Measure the longest side of the triangular plot starting from one property line marker and moving to the other. Record this measurement. For example, if you have a triangular plot of land three miles long on the longest side, the base is three.
Measure a perpendicular line from the opposite tip of the triangle to the base. In a normal triangle, this is the height. Assume a distance of six miles.
Multiply the base by the height, and halve the result. Three times six equals 18; and 18 divided by two equals nine. The plot has an area of nine square miles.
Convert miles to acreage by multiplying by 640; according to the University of North Carolina, exactly 640 acres exist in one square mile. Nine times 640 equals 5,760 acres.