With a median household income of more than $125,000, Loudoun County, Virginia is considered the richest county in the United States. Established in the 1700s, Loudoun County is right outside of Washington D.C. and is home to a population of around 400,000 people. It is Virginia's third biggest county by population.

In the list of wealthiest counties, Loudoun County is number one. Here's the rest of the top ten counties on that list, from richest to poorest:

The county with the lowest median income in the United States is McCreary County, Kentucky. The median income there is just over $19,000, more than $100,000 lower than the richest county. The average life expectancy in McCreary county is six years below the national average, and the unemployment rate is 5.9%, compared to the national rate of 3.7%.

McCreary County is first on the list of poorest counties in the United States. Here's the rest of the list, ordered from poorest to richest:

Wealth is typically measured by looking at the median household income in each county. The median is the middle value in a set of values. Imagine all incomes are lined up from smallest to largest. The median would be the value that falls right in the middle of the list. This is different from the average. In income data, the average can be misleading, since a few very high earners in one spot could artificially inflate the number.

The other way to measure wealth by area is by measuring the per capita income. The per capita income is the average income for an area. Per capita income is a useful way to measure income, but it is better for larger populations. In big groups, outliers with very high or very low income are less likely to sway the figure in one direction or the other.

People who live in wealthy counties are more likely to live longer and have higher education levels. Just because a county is rich, though, doesn't necessarily mean that its state is rich. Although Virginia has quite a few wealthy counties, the income levels in the other counties are lower than average, so Virginia is not at the top of the list of wealthiest states.

McCreary county's life expectancy is much lower than the national average. That's because poor health outcomes are more common in poorer counties. People in poorer counties often have a lower life expectancy and less education. Poorer counties also have higher unemployment rates, and people there are more likely to live below the poverty line.

On the list of 25 wealthiest counties, Virginia and Maryland both have five. Although these counties have high earners, they're not necessarily the highest earners. These counties have a high number of federal workers, lobbyists, defense contractors, and computer engineers that work with the government. Their salaries drive the median income figures that make these counties so wealthy.