Sometimes on a boat or just peering into the distance, you might wonder just how far you are from the horizon or how close things on the horizon really are. Calculating the distance of the horizon is easy once you know the formula. Of course, for most people, the complex calculations will require a calculator. Below the calculations are performed in meters and kilometers giving the straightline distance to the horizon.
Things You'll Need
 Tape measure
 Partner
 Paper
 Pen
 Calculator

Measure the distance between your eye and the ground beneath your feet. A tape measure or ruler held by another person will garner the most accurate results. Be sure to mark down this distance in feet. This is the height of your vision line above the surface of the planet.

Know that the calculations for the distance to the horizon must include your current elevation above sea level. Add this number to the number in step 1. If you are at sea level, skip step 2.

Input the total of steps 1 and 2 into the calculator and multiply by 1.5. Mark the answer down on a piece of paper.

Clear the calculator, input the answer from step 3, and then hit the square root button. Clearing the calculator ensures a more accurate answer. The square root answer will be in miles.

Understand that you have just used Pythagoras theorem to calculate the distance to the horizon.