# Gini Coefficient: How Income Inequality Is Measured

*Science & Technology · Priya Nair · 2026-08-06*

## TL;DR

The Gini coefficient measures income inequality within a population on a scale from 0 (perfect equality) to 1 (perfect inequality). It is calculated from the Lorenz curve, which plots cumulative income share against cumulative population share. While useful for comparisons, it ignores where inequality occurs in the distribution and can produce misleading results when comparing countries with different demographics.

## Key facts

- Corrado Gini, an Italian statistician, introduced the coefficient in his 1912 paper "Variabilità e Mutabilità."
- A Gini coefficient of 0 represents perfect equality, where everyone earns the same income, while a coefficient of 1 indicates that one person holds all income.
- The Lorenz curve, developed by American economist Max Lorenz in 1905, provides the graphical foundation for calculating the Gini coefficient.
- The World Bank and the OECD publish Gini coefficients by country, making cross-national comparisons possible for researchers and policymakers.
- The coefficient measures relative inequality, not absolute wealth, so a poor country and a rich country can share the same Gini value despite vastly different living standards.

## The Gini coefficient: How income inequality is measured

Every country has its wealthy and its struggling, but how do economists actually put a number on that gap? In 1912, Italian statistician Corrado Gini developed the Gini coefficient, a single figure between 0 and 1 that has become the world's most widely used tool for measuring income and wealth inequality. It's the number you see in headlines about economic fairness, and it starts with a simple question: how far is reality from perfect equality?

The idea behind it is beautifully visual. Imagine lining up everyone in a society from the poorest to the richest. Now plot their cumulative share of income against their cumulative share of the population. That curve, first drawn by American economist Max Lorenz in 1905, is called the Lorenz curve. If income were split perfectly evenly, the curve would be a straight diagonal line, the line of perfect equality. But real life never looks like that. The actual Lorenz curve bows below that line, and the size of that bow tells you everything.

Gini's stroke of genius in 1912 was to turn that visual gap into a precise ratio. The coefficient is calculated as the area between the Lorenz curve and the line of perfect equality, divided by the total area under that line. A value of 0 means everyone shares income identically. A value of 1 means one single person holds all the income while everyone else has nothing. Most real countries fall somewhere in between, and the closer the number gets to 1, the more concentrated the wealth.

What makes the Gini coefficient so appealing is its flexibility. It works whether you're measuring a household, a city, or an entire nation. But it comes with a catch that surprises many people: two societies with wildly different income distributions can produce the exact same Gini number. The coefficient compresses an entire curve of information into one digit, and that compression means real detail gets lost along the way. A society where everyone is moderately unequal can look identical to one where a tiny elite hoards everything.

## The Lorenz curve: A picture of inequality

In 1905, an American economist named Max O. Lorenz invented a simple graph that changed how we see wealth gaps. His creation, the Lorenz curve, lines up a country's population from poorest to richest along the bottom axis, then plots how much of the total income each slice of that population actually holds on the vertical axis.

Here is the key to reading it. Imagine a point sitting at (0.5, 0.2) on that graph. That single dot tells a stark story: the poorest half of the population earns just 20 percent of all the money in the group. If income were shared perfectly evenly, the curve would follow a straight diagonal line where the poorest 50 percent earns exactly 50 percent of the income. Economists call this the line of perfect equality, and it runs along the path where y equals x.

Real life never matches that perfect diagonal. The actual Lorenz curve bows downward, sagging away from the line of equality, and the size of that sag reveals just how lopsided things really are. The farther the curve dips, the more unequal the distribution. This visual tool became the foundation for the **Gini coefficient**, the single number that economists worldwide now use to measure inequality at a glance.

## How the Gini coefficient is calculated

The Gini coefficient is a single number, usually somewhere between 0 and 1, that tells you how unevenly income or wealth is spread across a group. It works by measuring the gap between how income is actually distributed and a hypothetical world where everyone gets an identical share. The bigger the gap, the higher the number.

To see it in action, picture the Lorenz curve, a graph that economist Max Lorenz introduced in 1905. The horizontal axis stacks the population from poorest to richest, showing the cumulative percentage of people. The vertical axis shows the cumulative percentage of total income they hold. If income were split perfectly evenly, the curve would trace a straight diagonal line, the line of perfect equality. In reality, the curve bows downward, because the poorest 50 percent of people rarely hold 50 percent of the money. In one common example, the lowest-earning half of a population might take home only 20 percent of total income, a point you could mark at (0.5, 0.2) on the graph.

The Gini coefficient, developed in 1912 by Italian statistician Corrado Gini, measures exactly how far the Lorenz curve sags below that diagonal. Label the area between the curve and the line of equality as B, and the entire triangle beneath the equality line as A plus B. The formula is simply Gini = B / (A + B). When the curve hugs the diagonal tightly, B shrinks toward zero and the coefficient drops. When one person hoards everything, the curve collapses against the axes, B swells, and the number climbs toward 1.

A Gini of 0 means perfect equality: everyone in the group earns the same amount. A Gini of 1 means perfect inequality: a single person holds all the income while everyone else earns nothing. Neither extreme is inherently good or bad. A household where one member earns the entire paycheck can register a very high Gini coefficient, yet that arrangement is completely ordinary in millions of homes. The coefficient alone cannot tell you whether a society is thriving or struggling; it only describes the shape of the distribution. Two communities with wildly different income structures can even produce the identical Gini number, because the measure compresses all the detail of the Lorenz curve into one summary figure.

## Why the Gini coefficient has critics

For all its usefulness, the Gini coefficient carries real blind spots that economists openly argue about. Critics point to three main weaknesses: it squeezes a whole income picture into one number, it ignores the stories behind people's finances, and it flinches at extremes.

The first complaint is about compression. The Gini coefficient takes the entire Lorenz curve, that sweeping visual of how income spreads across a population, and reduces it to a single decimal. Two societies with completely different distribution shapes can land on the very same coefficient, which means the measure hides as much as it reveals. A country where everyone clusters near the middle and another where a few soar while many struggle might both register a 0.35, yet their lived realities could hardly differ more. That loss of information frustrates researchers who want the full picture, not just a thumbnail.

The second issue cuts deeper into individual lives. The Gini coefficient treats people purely by their income at one frozen moment, with no regard for why that income looks the way it does. Consider two people with identical negative incomes. One is a 24-year-old medical student buried in student loans, someone whose red balance sheet is a temporary stepping stone to a lucrative career. The other is a long-term unemployed person with the same negative income and little prospect of change. The Gini coefficient lumps them together as if their situations were equivalent, even though their trajectories could not be more different. It cannot see that one is investing in a future while the other is trapped in a present.

Finally, the Gini coefficient has a documented sensitivity to outliers. A handful of extremely wealthy individuals, or a small cluster of desperately poor ones, can shift the coefficient noticeably even within a large sample. That means a single billionaire or a concentrated pocket of destitution can distort the headline number, making an economy look more unequal than its typical household experience would suggest.

## FAQ: What does a Gini coefficient of 0.5 mean?

A Gini coefficient of 0.5 signals a high level of inequality, sitting exactly halfway between perfect equality (0) and perfect inequality (1). It tells you that the Lorenz curve bows substantially away from the line of equality. But it doesn't tell you who has what, or how the income is actually split up.

Think of it this way: the Gini coefficient is a summary statistic, not a photograph of a country's income structure. Two nations can both land at 0.5 and yet have completely different economic realities. In one, the middle class might hold most of the wealth with a few very poor outliers. In another, a tiny elite might control nearly everything while the rest of the population shares the scraps. Same number, radically different stories.

That's because the Gini coefficient compresses the entire Lorenz curve into a single figure. The Lorenz curve, developed by economist Max Lorenz in 1905, plots the cumulative percentage of income against the cumulative percentage of the population, ordered from lowest to highest earners. When you collapse all that information into one number, you inevitably lose the shape of the distribution.

So when you encounter a Gini coefficient of 0.5, you know inequality is substantial. What you don't know is whether that inequality comes from a handful of billionaires at the top, a squeezed middle, or a large population living in poverty. The context matters enormously.


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