Total Cost and Marginal Cost

This particular total-marginal relation applies to both total cost and total variable cost. Because not only is marginal cost the slope of the total cost curve, it is also the slope of the total variable cost curve. The reason is that any changes in total cost resulting from changing output is matched by changes in total variable cost. This occurs because total fixed cost is FIXED.

This two-paneled graph for the production of Wacky Willy Stuffed Amigos (those cute and cuddly armadillos and rattlesnakes) visually illustrates the connection between total cost and marginal cost. For the first few quantities of output (Stuffed Amigos), total cost in the top panel is positive AND the slope of the total cost curve decreases--it becomes flatter. This corresponds with a positive and decreasing marginal cost in the bottom panel up to 4 Stuffed Amigos. For the next several quantities of Stuffed Amigos output, the slope of the total cost curve becomes increasingly steeper. This corresponds to an increasing marginal cost in the bottom panel.

The prime conclusion is the key role played by the law of diminishing marginal returns in the slope of both the marginal cost curve and the total cost curve.

• The U-shape of the marginal cost curve is a direct reflection of first increasing marginal returns, as marginal cost falls to a minimum, then decreasing marginal returns and the onset of the law of diminishing marginal returns as marginal cost rises.


• However, because the marginal cost curve is a plot of the slope of the total cost curve, then the shape of the total cost curve also reflects the law of diminishing marginal returns. The flattening slope of the total cost curve for small quantities of output is due to increasing marginal returns. Then the onset of the law of diminishing marginal returns causes the total cost curve to become increasingly steeper.