Your box 1 wages figure on your W2 will already contain the adjustment for your pre-tax amounts. The pre-tax amounts themselves are then generally reported in box 12 (or sometimes box 14).
Example: You earned $55,000 in 2017 but you contributed $5,000 pre-tax to a 401(k) account. On your Form W2, box 1 wages will have $50,000 for income and then in box 12 you will see a code D for $5,000 for pre-tax 401k contributions.
Its also not uncommon to see different wage figures for boxes 3 and 5 for social security and Medicare wages as some pre-tax deferral rules do not apply to these wages and social security wages are also capped at a certain amount.
