# Calculating the computed range

Calculating the Computed range of the light “X”.
Let’s say that light “X” has an elevation of 170 feet above the water and the observer’s elevation is 8 feet.
Using the light’s elevation, we calculate the light’s Geographic range using the formula:
1.17 times the square root of the height = distance to the horizon in nautical miles.
In our case 1.17 times 13.04 equals 15.25 nautical miles. This is the Geographic range.
Secondly, knowing the observer’s elevation we calculate the observer’s Horizontal distance using the same formula.
In our case 1.17 times 2.83 equals 3.31 nautical miles.
This is the observer’s Horizontal distance.
By adding the two values, 15.25 and 3.31 nautical miles, we determine the Computed range of
the light “X”.
In our case 18.56 nautical miles.
By comparing the Luminous range of the light “X”, 21 nautical miles, and the Computed range, 18.56 nautical miles, we can determine the maximum range at which the light “x” may be seen.
In our case this is 18.56 nautical miles.