the highest point of elevation in the United States is Mount McKinley at about 6193.54 meters above sea level

the lowest point is Death Valley at about 85.95 meters below sea level

the elevation difference between the highest point and the lowest point is equal to sum of the difference between about 6193.54 meters above sea level and the see level + the difference between the see level and about 85.95 meters below sea level that is

(6193.54 - 0) + (85.95 - 0) = 6193.54 + 85.95 = 6279.49 meters

the elevation difference between the highest point and the lowest point is about 6279.49 meters.