To find the latest population estimates on the US Census Bureau website follow these instructions:

Click here to open the US Census Bureau site in another window.
This should take you to Washington County, Utah 2010 Census population. If it does not, you will need to enter in Washington County, Utah under "Enter a state, county, city, town, or zip code"

To view the latest population, click on the "Population (Latest Estimate) tab to the left.