Draw a rough sketch of your acre. This will serve as a visual representation and reference for you when you calculate the outside perimeter.
Use stakes or other temporary placeholders to mark the four corners of the acre.
Measure the perimeter of the acre with a distance measuring wheel. Plant the measuring wheel into the ground at one corner of the acre. Run the wheel along the outside edge of the acre until you hit another corner. Write down your measurement for the first side. Repeat this for the other three sides of your acre.
Add up all the sides. For example, if the acre measures 200 feet on two sides and 300 feet on two sides, the perimeter is: 200 + 200 + 300 + 300 = 1,000 feet.