Definition
The tax rate applied to your next dollar of income: the rate of the bracket that dollar lands in, plus any state tax. Under the progressive system it is higher than your effective rate, which averages across all your income.
Why it matters
Marginal is the rate that prices decisions: what a raise, a side gig, a Roth conversion, or a pre-tax contribution is actually worth. Using the effective rate for those decisions understates the tax; believing the marginal rate applies to your whole income overstates it.
Example
A worker in the 22% bracket weighs putting $5,000 into a pre-tax 401(k). The contribution comes off the top, the marginally taxed dollars, so it saves $1,100 of federal tax this year. Their 12% effective rate is irrelevant to this choice.
Put it into practice
Related terms
This definition is educational, not financial, legal, or tax advice. U.S. rules and limits change; verify time-sensitive details with official sources. See our disclaimer.