Trace a map of the lake onto a piece of graph paper or other type of grid.
Count the squares that lie within the tracing. At the edges of the tracing, only count the squares that fall more than halfway inside the edge. Ignore the squares that are more than halfway outside the edge.
Determine the area represented by one square of the graph paper or grid by using the map scale. The University of Florida gives the example of a map scale in which one inch represents 1,000 feet. If the sides of the grid squares are one-half inch each, then each side of the square equals 500 feet (0.5 x 1,000 = 500). The area of each square can be found by multiplying length times width (500 x 500 = 250,000 sq. ft.).
Calculate the area of the lake, which is equal to the number of squares you counted from the grid (N) multiplied by 250,000. To figure the area of the lake in acres, divide that number by 43,560, which is the conversion factor to convert square feet to acres.