Total cost separates into fixed and variable cost

Only variable cost changes with current output.

TC=FC+VC. At zero output, variable cost is normally zero and total cost equals fixed cost. Rent on a current lease, a permit fee, or unavoidable interest may be fixed. Ingredients, hourly production labor, and energy used by machinery are variable. The same expense can be fixed for one decision and variable for another. Use the stated horizon.

Cost classification depends on whether the expense changes with current output, not on whether its dollar amount is large or paid regularly. A monthly oven lease can be fixed while a small amount of packaging is variable. A salaried manager may be fixed over today’s output range but variable when the firm can change staffing in the long run.

At zero output, VC=0 in the basic schedule, so the vertical intercept of total cost is fixed cost. The total variable cost curve begins at zero, and total cost runs parallel above it by the constant amount of fixed cost. If a table shows total cost of $300 at zero output, fixed cost is $300 at every short-run output row.

Average fixed cost is FC/Q, average variable cost is VC/Q, and average total cost is TC/Q=AFC+AVC. Because a constant fixed cost is spread over more units, AFC falls continuously. The vertical distance between ATC and AVC equals AFC and therefore narrows as output grows.

Suppose FC=$240 and variable cost at 20 units is $360. Then total cost is $600, AFC is $12, AVC is $18, and ATC is $30. The identity ATC=AFC+AVC checks the row: 12+18=30. Each average uses the same quantity denominator.

A horizon changes the label

A factory owes $8,000 monthly rent under a current lease. For today’s output decision, the payment is fixed and remains if output is zero. When the lease expires and the firm can choose a smaller plant or exit, the rent becomes avoidable. “Fixed” describes the current decision horizon, not an eternal property of rent.

Fixed cost affects profit but not the short-run marginal cost of another unit. Raising a fixed license fee increases TC, AFC, and ATC while leaving VC, AVC, and MC unchanged. This curve pattern is often more informative than the broad statement “cost rises.”

Do not confuse fixed cost with sunk cost. A fixed cost may be recoverable or avoidable later. A sunk cost is unrecoverable for the present choice. The short-run shutdown rule treats fixed cost as unavoidable during that horizon, but the conceptual tests remain distinct.

When a stem changes an expense, create a before-and-after map. If the expense varies with each unit, place it in variable cost and expect MC and AVC to change. If it is unavoidable regardless of current output, place it in fixed cost and expect only AFC and ATC to change. This classification predicts both the curves and the short-run response. A higher fixed fee lowers profit without changing the MR=MC quantity. A higher per-unit material cost can reduce that quantity.

The zero-output row is an especially valuable check. Total cost there identifies fixed cost, while average measures are undefined because division by zero is impossible. Do not report AFC or ATC at zero. Once output becomes positive, the fixed total is spread across units and AFC falls. Every cost answer should state whether it is a total dollar amount or dollars per unit.