Identify the change in total cost
Identify the change in quantity produced
Divide the change in total cost by the change in quantity
Use the formula: Marginal Cost = Change in Total Cost / Change in Quantity
Example: if total cost rises from 100 to 130 when output rises from 10 to 15 units, marginal cost = 30 / 5 = 6
If using calculus, take the derivative of total cost with respect to quantity: MC = dTC/dQ