Get an overview of the area you're trying to figure square mileage for, either on a map, a computer representation or a photograph. Make sure you have some kind of scale to gauge your measurements against---for example on some maps, 1 inch on the map equals 1 mile in the real world.
Decide which shape best fits the shape of the area you're measuring. Is it a square, rectangle, triangle, circle or parallelogram? If you can't quite fit the area you're measuring into any of these shapes, try to break it into several shapes---say, a square next to a triangle---and then calculate the square mileage of each area before adding them together to get the total square mileage.
Calculate the square mileage of squares and rectangles by measuring the length of two sides adjacent to each other---in other words, two sides that come together to form a corner. Multiply these measurements together to get the square mileage of the figures.
Calculate the square-mile area of a circle by measuring a straight line from one point on the circle to the other side, making sure you pass through the center of the circle. This is the circle's diameter. Divide the diameter in two to get the radius. Multiply the radius by itself---this is called squaring it---and finally multiply it by 3.14 to get the area of the circle.
Calculate the square mileage of a triangle by measuring how tall it is at its tallest point and how wide it is at its widest point. Multiply these figures together and divide the answer by two.
Figure the area of a parallelogram by multiplying the shape's height by the length of its base. Note that the figure's height is the distance straight across from whichever side you designate as the base to the other side, not the length of an adjacent side.