — across hundreds or thousands of addresses — creates significant operational overhead and is a common cause of stuck or failed transactions.

<CardGroup cols={2}>
  <Card title="Seamless User Experience" icon="wand-magic-sparkles">
    Your users and automated systems never need to think about gas. Transactions execute immediately without waiting for manual fee top-ups.
  </Card>

  <Card title="Cost Management" icon="chart-line">
    Pre-fund your Gas Tank when native token prices are lower. Manage your fee budget centrally rather than across thousands of individual addresses.
  </Card>

  <Card title="Transaction Automation" icon="robot">
    Automated sweeps, payouts, and contract interactions run without constant human intervention to keep addresses funded with gas.
  </Card>

  <Card title="Prevent Failed Transactions" icon="circle-xmark">
    Transactions submitted without sufficient gas fail — and you still pay any gas that was consumed. A funded Gas Tank eliminates this failure mode entirely.
  </Card>
</CardGroup>

## Automatic Gas Refill

When you create or update an address, you can set the `auto_refill` flag to `true`. With this flag enabled, Finrock automatically draws from your Gas Tank to fund that address with native token whenever its gas balance falls below the threshold required to execute a transaction.

```json theme={null}
{
  "auto_refill": true
}
```

This is the recommended configuration for any address involved in automated workflows — including omnibus addresses, sweep destinations, and high-frequency payout addresses.

<Warning>
  **Manual Sweep operations may fail if your Gas Tank has insufficient funds.** Always ensure your Gas Tank holds an adequate reserve before initiating bulk sweeps or high-volume transaction periods. Monitor your Gas Tank balance via the API and set up alerts to avoid interruption.
</Warning>

## Supported Blockchains

Gas Tanks are available for all major EVM and fee-token blockchains supported by Finrock:

<CardGroup cols={3}>
  <Card title="Ethereum" icon="ethereum">
    ETH
  </Card>

  <Card title="Binance Smart Chain" icon="coins">
    BSC (BNB)
  </Card>

  <Card title="Tron" icon="circle-nodes">
    TRX
  </Card>

  <Card title="Solana" icon="sun">
    SOL
  </Card>

  <Card title="Avalanche C-Chain" icon="mountain">
    AVAX
  </Card>

  <Card title="Polygon" icon="hexagon-nodes">
    POL (formerly MATIC)
  </Card>

  <Card title="Fantom" icon="ghost">
    FTM
  </Card>

  <Card title="Cronos" icon="link">
    CRO
  </Card>

  <Card title="Arbitrum" icon="circle-nodes">
    ARB
  </Card>

  <Card title="Harmony" icon="circle-half-stroke">
    ONE
  </Card>

  <Card title="Optimism" icon="fire">
    OP\_ETH
  </Card>

  <Card title="Klaytn" icon="layer-group">
    KLAY
  </Card>
</CardGroup>

## Managing Gas Tanks via the API

You can view your Gas Tank balances and manage funding programmatically through the Finrock REST API. Use the following endpoints:

| Action               | Endpoint                      |
| -------------------- | ----------------------------- |
| List all Gas Tanks   | `GET /gas-tanks`              |
| Get Gas Tank balance | `GET /gas-tanks/{id}/balance` |

Refer to the [Gas Tanks API reference](/api-reference/gas/list-gas-tanks) for full request and response schemas, authentication requirements, and examples.

<Tip>
  Set up a regular job or webhook listener to monitor your Gas Tank balance. Proactively top up before high-volume periods (e.g., batch payout runs, end-of-day sweeps) to avoid transaction failures.
</Tip>