Microeconomics
Lerner Index Calculator
Measure price-cost markup and implied market power using price and marginal cost.
Inputs and results stay in this browser. Currency symbols are illustrative; use any consistent currency.
Problem → model → reason → result
What problem does this model solve?
Measure price-cost markup and implied market power using price and marginal cost.
Why does the model apply?
The index ranges from zero under price-equals-marginal-cost pricing toward one as the proportional markup rises. Cost measurement and market definition strongly affect interpretation.
What assumptions does it make?
The variables must describe the same market, firm, period or decision context and use consistent units. Any behavioural condition implied by the formula—such as other factors remaining unchanged—must be reasonable for the question being asked.
Formula
Lerner index L = (Price − Marginal cost) ÷ Price; under monopoly optimum L = −1/Ed
Calculation and working
The calculator substitutes your inputs locally and displays the numerical result. Change one input at a time to test which relationship drives the result.
What does the result mean?
Interpret the result in the economic context named above, including its sign, magnitude, units and time period. A calculated relationship is not by itself evidence that one variable caused another.
Worked example
A $60 price and $20 marginal cost produce a Lerner index of 0.667 and an implied demand-elasticity magnitude of 1.5 at a monopoly optimum.
When does this model not apply?
Do not use the result when the input definitions, units or formula assumptions do not match the real situation. This is an educational model, not financial, investment, tax or policy advice; verify material decisions against primary data and professional guidance.
Clear answers
Frequently asked questions
What does the Lerner index do?
Measure price-cost markup and implied market power using price and marginal cost.
How does the Lerner index work?
The calculator applies this formula: Lerner index L = (Price − Marginal cost) ÷ Price; under monopoly optimum L = −1/Ed. The index ranges from zero under price-equals-marginal-cost pricing toward one as the proportional markup rises. Cost measurement and market definition strongly affect interpretation.
What can I learn from the Lerner index?
It helps you explore the relationship described by this tool: Measure price-cost markup and implied market power using price and marginal cost. Change one input at a time to observe how it affects the result.
Does MW SysArc receive or store what I enter?
No. The calculation runs locally in your browser. MW SysArc does not receive or store your calculation inputs.
How should I use the result?
Use the result as an estimate or educational aid. Check important financial, business or policy decisions with qualified sources and current data.
Last reviewed 2026-07-14. Calculations tested 2026-07-14.