---
title: "Lerner Index | Intermediate Microeconomic Theory"
description: "Lerner Index measures a firm's markup over marginal cost, showing market power and pricing ability in Intermediate Microeconomic Theory."
canonical: "https://fiveable.me/intermediate-microeconomic-theory/key-terms/lerner-index"
type: "key-term"
subject: "Intermediate Microeconomic Theory"
unit: "Unit 4"
---

# Lerner Index | Intermediate Microeconomic Theory

## Definition

The Lerner Index is a measure of market power, calculated as (P - MC) / P. In Intermediate Microeconomic Theory, it shows how far a firm's price sits above marginal cost.

## What It Is

The Lerner Index is a way to measure how much pricing power a firm has in Intermediate Microeconomic Theory. It is defined as (P - MC) / P, where P is price and MC is marginal cost. If price equals marginal cost, the index is 0, which is what you expect in a perfectly competitive market. If a firm can charge far above marginal cost, the index moves closer to 1.

That formula is doing something very specific: it compares the firm’s markup to the price customers actually pay. A higher number means the firm keeps a larger gap between cost and price, which usually signals less competition and more control over the market. A lower number means the firm has little room to raise price without losing buyers.

This makes the Lerner Index especially useful when you are studying monopoly and other imperfectly competitive market structures. A monopoly does not take price as given the way a competitive firm does. Instead, it chooses a price-quantity pair on its demand curve, and the Lerner Index helps summarize how much that choice can depart from marginal cost.

The index is closely tied to demand conditions. If demand is very elastic, buyers are quick to switch away when price rises, so the firm has less room to set a large markup. If demand is inelastic, the firm can often charge more above marginal cost without losing many sales. That is why the Lerner Index and price elasticity of demand are usually discussed together.

You can also read the index as a quick signal of how much competition is constraining the firm. In a competitive market, firms are pushed toward price equals marginal cost, so the index tends toward zero. In a monopoly, the firm has more freedom to mark up price, so the index is higher. In the real world, many firms sit between those extremes, which is why the Lerner Index is useful for comparing market power across industries.

## Why It Matters

The Lerner Index matters because it turns the abstract idea of market power into a number you can interpret. In monopoly problems, you often know that a firm sets output where marginal revenue equals marginal cost, but that alone does not tell you how much the firm is able to charge above cost. The Lerner Index gives you the pricing side of the story.

It also connects the firm’s optimization problem to the market environment around it. When demand is more elastic, the index tends to be lower because the firm cannot raise price very much without losing sales. When demand is less elastic, the firm can sustain a larger markup. That lets you connect consumer behavior, firm theory, and market structure in one place.

In class problems, the index is a quick way to compare industries, interpret monopoly outcomes, or explain why some firms earn high margins. It is also a good checkpoint for spotting whether a market is close to competition or shaped by stronger market power. If you can read the index well, you can move from equations to economic interpretation faster.

## Connections

### Market Power

The Lerner Index is one way to measure market power, but the two are not identical. Market power is the broader ability to influence price, while the Lerner Index gives you a formula that summarizes how far price sits above marginal cost. If a problem asks whether a firm has market power, the index is one piece of evidence, not the whole story.

### Marginal Cost

Marginal cost is the cost of producing one more unit, and it sits in the denominator of the Lerner formula through the markup gap. A firm with a price far above marginal cost has a higher index. In monopoly problems, you often find MC from the cost function first, then use it to judge the firm’s pricing power.

### Price Elasticity of Demand

Elasticity helps explain why the Lerner Index is higher for some firms than others. When demand is more elastic, the firm faces stronger resistance to price increases, so the markup over marginal cost is smaller. When demand is less elastic, the firm can charge more above cost and still keep enough buyers.

### [Monopoly Power](/intermediate-microeconomic-theory/key-terms/monopoly-power)

Monopoly power is the strongest setting where the Lerner Index usually gets discussed. A monopolist chooses output knowing it can set price above marginal cost, so the index is often positive and sometimes large. In homework, this helps you distinguish a monopoly outcome from a competitive outcome where the index is near zero.

## On the AP Exam

A problem set or quiz question may give you price, marginal cost, or a demand setup and ask you to compute the Lerner Index or interpret its value. You might also be asked to compare two firms or two market structures and explain which one has more pricing power. If the question gives elasticity, you may need to connect it to the markup rather than just plug into the formula. In a written response, the move is to say what the number means economically, not just calculate it. A low index means price is close to marginal cost, while a high index suggests the firm can sustain a larger markup because competition is weaker or demand is less elastic.

## Lerner Index vs Market Power

Market power is the broader idea that a firm can influence price. The Lerner Index is a specific measure of that power, calculated from price and marginal cost. So market power is the concept, while the Lerner Index is one way to quantify it.

## Key Takeaways

- The Lerner Index is calculated as (P - MC) / P, so it measures how far price sits above marginal cost.
- A value near 0 suggests a competitive outcome, while a value closer to 1 suggests stronger monopoly power.
- The index is useful because it turns market power into a number you can compare across firms or industries.
- More elastic demand usually means a lower Lerner Index, because the firm cannot charge a large markup without losing buyers.
- In monopoly analysis, the index helps connect cost, demand, and pricing into one interpretation.

## FAQs

### What is the Lerner Index in Intermediate Microeconomic Theory?

It is a measure of a firm's market power, calculated as (P - MC) / P. In micro theory, it tells you how large the firm’s markup is relative to the price it charges. A higher value means the firm can keep price farther above marginal cost.

### How do you interpret a Lerner Index of 0?

A Lerner Index of 0 means price equals marginal cost. That is the competitive benchmark, where firms do not have room to mark up price above cost. It does not always mean perfect competition in every detail, but it points in that direction.

### Why is the Lerner Index higher for a monopoly?

A monopoly faces less competitive pressure, so it can set price above marginal cost by a larger amount. Since the index measures that gap relative to price, monopoly pricing usually produces a higher number. The exact value still depends on demand and costs.

### Is the Lerner Index the same as market power?

No. Market power is the broader ability to influence price, while the Lerner Index is one numeric measure of that ability. You can think of market power as the idea and the Lerner Index as the calculation.

## Related Study Guides

- [4.2 Profit maximization under monopoly](/intermediate-microeconomic-theory/unit-4/profit-maximization-monopoly/study-guide/7pcbOXANr0anAouQ)

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