Lerner Index Calculator

The Lerner Index Calculator measures pricing power by comparing a product’s price with its marginal cost. The index is the price-cost gap divided by price. A result of 0 means price equals marginal cost, the benchmark associated with perfect competition in the simplest model. A result closer to 1 means a larger share of the price is above marginal cost.

This calculator is for microeconomics and pricing analysis. It is different from a general margin calculator, which usually compares profit with revenue using accounting costs, and different from a markup calculator, which scales the gap by cost rather than price. If you are deciding whether a product covers fixed and variable costs, use the break-even calculator. If taxes affect the final price customers see, compare the posted amount with the sales tax calculator.

What the Lerner index means

The Lerner index asks how far price is above marginal cost. Marginal cost is the cost of producing one additional unit. If a competitive market pushes price down to marginal cost, the index is zero. If a firm can charge substantially more than marginal cost, the index rises. Economists often use that price-cost wedge as a compact measure of market power.

The index is not the same as profit margin. Profit margin may use average cost, accounting cost, or total profit after fixed costs. The Lerner index uses marginal cost because the theory behind it concerns pricing one more unit. A firm with high fixed costs can have a high Lerner index and still face difficult profitability questions, especially if it must recover research, capacity, marketing, or platform costs.

Formula used by this calculator

The calculator applies:

$\text{Lerner index} = \frac{P - MC}{P}$

It also computes the price-cost gap:

$\text{price-cost gap} = P - MC$

When marginal cost is greater than zero, it reports markup on marginal cost:

$\text{markup on marginal cost} = \frac{P - MC}{MC} \times 100\%$

P is price, and MC is marginal cost. Both must be measured in the same currency and for the same unit. Price per subscription must be paired with marginal cost per subscription; price per case must be paired with marginal cost per case.

The form requires price greater than zero, marginal cost at least zero, and marginal cost no greater than price. That input rule matches the usual interpretation of the Lerner index. Temporary below-cost pricing, loss leaders, or introductory offers may be strategically important, but they do not fit this calculator’s standard range.

Worked example

Use the default inputs: price 500 dollars and marginal cost 350 dollars. The price-cost gap is:

$500 - 350 = 150$

The Lerner index is:

$\frac{500 - 350}{500} = \frac{150}{500} = 0.30$

The calculator rounds the primary index value to 0.30. It then computes markup on marginal cost:

$\frac{150}{350} \times 100\% = 42.857142857\%$

Rounded for display, the markup on marginal cost is 42.86%. The interpretation line says moderate pricing power because the code labels values below 0.2 as limited, values below 0.5 as moderate, and values of 0.5 or higher as high. If price and marginal cost were both 100 dollars, the index would be 0.00 and the interpretation would be price equals marginal cost.

How to use the result

For a manager, the index can describe how much room price has above the incremental cost of one more unit. A high index may reflect differentiation, brand loyalty, capacity limits, patents, network effects, switching costs, or a narrow market. A low index may reflect intense competition, easy entry, commodity pricing, or customers who can quickly substitute away.

For an economics student, the result connects directly to monopoly and competition models. In the textbook monopoly model, a firm with market power chooses output where marginal revenue equals marginal cost, then charges a price above marginal cost. The Lerner index summarizes that wedge. In empirical work, researchers may estimate marginal cost rather than observe it directly, which means the index depends heavily on data quality.

For policy or antitrust discussion, treat the number as an input, not a verdict. Market definition matters. A high index for one branded product may be less concerning if close substitutes are available. A moderate index in a market with high entry barriers may deserve more attention than the same number in a market where new competitors can enter quickly.

Limits and common mistakes

The biggest mistake is using average cost instead of marginal cost. Average cost includes fixed costs spread across units; marginal cost asks what one more unit costs. Another mistake is mixing units, such as price per package and marginal cost per individual item. Also avoid comparing index values across unrelated industries without considering fixed costs, product differentiation, regulation, and capacity.

The calculator’s interpretation labels are intentionally simple. They help read the output, but they do not replace economic judgment. A Lerner index of 0.30 can mean very different things in software, electricity, pharmaceuticals, retail, or banking. Use the result with market evidence and, when relevant, legal or regulatory expertise.

Sources

• FRED, Lerner Index in Banking Market for United States — data series illustrating an empirical Lerner index application.
• Federal Reserve Bank of St. Louis, Do Banks Have Pricing Power? — discussion of bank market power and Lerner index use.
• OpenStax, How a Profit-Maximizing Monopoly Chooses Output and Price — textbook background on monopoly pricing above marginal cost.
• OpenStax, Explicit and Implicit Costs, and Accounting and Economic Profit — cost concepts relevant to marginal-cost interpretation.