Definition
Marginal utility is an economic and psychological concept quantifying the additional satisfaction, benefit, or value a consumer derives from consuming or acquiring exactly one more unit of a good or service. Mathematically, it is the first derivative of the total utility function with respect to quantity.
How to read: Marginal utility equals the change in total utility divided by the change in quantity, which is approximately the derivative of utility with respect to quantity. Meaning / when to use: Used to model consumer choice. It measures the slope of the total utility curve at any given point, showing how much extra value the next single action provides.
Why It Matters
Marginal utility is the engine of economic decision-making. People do not make choices based on total historical value; they make decisions at the margin (“Should I eat one more slice of pizza?”). The principle of Diminishing Marginal Utility explains why demand curves slope downward: the first slice of pizza is incredibly valuable, but the tenth slice provides zero or negative value. It resolves the diamond-water paradox (water is essential but cheap because it is abundant, making its marginal utility low).
Core Concepts
- Law of Diminishing Marginal Utility: As consumption increases, the marginal utility derived from each additional unit declines.
- Optimization (Equimarginal Principle): A consumer maximizes total utility when the marginal utility per dollar spent is exactly equal across all goods ().
- Continuous vs. Discrete: While goods are often discrete (you buy whole apples), economists model utility as a continuous differentiable function to apply calculus for optimization.
- Subjective Value: Utility is not an objective physical property; it is highly context-dependent and varies wildly between individuals and moments in time.