Identify the total cost function, C(q), where q is the quantity produced
Differentiate the total cost function with respect to quantity, q
Write the result as MC(q) = dC(q)/dq
If using discrete data, compute the change in total cost divided by the change in quantity: MC ≈ ΔC/Δq
Evaluate the derivative at the specific quantity of interest if needed
Use the resulting value as the marginal cost