---
title: "Law of Equimarginal Utility | Microeconomics"
description: "Law of equimarginal utility means consumers spread spending so marginal utility per dollar is equal across goods, maximizing satisfaction in microeconomics."
canonical: "https://fiveable.me/principles-microeconomics/key-terms/law-equimarginal-utility"
type: "key-term"
subject: "Principles of Microeconomics"
unit: "Unit 6"
---

# Law of Equimarginal Utility | Microeconomics

## Definition

The law of equimarginal utility says you get the most satisfaction from a fixed budget when the marginal utility per dollar is equal across all goods you buy. In Principles of Microeconomics, it explains consumer choice.

## What It Is

The law of equimarginal utility is the rule that a consumer should divide spending so the last dollar spent on each good gives the same marginal utility. In Principles of Microeconomics, that is the condition for choosing the mix of goods that maximizes total utility under a budget constraint.

The idea is not that every good gives the same satisfaction per unit. A pizza slice and a movie ticket can feel very different. What has to match is the utility you get from the last dollar spent on each one. If pizza gives you more utility per dollar than a movie ticket, you should shift some spending toward pizza until the gap closes.

This rule works because of diminishing marginal utility. The more of one good you already have, the less extra satisfaction the next unit usually gives you. So if you keep buying only one item, the marginal utility from that item falls, while another good may still offer a higher utility return for the same dollar.

A simple example makes it clearer. Suppose a student has $20 to spend on coffee and sandwiches. If the first few dollars spent on coffee produce more satisfaction per dollar than sandwiches, the student should buy more coffee. But once the utility per dollar from coffee drops enough, the next dollar should go to sandwiches instead. The best bundle is reached when one more dollar on coffee would not improve total satisfaction more than one more dollar on sandwiches.

You can think of the law as a balancing rule for choice. It does not tell people what they should like. It describes how a rational consumer spreads a limited budget across options based on preferences and prices. If prices change, the balance changes too, so the consumer may reallocate spending and choose a different bundle.

This also explains why the law is useful in demand theory. When income or prices shift, consumers compare the marginal utility per dollar from each good again. That comparison helps predict why people substitute toward cheaper goods or away from goods that become relatively expensive.

## Why It Matters

The law of equimarginal utility matters because it shows the logic behind consumer choice, not just the final purchase. In microeconomics, you are often asked why someone buys more of one good and less of another, and this rule gives the answer in terms of marginal analysis.

It also connects individual preferences to the budget constraint. A consumer may want many things, but limited income forces tradeoffs. The equimarginal rule explains how the consumer can spend each dollar where it does the most good, which is the basic decision process behind utility maximization.

This concept shows up whenever prices or incomes change. If one product becomes cheaper, its marginal utility per dollar rises relative to the others, so consumers may shift spending toward it. That change in behavior is part of how demand curves make sense in the first place.

The term also gives you a cleaner way to read scenarios. If a problem says a consumer is still getting more utility per dollar from good A than good B, you know the consumer is not at the best allocation yet. That lets you diagnose what adjustment should happen next instead of guessing at the answer.

A lot of textbook consumer choice questions are really asking whether the spending pattern is balanced. Once you can spot that balance, you can move from memorizing definitions to actually analyzing choices, bundles, and price changes.

## Connections

### [Marginal Utility](/principles-microeconomics/key-terms/marginal-utility)

Marginal utility is the extra satisfaction from one more unit of a good, and the law of equimarginal utility depends on it. You compare the marginal utility of each good, then adjust spending until the last dollar spent on each option gives the same payoff. Without marginal utility, the rule has nothing to measure.

### Budget Constraint

The budget constraint limits what a consumer can afford, so the equimarginal rule works within that limit. You are not picking the highest total utility in the abstract, you are picking the best bundle that fits income and prices. If the budget changes, the utility-maximizing mix can change too.

### [Utility Maximization](/principles-microeconomics/key-terms/utility-maximization)

Utility maximization is the broader goal, and the law of equimarginal utility is one way to reach it. The rule tells you how to allocate spending efficiently across goods. If marginal utility per dollar is unequal, the consumer can still improve total utility by shifting spending.

### [Indifference Curves](/principles-microeconomics/key-terms/indifference-curves)

Indifference curves show combinations of goods that give the same satisfaction, while the equimarginal rule gives the spending logic behind the chosen combination. One is a graphing tool, the other is a decision rule. Together, they describe how consumers settle on an optimal bundle.

## On the AP Exam

A quiz problem will usually give you prices, income, and marginal utilities, then ask you to choose the best bundle or explain a spending change. Your job is to compare marginal utility per dollar across the available goods and see whether money should move from one item to another. If one good gives more utility per dollar, the consumer has not yet reached the best allocation.

You may also see a short scenario with a consumer buying two goods, then changing purchases after a price change. The right answer usually comes from checking whether the new pattern restores equality in marginal utility per dollar. If the rule is violated, you can say total utility can still increase by reallocating spending. In a graph or written explanation, connect the choice to diminishing marginal utility and the budget constraint.

## Key Takeaways

- The law of equimarginal utility says a consumer maximizes satisfaction by making the marginal utility per dollar equal across goods.
- The rule works because diminishing marginal utility makes the extra satisfaction from additional units fall over time.
- If one good gives more utility per dollar than another, the consumer can improve total utility by shifting spending toward the better deal.
- The law fits inside the budget constraint, so it explains the best possible bundle a consumer can afford with limited income.
- When prices change, the utility balance changes too, which is why consumers may substitute one good for another.

## FAQs

### What is the law of equimarginal utility in Principles of Microeconomics?

It is the rule that a consumer should spend so the last dollar spent on each good gives the same marginal utility. That allocation makes the consumer as satisfied as possible given a fixed budget. If one good gives more utility per dollar, spending is not yet balanced.

### How does the law of equimarginal utility work?

You compare the marginal utility per dollar from each good, then move spending toward the option with the higher return. As you buy more of one good, diminishing marginal utility lowers its extra satisfaction, which helps the balance settle. The process stops when no reallocation can raise total utility.

### What is the difference between marginal utility and the law of equimarginal utility?

Marginal utility is the extra satisfaction from one more unit of a good. The law of equimarginal utility uses that idea to decide how to divide spending across several goods. So marginal utility is the measure, and the law is the spending rule built from that measure.

### How do you solve a problem using the law of equimarginal utility?

List the marginal utility and price for each good, then calculate marginal utility per dollar. The consumer should spend more on the good with the higher ratio until the ratios are equal, or as close as possible with the available budget. If the numbers are not equal, the bundle is not optimal yet.

## Related Study Guides

- [6.1 Consumption Choices](/principles-microeconomics/unit-6/1-consumption-choices/study-guide/abFqOeA7DbSCoNXZ)

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