The marginal tax rate is the rate on the last dollar you earn, set by the bracket that dollar falls into. The effective tax rate is your total tax divided by your total income. Because the US system taxes income in layers, with the first slice at 10%, the next at 12%, and so on, the effective rate is always lower than the marginal rate.
A single filer with $150,000 of taxable income in 2026 has a marginal rate of 24%. But the 24% applies only to the dollars in that bracket. The first portion was taxed at 10%, 12%, and 22%. The total federal tax comes to roughly $26,000, an effective rate near 17%.
Which one to use depends on the question. Deciding whether to take on extra income, make a traditional or Roth contribution, or exercise options is a marginal question: what happens to the next dollar. Understanding how much of your year went to tax is an effective question.
The common mistake is the raise myth: worrying that a raise into a higher bracket will reduce take-home pay. It cannot. Only the dollars above the threshold are taxed at the higher rate, so more gross always means more net. A second mistake is applying the effective rate to a bonus or vest. That extra income is taxed at the marginal rate, which is why a $50,000 bonus at a 32% marginal rate nets far less than a 17% effective rate would suggest.