The standard atmosphere

The standard atmosphere can be thought of as the average pressure, temperature and air density for various altitudes. It is useful for engineering calculations for aircraft. It also shows in a general way the pressures and temperatures to be expected at various altitudes. The standard atmosphere is based on mathematical formulas that reduce temperature and pressure by certain amounts as altitude is gained. But, the results are close to averages of balloon and airplane measurements at various altitudes.

This table give density in slugs per cubic foot because it uses the American system of altitude in feet, pressure in inches of mercury and temperature in degrees Fahrenheit. While people often use pounds per cubic foot as a measure of density in the U.S., pounds are really a measure of force, not mass. Slugs are the correct measure of mass. You can multiply slugs by 32.2 for a rough value in pounds.

Source: Aerodynamics for Naval Aviators

Note: This is a summary table. For a more detailed table or a table in metric units, consult an aerodynamics textbook. Many weather textbooks also have tables of the standard atmosphere.