In mathematics, the rectangle method of integral calculus uses an approximation to a definite integral, made by finding the area of a series of rectangles.

Either the left or right corners, or top middle of the boxes lie on the graph of a function, with the bases run along the x-axis. The approximation is taken by adding up the areas (base multiplied by height, a function value) of the rectangles that fill the space between two desired x-values.