States calculate corporate income tax by state by multiplying the state’s corporate tax rate against the corporation’s apportioned state taxable income. Specifically, apportioned state taxable income equals total taxable income multiplied by the state’s apportionment percentage. Consequently, a corporation with $10M of total taxable income and a 30% California apportionment percentage owes California tax on $3M — not the full $10M. Furthermore, each state uses its own apportionment formula — most states now use single sales factor, meaning only the sales percentage determines apportionment. Additionally, state taxable income often differs from federal taxable income due to conformity gaps — particularly around bonus depreciation and interest limitations.