How Much ETH Do You Need for a Cross-Chain Swap?

There is no fixed ETH amount for a cross-chain swap: you need enough to pay Ethereum’s source transaction fee if the swap starts on Ethereum, and the destination fee may be deducted from the amount you receive. The steps are simple: a wallet sends the input asset to the swap protocol, the protocol processes the trade, then a destination-chain transaction delivers the output. Each network charges for its own work.

On Ethereum, that first transaction costs gas, paid in ETH. The fee depends on the work the transaction performs and current network demand, as Ethereum.org explains in its “Gas and fees” documentation. A token deposit may also require an approval transaction if the wallet or protocol has not already been authorized to use the token. For a THORChain route from Bitcoin to Ethereum, the input-side fee is paid in Bitcoin; the detailed thor swap account explains how the protocol handles the cross-chain steps.

Do I need ETH if the swap starts on another chain?

No. If you send Bitcoin or another non-Ethereum asset into a cross-chain route, you pay the source chain’s transaction fee in that chain’s native asset. You do not need ETH just to start that transaction. The route’s quote will show how much of the input is expected to become output after swap fees and any destination cost.

Think of the swap as a parcel crossing two toll roads: you pay the toll at the road where you enter, while the delivery service can take its own charge from what arrives. In THORChain’s documented fee model, the user pays the source-chain network fee, and the protocol deducts an outbound fee from the swap output to cover gas on the destination chain. If the output is ETH, that charge reduces the ETH delivered; it is not an extra ETH balance you must already hold.

What if the swap starts with ETH or an Ethereum token?

Keep ETH in the sending wallet for Ethereum gas, in addition to the amount you intend to swap. If you are swapping an ERC-20 token, the wallet may need to approve the token before it can send it, and both approval and deposit actions can require gas. Ethereum’s gas documentation notes that contract interactions use gas and that gas fees are paid in ETH. The wallet’s transaction screen gives the fee estimate for the transaction it is preparing.

Do not treat the swap amount as your whole balance. If you send nearly all your ETH as the input, there may be too little left to pay the transaction fee. The fee estimate can change with network demand, so check it again when you sign. The same distinction applies when a swap begins with an Ethereum token: token value pays for the trade, but ETH pays Ethereum’s gas.

How much should I leave for the swap?

Use the route quote and wallet estimate rather than a fixed rule of thumb. The quote estimates the output and fees for the selected route; the wallet estimates the source transaction’s gas. Those are separate costs, and the destination fee can vary with destination network conditions. THORChain’s “Fees” documentation describes the source fee, liquidity fee, optional affiliate fee and output-side outbound fee as distinct parts of its fee model.

• Check which chain holds the asset you are sending. That chain determines the source gas currency.
• Leave enough of the source chain’s native asset to cover the wallet’s displayed transaction fee.
• For an Ethereum token, check whether an approval is needed and account for its gas separately.
• Review the quoted output after fees, and confirm the destination address and network before signing.

The practical answer is route-specific: keep ETH for gas when Ethereum is the source, but do not buy ETH solely to cover a destination fee that the protocol deducts from the output. If the quote leaves too little value after fees, choose a different time or route, or wait until the swap makes economic sense.