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Diminishing marginal utility

Calculate marginal utility from a total, and explain why falling marginal utility makes demand slope down

Key terms
Diminishing marginal utility
Diminishing marginal utility is the fall in the extra satisfaction a person gets from each additional unit of a good as they consume more of it.
Total utility
Total utility is the whole satisfaction a person gets from all the units of a good they consume, the sum of the marginal utilities.

Marginal utility is what one more unit adds to total utility

Utility is the satisfaction a person gets from what they consume. Economists imagine measuring it in units called utils, which only mean something to one person: one person's 20 utils cannot be compared with another's 10. Total utility is the satisfaction from all the units consumed. Marginal utility is the extra utility from one more unit, the change in total utility divided by the change in quantity.