How to Measure the Square Feet in an Acre That Has Four Unequal Sides
Parcels of land can end up having four sides that are not always equal as they are divided to conform to roads and natural obstacles. This can make determining the area of the land more difficult than for just a simple square or rectangle. In addition to measuring the four sides of the area, the measurement of either one of the diagonals allows the use of Heron's formula to calculate the area of the two created triangles.

Measure the four sides of the area with the measuring tape and one of the diagonals and record the measurements. The choice of diagonal does not matter.

Add up the lengths of the two sides and the diagonal that form a triangle to find the perimeter of the triangle. Do the same for the other two sides that also form a triangle.

Divide the perimeter of each triangle by two and record the value.

Calculate the area of each of the two triangles with this formula:
area = square root (s * (sa) * (sb) * (sc))
Where s is the value of one half the perimeter found in Step 3 and a, b, and c are the lengths of each side of the triangle.

Add the area of the two triangles created by the diagonal together to find the total area of the land.
Things You Will Need
 Measuring tape with markings in feet