After-tax income is the amount of income left after direct taxes are paid. It is one of the most useful household-level measures in economics because it is closer than gross income to the money people can actually spend or save.
Basic Calculation
A simple definition is:
[ Y_{AT} = Y_G - T ]
where Y_G is gross income and T is direct taxes such as income tax and payroll tax.
A broader disposable-income measure may also include government transfers:
[ Y_D = Y_G - T + TR ]
where TR stands for transfers received. That is why after-tax income and disposable income are related but not always identical.
Why Economists Care
After-tax income matters because it influences:
- household consumption and saving
- labor-supply incentives
- the distributional effects of tax reform
- how much stabilization policy reaches different income groups
A rise in taxes reduces after-tax income directly, while a tax cut raises it. The impact on spending then depends on how much of the extra income households choose to consume.
The Marginal Incentive Angle
If the marginal tax rate is t, then each extra dollar of gross income raises after-tax income by:
[ \frac{dY_{AT}}{dY_G} = 1 - t ]
That is why economists distinguish between average tax burdens and marginal tax incentives. The marginal rate affects the payoff from earning one more dollar.