taxable income vs book income

Imagine keeping two scorecards for the same game. One scorecard follows the rules your club uses to decide who really played best; the other follows the referee's official rulebook for the league standings. Both are honest, but because they use different rules, they can show different scores. A company keeps two such scorecards for its profit: book income for its financial statements and taxable income for the tax authority.

Book income (also called accounting income or pretax book income) is the profit a company reports to investors, calculated under accounting standards such as GAAP or IFRS. Taxable income is the profit figure the tax law tells you to use when computing tax. They start from the same business activity but diverge because the two rulebooks recognize some revenues and expenses at different times, or count some items in one but not the other. For example, accounting might spread a machine's cost over ten years while tax rules let the company deduct it faster; or a fine paid by the company reduces book income but is not allowed as a tax deduction at all. So a company with 200,000 of book income might have 170,000 or 230,000 of taxable income, depending on these adjustments.

This gap is the root of much of tax accounting. The difference between book and taxable income is sorted into permanent differences (items that never line up, like that non-deductible fine) and temporary differences (timing gaps that reverse over time, like depreciation). Temporary differences create deferred tax assets and liabilities on the balance sheet. Understanding that book income is not the number the company is taxed on — and why — is the single most important idea in this field.