Calculating the size, or area, of a lake can be done a couple of different ways. You can estimate the size by treating the lake as a the closest geometric figure, such as a circle, rectangle or triangle, or you can try to directly count the square footage of the lake by using graph paper on a scaled area map. The latter method may be more precise and represent the only method when a lake doesn't practically fit any basic shape.
Items you will need
Transparent graph paper
Determine the lake's basic geometric figure, such as a circle, rectangle or triangle.
Measure the dimensions of the shape using units of feet. For circles, you want the diameter. For rectangles, you want the length and width. For a triangle, you want the width at its base and the height to the top point. Gain these measurements by consulting the area map and using the scale to estimate distances.
Plug in your measurements to the appropriate formula:
Area (Circle) = 3.14 * (0.5 * Diameter) * (0.5 * Diameter)
Area (Rectangle) = Width * Height
Area (Triangle) = 0.5 * Width * Height
Convert square feet to acres by dividing the area by 43,560.
Overlay a piece of transparent graph paper on top of the area map.
Count the number of graph squares that are positioned over the lake. If the squares are on the edge of the lake, only count the ones where some part of the lake is positioned under at least half the square.
Determine the scaled size of the graph squares. Do this by overlapping a square on the map scale reading, and using that similar to a ruler. This will give you distance on one side. Since it is a square, multiply that figure by itself to get the area. As an example, if the scale says one side of the square is the equivalent of 100 feet, then the entire square represents 100 * 100 feet, or 10,000 square feet.
Multiply the area of each square by the number of squares you counted earlier. As an example, if you counted 50 squares, the lake area would be 50 squares * 10,000 square feet/square, or 500,000 square feet.
Convert square feet to acres by dividing the area by 43,560. In the example, 500,000 square feet divided by 43,560 would yield 11.48 acres in area.