Bottom line: do not calculate the ETH reserve from the exchange amount alone. First identify every on-chain transaction that must be completed, obtain a fresh fee estimate for each one, and make sure the wallet can cover the displayed maximum network fee as well as the amount being sent. There is no permanently correct reserve in ETH because network demand, transaction type, fee settings, and the selected network can change the required amount.

How the claims were checked

The evidence hierarchy for this analysis prioritizes Ethereum protocol specifications, current ethereum.org technical documentation, the Ethereum JSON-RPC reference, and the ERC-20 standard. Protocol rules are treated as stable only where the specification establishes them. Gas prices, wallet estimates, supported exchange directions, and transaction-specific gas usage are treated as dynamic or conditional.

Source freshness matters differently for each claim. EIP-1559 defines the fee mechanism and remains the primary protocol reference, while the ethereum.org gas documentation—last updated June 24, 2026—provides a more recent explanation of how users and wallets apply that mechanism. A live wallet estimate or blockchain explorer is still required immediately before signing because documentation cannot provide the current fee for a future transaction. ^[1]

Confirmed facts behind the ETH reserve

On Ethereum, gas is measured separately from its price, but the resulting network fee is paid in ETH. Under EIP-1559, a standard type-2 transaction specifies a gas limit, a maximum fee per gas, and a maximum priority fee per gas. The actual price includes the block’s base fee and an eligible priority fee, subject to the sender’s maximum fee setting. ^[1]

The practical upper-bound calculation for one transaction is:

Maximum fee reservation = gas limit × maximum fee per gas.

This is a reservation ceiling rather than a prediction of the final charge. Ethereum’s protocol requires the sender to be able to afford the transaction value plus the gas limit multiplied by the maximum fee per gas. Gas that is not consumed is not charged, and the difference between the permitted maximum fee and the effective fee is returned to the account. ^[1]

That distinction matters when sending ETH itself. If the balance is B and the wallet displays a maximum network fee of F, the proposed ETH transfer must not exceed B − F. A wallet’s “maximum” or “send all” function may perform this subtraction automatically, but the confirmation screen should still be checked before signing.

Sending an ERC-20 token from a self-custody wallet does not remove the need for ETH on Ethereum. The token transfer changes the state of a smart contract, and Ethereum transactions involving smart contracts require gas. If a workflow also requires an approval transaction, that approval is a separate contract operation under the ERC-20 interface and may create another gas-paying transaction. A simple transfer to a deposit address does not necessarily require approval, so the number of steps must be determined from the actual workflow rather than assumed. ^[2]

Claim Register

Claim	Confirmation status	Primary source type and name	Source date	Limitation	What could change the conclusion
Ethereum execution gas is paid in ETH.	Confirmed	Official technical documentation: “Gas and fees”; protocol specification: EIP-1559	Documentation updated June 24, 2026; EIP created April 13, 2019	This statement concerns Ethereum itself, not every network on which an asset called ETH or an Ethereum-compatible token may appear.	A different selected network may use another native asset or a different fee mechanism.
A single fixed ETH reserve is sufficient for every exchange transaction.	Not supported	Official technical documentation: “Gas and fees”	Updated June 24, 2026	Base fees respond to block demand, priority fees depend on inclusion preferences, and gas usage depends on the transaction being executed.	Current congestion, wallet fee settings, transaction complexity, and additional required transactions.
The wallet’s estimate is the exact final fee.	Condition-dependent	Official Ethereum JSON-RPC reference: eth_estimateGas; EIP-1559	No publication or update date displayed in the retrieved JSON-RPC page; EIP created April 13, 2019	eth_estimateGas returns an estimate without submitting the transaction and may differ from actual gas usage. The effective fee per gas is determined when the transaction is included.	State changes, node implementation, base-fee movement, fee-cap settings, or transaction failure.
An ERC-20 exchange workflow always requires both approval and transfer transactions.	Not supported as a universal claim	Token standard: ERC-20	Created November 19, 2015	ERC-20 defines transfer and approval functions, but a direct deposit may require only a token transfer. Contract-based swaps may require approval or another authorization method.	The receiving method, existing allowance, token implementation, wallet design, and exchange instructions.
The requested asset, direction, and network are currently available through the exchange service.	Unknown until checked	Live order interface and current operating requirements	Must be checked at the time of the planned operation	No static article can establish current availability for every pair, network, or direction. The commercial link is not evidence for the technical gas claims above.	Service updates, maintenance, liquidity conditions, compliance results, or changes to supported directions.

After reviewing the technical evidence, use the live interface to check currently available exchange directions and networks. Confirm availability and any applicable verification requirements before creating an order; these requirements can depend on the operation and the results of compliance checks.

A practical method for deciding how much ETH to leave

1. Confirm the network first. The deposit instructions and the sending wallet must refer to the same network. Do not infer the network from the asset ticker alone.
2. List the remaining on-chain actions. A direct ETH deposit may involve one transfer. An ERC-20 deposit involves a token-contract transaction. A decentralized swap may involve approval followed by a swap, and a later transfer to the exchange would be another transaction.
3. Prepare the actual transaction without signing it. Enter the real destination address and amount so the wallet can estimate the relevant gas limit and fee parameters. A generic gas figure for a different action is not an adequate substitute.
4. Read the maximum network fee, not only an “average” fee label. For an ETH transfer, subtract the displayed maximum from the wallet balance before choosing the send amount. For a token transfer, retain enough ETH to cover that maximum separately from the token balance.
5. Repeat the estimate for every separate transaction. If an approval and a transfer are both required, one estimate does not cover both. Add their displayed maximum fees rather than multiplying a generic transfer fee by an assumed number of steps.
6. Leave a contingency amount. The appropriate margin is an operational choice, not a protocol constant. It should reflect how long the transaction may remain unsigned, whether another transaction could be needed, and whether the fee cap might need adjustment. A fixed percentage presented without current transaction data would create false precision.

Example without a fabricated gas price

Suppose a wallet must complete two transactions before the exchange deposit: transaction A authorizes token spending and transaction B performs the intended transfer or swap. Let the wallet show maximum network fees of F_A and F_B. The minimum calculable reservation is F_A + F_B. A separate contingency amount may then be added. If transaction A is unnecessary because the workflow is a direct token transfer, including it would overstate the reserve; if a later deposit transfer was omitted, the reserve would be understated.

Risks and the final recheck

Wrong network or address: compare the full destination address and the named network in both the receiving instructions and the wallet. Ethereum transactions cannot normally be reversed after they are sent, and ethereum.org specifically warns users to verify recipient addresses before confirmation. ^[3]

Fee movement: refresh the wallet quote immediately before signing. The base fee changes according to block usage, while the chosen priority fee can affect inclusion. A quote copied earlier is therefore evidence of a past estimate, not the cost of the transaction now awaiting approval. ^[1]

Failed execution: a failed on-chain transaction can still consume gas because validators performed computation before the failure was recorded. Check the transaction details, expected contract action, balance, allowance where relevant, and wallet warning messages before signing. ^[4]

Phishing: access the service through a verified domain, distrust unsolicited support messages, never disclose a seed phrase, and inspect the address and transaction message inside the wallet rather than relying only on text shown by a website. ^[3]

Volatility and local rules: the fiat value of the retained ETH can change, and exchange or compliance requirements can differ between countries and transaction directions. Those considerations do not alter the Ethereum gas formula, but they can affect whether and how an operation may proceed.

The final check should therefore be performed in this order: verify the live exchange direction and network, copy and independently compare the deposit address, prepare the exact transaction, refresh the fee estimate, confirm that the wallet balance covers both the transfer value and the maximum fee reservation, and only then sign. This procedure does not guarantee a particular fee or processing result, but it avoids relying on a stale universal ETH amount that cannot account for the actual transaction.