Mesauring gas consumption

You can measure the gas used in a block of code by calculating the difference between gasleft() at the start and gasleft() at the end:

Example:

// SPDX-License-Identifier: MIT
pragma solidity 0.8.10;

contract Gas {
    uint public i = 0;
    uint public gasUsed = 0;
    uint public testCalculation;
    uint public gasPrice = 0;

    // Using up all of the gas that you send causes your transaction to fail.
    // State changes are undone.
    // Gas spent are not refunded.
    function forever() public {
        // Here we run a loop until all of the gas are spent
        // and the transaction fails
        while (true) {
            i += 1;
        }
    }

    function checkGasUsed() public {
        uint startGas = gasleft();

        testCalculation = 25/5;

        gasUsed = startGas - gasleft(); // 22121
    }

    function checkGasUsed2(uint a) public {
        uint startGas = gasleft();

        testCalculation = a/5;
        gasPrice = tx.gasprice;
        gasUsed = startGas - gasleft(); // 27315
    }

}