For the most simple case: $42,000 income minus $12,200 standard deduction = $29,800 taxable income. The tax on that, from the tax table is $3385. If your federal withholding is more than that; the difference will be your refund.
But for most people, There are too many variables for the simple answer; such as the type of income, your dependents, filing status, credits, withholding etc..
