Estimate gas usage in TON contracts

Let’s call contracts that receive user’s message receiver contracts. Other contracts in a contract system are called internal contracts. That is not global terminology, just local to the current article. Imagine a contract system with three contracts: receiver, A and B. In that system the typical trace looks like this (transactions are going from left to right):

On the diagram A and B are internal contracts.

It is crucial to understand that there is no separate message balance and contract balance. After the message is received, coins are added to the contract balance, and then the contract is executed. Sending message mods and reserve actions help to properly divide contract balance in the action phase. This diagram of a possible value flow illustrates this. Note, that this diagram is not connected to the diagram above, it illustrates different fact.

The reason for this requirement is that reverting the contract system state is usually not possible, because the toncoins are already spent.

In case you are writing a contract system, where correctness depends on successful execution of the rest of the transaction trace, then you need to guarantee that there are enough attached toncoins in an incoming message to cover all fees. This article describes how to compute those fees.

Define variables for limits and initialize them with zero. We will raise them further to actual values.

Use descriptive names indicating the operation and contract. We recommend creating a dedicated constants file.

const GasSwapRequest: Int = 0;

• Run tests covering all execution paths. If you miss a path, it might be the most expensive.
• Extract resource consumption from send() method return value. The sections below describe ways to compute consumption of different kinds of resources.
• Use expect(extractedValue).toBeLessThanOrEqual(hardcodedConstant) to verify that the hardcoded limit is not exceeded.

import {findTransactionRequired} from "@ton/test-utils";

const result = await contract.send(/* params */);
const vaultTx = findTransactionRequired(result.transactions, {
    on: contract.address,
    op: 0x12345678,
});
expect(getComputeGasForTx(vaultTx)).toBeLessThanOrEqual(GasSwapRequest);

On the first run, use an error message to set the constant to the actual value used.

expect(received).toBeLessThanOrEqual(expected)

Expected: <= 0n
Received:    11578n

const GasSwapRequest: Int = 12000;

Compute fees

There are two kinds of values: gas units and toncoins.

The price of contract execution is fixed in gas units. However, the price of the gas itself is determined by the blockchain configuration.

One can convert to toncoins in the contract code using current blockchain config parameters:

let fee = getComputeFee(hardcodedGasValue, isAccountInMasterchain);

This function uses the GETGASFEE TVM opcode.

Forward fees

In general, you can calculate message size at runtime using computeDataSize() which uses CDATASIZE: And then, calculate forward fee using getForwardFee() which uses GETFORWARDFEE

ComputeDataSize have the second argument - maximum number of cells to visit. If it is ok to set in in 8192 since it is the limit for message size.

let size = computeDataSize(msg.toCell(), 8192);
let fwdFee = getForwardFee(size.cells, size.bits, isAccountInMasterchain);

Optimized forward fee calculation

If the size of the outgoing message is bounded by the size of the incoming message, we can estimate the forward fee of an outgoing message to be no larger than the forward fee of the incoming message, that was already computed by TVM. Thus, we don’t have to calculate it again. Note, that this estimation is correct only for contract system in the same workchain.

fun onInternalMessage(in: InMessage) {
    val fwdFee = in.originalForwardFee;
    // ...
}

Additional forward fee calculation

Forward fee is calculated using such formula

fwdFee = basePrice + priceForCells * cells + priceForBits * bits

So, when one want to send message, with a + b cells and x + y bits, the forward fee won’t be getForwardFee(a + b, x + y), but rather basePrice + priceForCells * (a + b) + priceForBits * (x + y).

For this case, we can use getSimpleForwardFee() which uses GETFORWARDFEESIMPLE. This function does not add basePrice (called lump_price in config) into account.

So the price of sending message with a + b cells and x + y bits is getForwardFee(a, x) + getSimpleForwardFee(b, y).

For example, when deploying contracts as part of the operation:

deploy(DeployParameters{
    init: initOf TargetContract(params),
    value: 0,
    mode: SendRemainingBalance,
    body: msg.toCell(),
});

The init field adds significant message size. Calculate forward fees using actual cell and bit counts, summing the base message and the StateInit: getForwardFee(msgCells, msgBits) + getSimpleForwardFee(stateInitCells, stateInitBits).

Complex forward fee calculation

Sometimes, out message is larger than the input one. In that cases combined approach can be used.

fun onInternalMessage(in: InMessage) {
    val origFwdFee = in.originalForwardFee;
    // Out message will consist of fields from in message, plus some extra fields.
    // We can estimate forward fee for out message using forward fee for in message.
    val additionalFwdFee = getSimpleForwardFee(additionalFieldsSize.cells, additionalFieldsSize.bits, isAccountInMasterchain);
    val totalFwdFee = origFwdFee + additionalFwdFee;
    // Remember to multiply those by the number of hops in the trace.
}

Storage fees

We cannot predict storage fees that we have to pay for sending messages because it depends on how long the target contract didn’t pay storage fee. Storage fees are different from forward and compute fees in that term, they should be handled in receiver contracts and in internal contracts.

Two distinct approaches exist:

Approach 1: Maintain a positive reserve

Always keep a minimum balance on the all contracts in your system. Storage fees deduct from this reserve, which replenishes with each user interaction. Do not hardcode TON; instead, hardcode the maximum possible contract size in cells and bits.

Note, this is supposed to be the code in internal contracts.

const secondsInFiveYears: Int = 5 * 365 * 24 * 60 * 60;
receive(msg: Transfer) {
    let minTonsForStorage: Int = getStorageFee(maxCells, maxBits, secondsInFiveYears, isAccountInMasterchain);
    nativeReserve(max(oldBalance, minTonsForStorage), ReserveAtMost);
    // Process operation with remaining value...
}
// Also this contract probably will require some code, that will allow owner to withdraw TONs from this contract.

In this approach, a receiver contract should calculate maximum possible storage fees for all contracts in trace.

const secondsInFiveYears: Int = 5 * 365 * 24 * 60 * 60;
receive(msg: UserIn) {
    // Suppose trace will be *in* -> *A* -> *B*
    let storageForA = getStorageFee(maxCellsInA, maxBitsInA, secondsInFiveYears, isAccountInMasterchain);
    let storageForB = getStorageFee(maxCellsInB, maxBitsInB, secondsInFiveYears, isAccountInMasterchain);
    let totalStorageFees = storageForA + storageForB;
let otherFees = 0; // compute compute + forward fees here
    require(messageValue() >= totalStorageFees + otherFees, "Not enough toncoins");
}

Approach 2: Cover storage on demand

In the worst case the storage fee for a single message is freeze_due_limit. Otherwise, the contract likely is already frozen and a transaction chain is likely to fail anyway.

So if we reserve storage debt from incoming messages. Allow the balance to remain at zero or with small debt.

Note, this is supposed to be the code in the internal contracts.

receive(msg: Operation) {
    // Reserve original balance plus any storage debt
    nativeReserve(myStorageDue(), ReserveAddOriginalBalance | ReserveExact);

    // Send remaining value onward
    send(SendParameters{
        value: 0,
        mode: SendRemainingBalance,
        // ...
    });
}

This simplifies fee calculation at the start of the operation—you do not need to pre‑calculate storage fees. The myStorageDue() function returns the amount needed to bring the balance to zero (or zero if it is already positive).

If we expect that the rest of trace uses n unique contracts, then it won’t take more than n freeze limits to pay their storage fees. So, in the receiver contract, the check should be:

receive(msg: Operation) {
    // The trace is still *in* -> *A* -> *B*
    let freezeLimit = getFreezeLimit(isAccountsInMasterchain);
    let otherFees = ...;
    // n equals 3 because *in* -> *A* -> *B*
    require(messageValue() >= freezeLimit * 3 + otherFees, "Not enough toncoins");
}

For contracts using this approach, confirm there is no excess accumulation:

it("should not accumulate excess balance", async () => {
    await pool.sendSwap(amount);

    const balance = (await blockchain.getContract(pool.address)).balance;
    expect(balance).toEqual(0n);
});

This confirms that all incoming value was consumed or forwarded, with none left behind. It helps identify any bugs that cause accumulation of TON on any contract.

Implement fee validation

So, the final code in the receiver contract could look like this:

receive(msg: SwapRequest) {
    let ctx = context();
    let fwdFee = ctx.readForwardFee();

    // Count all messages in the operation chain
    // IMPORTANT: We know that each of messages is less or equal to `SwapRequest`.
    let messageCount = 3;  // *in* -> vault → pool → vault

    // Calculate minimum required
    let minFees =
        messageCount * fwdFee +
        getComputeFee(GasSwapRequest, isInMasterchain) + // Operation in first vault
        getComputeFee(GasPoolSwap, isInMasterchain) + // Operation in pool
        getComputeFee(GasVaultPayout, isInMasterchain) + // Operation in second vault
        3 * getFreezeLimit();

    require(ctx.value >= msg.amount + minFees, "Insufficient TON attached");

    // Send remaining value for fees...

    // Also, you may need to handle fee on this exact contract, if this contract is supposed not to hold users TONs.
    // You can do that in any of 2 ways
}

Helper functions

Getting gas for transaction in sandbox is quite easy:

function getComputeGasForTx(tx: Transaction): bigint {
    if (tx.description.type !== "generic") {
        throw new Error("Expected generic transaction");
    }
    if (tx.description.computePhase.type !== "vm") {
        throw new Error("Expected VM compute phase");
    }
    return tx.description.computePhase.gasUsed;
}

To calculate the size of a message in cells, use this function:

const calculateCellsAndBits = (root: Cell, visited: Set<string> = new Set<string>()) => {
    const hash = root.hash().toString("hex")
    if (visited.has(hash)) {
        return {cells: 0, bits: 0}
    }
    visited.add(hash)

    let cells = 1
    let bits = root.bits.length
    for (const ref of root.refs) {
        const childRes = calculateCellsAndBits(ref, visited)
        cells += childRes.cells
        bits += childRes.bits
    }
    return {cells, bits, visited}
}

To extract a contract’s size in tests, use this function:

export async function getStateSizeForAccount(
    blockchain: Blockchain,
    address: Address,
): Promise<{cells: number; bits: number}> {
    const accountState = (await blockchain.getContract(address)).accountState
    if (!accountState || accountState.type !== "active") {
        throw new Error("Account state not found")
    }
    if (!accountState.state.code || !accountState.state.data) {
        throw new Error("Account state code or data not found")
    }
    const accountCode = accountState.state.code
    const accountData = accountState.state.data
    // Code and data likely do not share cells
    const codeSize = calculateCellsAndBits(accountCode)
    const dataSize = calculateCellsAndBits(accountData, codeSize.visited)

    return {
        cells: codeSize.cells + dataSize.cells,
        bits: codeSize.bits + dataSize.bits,
    }
}

Remember to verify your message-size constants across all possible paths in tests. Otherwise, your gas estimates might be wrong.